KT88 vs EL34 vs 6L6 Substitution In Vacuum Tube Amplifier.wmv


  • Опубликовано:  6 лет назад
  • This video was inspired by a question I was asked about swapping KT88's in place of EL34/6CA7's. I have used a couple of amps in the video and made cathode current and power output measurements on three types of tubes -- 6L6GC, KT88 and EL34. This video is not meant to encourage anyone to swap the output tubes in their amplifier out for other types but in the case of musicians, I know they are always looking for a unique sound that they may be able to get through vacuum tube substitution. Remember also, swapping around the 12AX7, with a 12AU7 or 12AT7, etc. may also change the sound and no over current conditions are going to exist with these tubes. In rare cases I have seen instabilities introduced by substituting these tubes resulting in oscillations. In any case, I hope the video does provide some (safe) ideas on tube substitution and helps prevent destruction of equipment.
  • КатегорииРазвлечения
  • Длительность: 15:53
  • Тэги для этого Видео:  KT88  6550  EL34  6CA7  6L6  6L6GC  Substitution  Vacuum  Tube  Amplifier  tube substitution  vacuum tube amp  Guitar Amplifier (Literature Subject)  Vacuum Tube (Invention)  vacuum tube substitution  guitar amplifier tube  Valve Amplifier  

Комментарии: 192

  • Jamie Avery
    Jamie Avery 1 месяц назад

    Hi there! Very interesting video! I am new to tube amps and just got a Peavey 5150. Eurotubes says that if you install an integrated quad set of 2 JJ KT88's or 2 JJ KT66's and 2 JJ 6L6's then the amp does not need to be biased and it's basically "plug and play". They said they did a lot of research on this and that it not only sounds great but saves you from having to bias the amp. They recommend you put the 2 KT's on the outside sockets and the 2 JJ 6L6's in the 2 middle sockets. You seem to know what you're talking about so I was just wondering if you think this is safe and is a good idea. I know this is an old video so I hope to get a response cause I just ordered the tubes today! Thanks!

  • Tom LaMar
    Tom LaMar 2 месяца назад

    Why does he not test and mark the tubes for matching first...and talk about screen voltage, which is a major player in substitutions of this type ? ...wish I had that much equipment !

    • ElPaso TubeAmps
      ElPaso TubeAmps 2 месяца назад

      Hi, and thanks for the comment - It has been some time since I have had comments on this video. I did post videos where I measured screen current and I agree that it is very important. In all of the triode connected amps like this one and the ultra-linear design, the screen voltage is at the same potential as the plate. I made a video about what happens to the screen if the plate voltage is removed (bad socket pin, etc) which ends up very destructive for the screen. The screen grid can be damaged very quickly and go unnoticed except that performance deteriorates. The message that I hope this video conveys is that "rolling tubes", as it is called, is generally a bad idea. The idea is putting a KT88/6550 in place of a 6L6 will provide more power and that is not true. I hope it is noticed that the control grid bias voltage is much lower for the 6550 (around -55 or so) than it is for the 6L6 (-37 or so) and that in an opposite manner than what most people think, you are better off putting a 6L6 in a KT88 amp than the other way around as the lower bias voltage in the KT88 amp will actually protect the 6L6 from over current.


  • JONNY HAFER
    JONNY HAFER 3 месяца назад

    what about the primary impedance of your output transformer. all those tubes call for different primary impedance. yes / no

    • ElPaso TubeAmps
      ElPaso TubeAmps 3 месяца назад

      Yes, the tubes do call for a different primary impedance but this video was to try to address the idea of "tube rolling" as it is sometimes referred to by musicians with the idea of simply swapping one tube for another in guitar amplifiers. I hope the interesting characteristic that can be seen from this video is that, from a broad view, a 6550 requires about -55 volts bias whereas a 6V6 requires about -15 volt bias so, again on a broad scale, if a 6V6 is placed in a 6550 amp, the (-55 volt bias) will likely be so high (negative) that the 6V6 will not self destruct. On the other hand, if a 6550 is placed in a 6V6 amp (with -15 volts bias) the 6550 will try to draw so much current that everything will go terribly wrong. (A 6L6 requires about -37 volts bias) So, one lesson to be learned is that putting a higher power tube in a lesser power amplifier will not give the power level increase that is desired and this is just from the DC level performance of the amplifier not considering the AC impedance characteristics. The other important factor is a 6L6 requires 900 mA of filament current (a 6V6 requires 450 mA) whereas a 6550 or an EL34 requires twice that much filament current as a 6L6 and is likely to overload the filament winding of the power transformer..


  • Alan Torrance
    Alan Torrance 1 год назад +1

    At 1% distortion -- this is NOT good. The old Quad Amplifier (from UK) using KT66's (mind you only 15 Watts max) had less than 0.1% total distortion.

    The sound with 1% or more distortion must have been terrible. No good whatsoever for classical music, but perfect for Pop, Metal, etc and other modern "noise".

    • stuknda80z
      stuknda80z 7 месяцев назад

      the queen eats bangers in bed in her panties


    • Charles Ayala
      Charles Ayala 8 месяцев назад

      I was actually referring to Mr. Classical Music Elitist Alan Torrance. but your setup sounds pretty sweet


    • ElPaso TubeAmps
      ElPaso TubeAmps 8 месяцев назад

      I prefer CD over vinyl. I use Mcintosh C20, MC275 and MVP831with Klipschorns. As you might see from my videos, I like to experiment with other vacuum tube amplifiers. I have never been charmed with SE amplifiers. LF response on SE amplifiers is usually poor.

      I have another system with Dahlquist speakers that sound pretty good but are terribly inefficient. I have to drive them with a MC2205 and MX112.

      The stereo amp I built not long ago with the Acrosound TO-305 transformers from the Williamson/Heathkit circuit is a beautiful sounding 25 watt system on the Klipschorns.

      I have some nice turntables, Rek-O-Kut and Thorens and others but acoustical feedback is always lurking when playing vinyl records.

      And yourself? What do you prefer?


    • Charles Ayala
      Charles Ayala 8 месяцев назад

      what medium are you playing your classical music with?


    • ThisName DoesntMatter
      ThisName DoesntMatter 8 месяцев назад

      passive-aggression isn't cool. in a hi-fi it doesn't matter what "noise" is put through it, everyone would want the lower THD.


  • Don Molenda
    Don Molenda 1 год назад +1

    soo?? this second amp has a B+ of around 350v when the 88s load it down...in order to qualify 70ma each. i am looking to convert a marshall superlead (34s) to 88s.. could you verify the needed change in splitter resistors and screen resistors...?

  • R. Weaver
    R. Weaver 1 год назад +1

    I too am contemplating a KT88/6550 tube change in a older marshall 1988 for more clean headroom, none of the new amps have the tone i’m after and I can’t afford a Dumble or Bludotone!

    • R. Weaver
      R. Weaver 1 год назад

      +ElPaso TubeAmps unless I'm mistaken I should be able to modify the circuit so as to get the bias where needed


    • R. Weaver
      R. Weaver 1 год назад

      +ElPaso TubeAmps Tranny is shorted at present, so changing to accommodate tube change should help. I am currently looking to change mid control to enhance 500k a little more. 


    • ElPaso TubeAmps
      ElPaso TubeAmps 1 год назад

      +R. Weaver It could be OK and you may like the change in tone. If the Marshall runs EL34's the filament current change to the 6550 will be OK. The filament of the 6550/KT88 and EL34 draw about 1.5 amps each whereas the 6L6 only requires 0.9 amp each) What I hope was noticed in the video is that the bias voltage for a 6L6 is approx -37 volts, for a EL34 approx -44 volts and for a 6550/KT88 approx -55 volts ( all approximately). What the point is, if your Marshall will allow you to adjust the bias a bit more negative, the 6550 could be OK but you must check it or you will just ruin tubes. Another point is, with a 6550 amp with a bias of approx -55 volts, even with the higher plate voltage in the EL34/6550 amp, you can, for a short time, put a 6L6 in the socket and it won't go up in flames since the 6L6 typically needs only -37 volts bias to set the cathode current at a safe level. But, in a 6L6 amp with -37 volts negative bias if a 6550/KT88 is placed in the sockets, the tube will draw Way too much plate/cathode current and things will not go well. Also, placing either EL34 or 6550 in a 6L6 amp will overload the filament circuit of the power transformer. Good luck with your project.


  • GraffitiPhysical
    GraffitiPhysical 1 год назад +1

    In a nutshell, you have to re bias your amp if you change the tubes. Some amps have switching on the back, others the bias has to be manually done. So, the rule of thumb is just use the recommended tubes. My experience is the better the tubes the better your amp will sound. New tubes have to be broken into, they will sound very clean. I was upset when I put new tubes into my amp, it didn't have the same distortion and sounded too clean. Well, a lot to be said about old tubes and good distortion.

  • Christopher Rowe
    Christopher Rowe 1 год назад

    hit a power chord with each of'em.

  • Giuseppe Loriga
    Giuseppe Loriga 1 год назад +1

    Can i Replace in a Marshall 1986 or 1987 the EL34 with the KT88-94 for More clean Headroom ?
    The Heater current of the KT88 is about the same of the EL34 ....I know i have to lower the bias feed resistor from 220k to 100k to avoid tube problems ..the question is : the transformer can handle the bias current draw with the KT88... i have read that the tube should be biased like a EL34...but if bias this way they should be biased too cold !!!!! Can i bias at 60% with a plate voltage of 470v-480v without overheating the transformer ?

    • Giuseppe Loriga
      Giuseppe Loriga 1 год назад

      +ElPaso TubeAmps Thanks for your Help ... So what is the effect of the reduced plate current in the tube ? Can i bias the tube at 60% ( 43.8ma with a plate volge of 480v Class AB Fixed Bias) or it will run colder ? Last question the pin1 in my el34 tube socket is grounded to the chassis through the 1 Ohm resistor , is that ok for the KT88 ?


    • ElPaso TubeAmps
      ElPaso TubeAmps 1 год назад

      +Giuseppe Loriga You are right about the filament current being close in the EL34 and 6550/KT88 so that should be OK. Plate voltage on both tubes can be as high as 800 volts. I doubt it will damage your transformers but I would limit the plate current in each KT88 to 70 mA (140 mA for both KT88's) by adjusting the grid bias. As the amplifier is built for an EL34 the bias is probably not negative enough for the KT88 so it will have to be adjustable to a more negative voltage. Pin 1 of the EL34 is g3 and is probably grounded at the socket. Pin 1 of the KT88 is attached to the metal ring of the 6550/KT88 - make Sure pin 1 of the KT88 socket is grounded to the chassis - as it probably already is..


  • Greg Graham
    Greg Graham 1 год назад +1

    This seems to be a very informative video -- thank you for posting it!
    In the spirit of improving things, I must say that I did find it difficult to watch because of the head-mounted camera, and had to stop watching part-way through. Perhaps a tripod would help, and as would arranging your test equipment so that the DVM and scope are both near the amp can be seen with minimal camera movement. Not trying to be mean-spirited, just a suggestion for your consideration that might get you more viewers.

  • Jacksonkellyfreak
    Jacksonkellyfreak 1 год назад +3

    One of the more interesting videos I've seen in a long time. Excellent.

  • koubou kankyu
    koubou kankyu 1 год назад

    I like EL34

  • erwin flores
    erwin flores 1 год назад

    you just change your - voltage ex: KT88 or 6550 -55, KT77 or 6CA7 -38, KT66 or 6L6 -44.

  • erwin flores
    erwin flores 1 год назад

    fix bias amp

  • joe zé
    joe zé 1 год назад

    https://www.youtube.com/watch?v=T12PmkRqh1I

  • AL theDoctorWho
    AL theDoctorWho 2 года назад

    Those 6CA7s tubes are very dependable and they sound great I have a Quid set in my plush amp we tinkered with they are GEs however so the tone is there. EL34s were very unstable in the plush. I did put in a presence pot in the circuit. so the amp sound like a fender on steroids a cross between English & American. I like the math of it but I am not a tech. Thank you for posting this video. c];-D

  • Crobular I
    Crobular I 2 года назад +1

    I don`t know if you are using grid or cathode bias but check the manufacturers tube spec. Also remember that with EL34`s (6CA7)  you must earth the screen pin (G3) seperately.

  • Hector J
    Hector J 2 года назад

    huhh?

  • Mike Muratagic
    Mike Muratagic 2 года назад

    Thankyou soo much for your input on that, as that is one of the most common questions people ask me when buying used amps is how can they make sure they are getting what they are paying for, or if there is less output then what the amp should be putting out.. In your experiences have you ever come across amps that should put out more power, but for some reason don't? Or are all manufacturers honost when they publish rms power output? (I know the peak power output of some amps are a farce,) haha I've seen computer speakers that say 1500watts pmpo output! hahaha... Maybe they are like 10watts at most probably less i'm guessing and yet no one does anything about them putting those crazy numbers as far as i'm concerned they should get sued for putting that crap.. - I've bookmarked you and subscribed, you are very knowledgable in your field as I can see and your definately my new mentor, please keep making your videos they are very informative. If you wouldn't mind I would like to keep in contact and probe you for knowledge now and then if thats okay with you..? I think i'm going to start with tube amps, as their circuts seem so simplistic compared to solid state.
    Always look forward to your reply. - Mike.

  • Mike Muratagic
    Mike Muratagic 2 года назад +1

    question, when your figuring out the wattage output of an amp, i know you use voltage squared divided by the ohms gives you the wattage, but when your checking the voltage do you have to have to use a true rms meter, or is there a way to do it with a regular multi meter?also, do you know of any cheap devices that I can get to verify the actual output of an amp before clipping? or do you basically have to have a osciloscope, frequency driver, rms volt meter, resistive load(to emulate speaker), and distortion analyzer? or can i not use an osciliscope and just get away using a distortion analyzer?(somehow i have a feeling a dist analyzer is more expensive then a scope)

    sorry for my igrorance but i'm relatively new at electronics repair and am trying to set myself up for working on tube/ss power amps.  

    • ElPaso TubeAmps
      ElPaso TubeAmps 2 года назад

      You have described pretty much the whole setup. A distortion analyzer and an audio spectrum analyzer is probably the most expensive things to have. An oscilloscope, a signal generator, a RMS reading volt meter, and dummy loads are necessary. You can drive the amp up to just below clipping and get a pretty good idea of its performance with a THD around 1%.  Noticing the shape of the sine wave is important also. Pretty much any volt-ohm meter will work for reading RMS voltage at 1 KHz like the vintage Triplett and Simpson analog meters Knowing the frequency response of the meter before purchasing is a good idea and ideally you would want one that is flat from at least 20 to 20 KHz or better. The Tektronix TM500 series instruments will let you have an oscillator, voltmeter, distortion analyzer all in one mainframe. The most expensive items for this type of setup would be a SG505 oscillator and the AA501 distortion analyzer - you could pay $1K for those items but possibly less. You can buy good used Tektronix scopes for little money and in my opinion they are definitely the best to have. Of course new ones with a spectrum analyzer and oscilloscope built together (called Mixed Domain Oscilloscopes - MDO) are lots of thousands of $$$. Good luck with your enterprise. If there is anyway I can help, please let me know.


  • vargasmongo3435
    vargasmongo3435 2 года назад +1

    hello sir this video is really teaching a lot and it does help me a lot I still reading and comprehending more about tube amplifiers, I have a question if you don't mind, I still in the process of making a KT88 push pull ultra linear hi fi amp "not for guitar" home use on the cathode of the KT88 what resistor should I use 10R 3W is fine or should I get a 5W ? many people have use 5W and 3W what is correct ?

    the B+ will be around 460V DC from a Edcor transformer model XPWR247-120 330 - 0 - 330 you mention already "long time ago" that I should use 4.2K output transformer one more question about output transformer, should I get one that can go up to 100W or one that supply about 60W ? I was thinking about the B+ and I'm not sure what transformer should I get because of the 460V DC B+ I'm a bit confuse to be honest

    Regards
    Juan

    • vargasmongo3435
      vargasmongo3435 2 года назад

      +ElPaso TubeAmps
      good idea about the HT fuse and the cathode resistor to be 3W and yes this does helps about the "Kill-A-Watt" really good idea I didn't  think  about that, I'm going slow with the build so when I have something done I will let you know thanks for the help :D and yes I have been aware of high B+ dangers don't worried I be really careful  my MOS was 94F Special Electronics Device Repair you have a great day sir :D

      Regards
      Juan


    • ElPaso TubeAmps
      ElPaso TubeAmps 2 года назад

      Hi Juan - here is the way I would look at it - absolute maximum cathode current for a pair of KT88's is about 275 mA - that is the current if the tube is being driven to maximum sustained sine wave power. (from a 1962 RCA tube manual) That current flowing through the 10 ohm cathode resistor would dissipate power of (I^2*R) which would be about 0.76 watts so a three watt resistor would be OK - even a 1 watt resistor would not burn up but would get pretty hot. Another possible way to look at it is, in case something goes terribly wrong in the tube and there is an excessive amount of current flowing through the tube, a 3 watt resistor would burn up before a 5 watt resistor would and "might" help protect the output transformer from excessive over current. So, we can see that making everything larger is not necessarily a good way to go. On the other hand, if the primary of the power transformer is fused properly, then it should blow and protect the amplifier. Another way to protect the circuitry is with a fuse in the high voltage line - a 0.5 A should be sufficient. Many guitar amplifiers have a HT fuse. Once you complete the amp, if you have one of the handy "Kill-A-Watt" meters that you can get pretty cheap at Lowes' you can actually measure the power up current of the amp and set the line fuse to some value above that point, somewhere around twice the max current that you measure. Hope this helps.


  • Nikos Damigos
    Nikos Damigos 2 года назад

    Hi my friend thanks for your videos
    i have one old  70's  Sound City-50plus  amp, with JJ EL34 power tubes
    I want to make his headroom bigger and stop the break up with bigger volumes (i have allready check my preamps and i find the right for me)
    could i change them with KT 88. KT66  or other powertubes.                                 How this could change the sound with the KT88 or other set?.
    Thanks a lot again

    • Nikos Damigos
      Nikos Damigos 2 года назад

      +ElPaso TubeAmps
      you are very helpfoul!thanks alot again, can i put a pair of kt88 with out change the socket in of the tubes?
      also could i change them with out a bias just for a listen?does it cost damage if i leave them with out bias? i am from an island and is really difficult to find a tech...
      thanks agaian sorry for my English :)


    • ElPaso TubeAmps
      ElPaso TubeAmps 2 года назад

      The filament current of the EL34 and 6550/KT88 are close enough that the filaments will be OK to switch to KT88's - but, the question remains if the bias supply has sufficient range to lower the cathode currents of the KT88's to a proper level. Only actually checking bias voltage levels and cathode currents will tell for any particular amp. I have successfully changed EL43's for KT88's in client's amps and one gentleman loves it. KT66's are just fancy 6L6's - I don't recommend using them in an EL34 or KT88 amp as the plate voltage is likely to be too high for safe operation.


  • orange70383
    orange70383 3 года назад

    Tubes are great, there's honesty in them, you get what you get.

  • Jyme Bale
    Jyme Bale 3 года назад

    Yes, Tim, your question will work with a JTM 45, you can use a dozen different tubes, you do have to change the bias resistor though.

  • Tim Stanley
    Tim Stanley 3 года назад

    Do you think this will work with any Kt88-6550 Amp?  I have a 50 watt Ampeg VL502 and I wanna try a set of 5881"s or Kt66"s. I cant get a good yes or no from any one... the amp has the switch to run either EL34"s or KT88"s thanks  

    • Tim Stanley
      Tim Stanley 3 года назад

      Lol I really appreciate you taking the time to break it down for me. I think it mite be better for me to maybe wait and experiment with an amp that's already set up to run the Kt66's or maybe find a good tech that could check my amp out and see just what it is actually putting out and how hard it would be to set it up to run them. With the price of tube amps coming down everyday I could get a decent 50 watt head for Under $600/ 700 bucks. Thanks again!!


    • ElPaso TubeAmps
      ElPaso TubeAmps 3 года назад

      +Tim Stanley A KT66 is just a "HiFi" version of a 6L6GC. A KT88 is the "HiFi" version of the 6550. EL34 and 6CA7 are the same as is an EL84 and a 6BQ5. You can experiment but to keep from frying things you must be able to measure (and adjust) cathode current. There are a number of "bias probes" available to do this and max cathode currents for all tubes can be found on the Internet. A static (quiescent) state for all the tubes is generally around 20-30% of the maximum plate dissipation for each tube. The higher the static plate dissipation, generally, the lower the THD up to the point of tube destruction. It is not terribly complicated but these things are what need to be taken into consideration otherwise a person is just plugging in tubes and watching what happens which often times is not what you want to happen.


    • Tim Stanley
      Tim Stanley 3 года назад

      How about the Kt66's? Would if fry those as well?


    • ElPaso TubeAmps
      ElPaso TubeAmps 3 года назад

      Tim - some things to consider - first off 5881's have a lower plate dissipation than the 6L6GC - I had a McIntosh MC240 for many years, 30 years ago, and new USA made 5881's would not last more than a week. 6L6GC's would last forever.
      The EL34 and 6550 can handle as much as 800 volts on the plate - a 6L6GC is more like 550 max so, you may have a serious problem there if you put them in place of your other tubes and may end up having a problem like I had with the 5881's in my Mac amp. One thing in your favor is the bias voltage for EL34's and 6550's is high enough that it will probably prevent your 6L6GC's from being destroyed immediately. Keep these ball-park figures in mind, 6L6 needs about -37 volts for 450 on the plate, the EL34 needs about -42 volts for 500+ volts on the plate and the 6550 needs about -54 volts for 600 or so volts on the plate. These are not exact numbers. What you can see is that if you plug a 6L6 into a 6550 amp you will probably have 600 volts on the plate with -50 volts bias on the control grid and it will probably work but I don't think the 6L6's will last long. Hope this helps.


  • jamesterry51
    jamesterry51 3 года назад

    thats what is guitar players love is the distortion on a tube amp when you crank it

  • Bernard Mahoney
    Bernard Mahoney 3 года назад

    i had an 80 watt valve amc power amplifier using kt88 it was awesome a great sound for the money .

  • AeroKitsune
    AeroKitsune 3 года назад

    So you could potentially run a pair of 6L6GC's with a pair of KT88's? Or am I mistaken?

  • Alexander Shibilski
    Alexander Shibilski 3 года назад

    What is that piece of equipment that tells you the voltage and percentage of distortion?
    I've got a scope and would love something like that!!

    • ElPaso TubeAmps
      ElPaso TubeAmps 3 года назад

      The distortion analyzer I use in this video is a Tektronix AA501. It also has a voltmeter built into it but for convenience I use a separate voltmeter. The Tek stuff for the 500 series modules are not too expensive except for the SG505 and AA501. Possibly another good instrument for you would be the HP 8903. It comes in model A and model B. It doesn't matter which one you get as they are both good. Check out these on Ebay and maybe you can find a good one - good luck.


  • lukefenderrhodes
    lukefenderrhodes 3 года назад

    hello boss on   a line balance potentiometer i dont have a 27 ohm resistor but i hve a thirty ohm resistor, how would that effect the line balance on the plate, or could i take a twenty ohm and a 8 ohm put them in parrallel together? thanks for listing to my nonsense

  • Mark70609
    Mark70609 3 года назад

    Could you list the equipment you used?

  • Tomii S Sonic
    Tomii S Sonic 3 года назад

    This was a great video, articulated very well! Thank you!

  • Bobby Dhal
    Bobby Dhal 3 года назад

    you are awesome !!!!!!    thank you thank you 

  • munja2111
    munja2111 3 года назад

    Great video Sir! Please advise me, I have HiFi amplifier Cayin A55T which have 4 KT88 in output in push pull configuration. Anode voltage B+ is about 400 V, Cathode is connected to 10 ohm resistor to GND. Bias is adjusted by negative voltage on grid and can go from -70V to -10V. I biased KT88 of about 45 mA (0.45 V across cathode resistor). Can I put EL34 valve in my amp and rebias it to about 40mA cathode current (voltage on grid can go up to -10 V)? Pin 1 and 8 are connected together in valve socket. I know that I will lose some power but I love EL34 tube a lot. Thanks :)

    • Tboy2000millenniumX
      Tboy2000millenniumX 2 года назад

      +ElPaso TubeAmps
      Hi!  Do you find the EL34 700-800v plate rating also to be true of the Chinese and Russian imports?  Thanks!


    • ElPaso TubeAmps
      ElPaso TubeAmps 3 года назад

      +ElPaso TubeAmps Sorry - I see I replied long ago concerning this. 


    • ElPaso TubeAmps
      ElPaso TubeAmps 3 года назад

      With that wide range of bias adjustment, you should be able to safely install EL34's in your KT88 amp. Both the EL34 and KT88 can take 700-800 volts on the plate, the EL34 draws slightly less filament current so that is good. Overall, it should be a nice experiment and you may like it better than the KT88. You are very rare as you are interested in going "downward" in power rather than trying to put KT88's in place of 6L6's just to get more power which is a really bad idea. Good luck with your project, I think it could go well - please let me know what you discover and how it sounds compared to the KT88's.


    • ElPaso TubeAmps
      ElPaso TubeAmps 3 года назад

      For some reason, I like the EL34 also. With the wide range of bias adjustment you have, you should be able to play with all three of the output tubes. Ballpark bias for the 6L6 is -37V, the EL34 is -45V and 6550 is -55V. For a HiFi amp Dynaco runs their 6550's at 70mA per tube. Pin 1 for the EL34 is the screen grid which Is Not internally connected as the 6550 is, so, pin 1 Must be grounded. You can leave the 10 ohm resistor in the cathode (pin 8). For the EL34, 40mA per tube is a good HiFi setting. Have fun - be careful.


  • wqpeb
    wqpeb 3 года назад

    Wonder what this MK III would actually sound like? 

  • A Colin Flood
    A Colin Flood 3 года назад

    shared on my Audio Tube Amp page on Facebook

  • wreckoningday
    wreckoningday 3 года назад

    I have a Sunn TC-50 50/25 watts w 6L6's and it has a bias adjustment so could I swap those w EL34's? Sorry if you already answered this roundabout in ur video, I'm just learning about all this and find it fascinating. Thanks for any help u can provide

  • jpalberthoward9
    jpalberthoward9 3 года назад

    You got that right. I sold my '66 Marshall plexi 50w years ago because if I turned it up to where it sounded good, everybody in the world hated my guts. These days, I use an old Tweed Fender Princeton and everybody's happy, including me. That's it in the little picture to your left. It works great.

  • ElPaso TubeAmps
    ElPaso TubeAmps 3 года назад

    have a bias supply with enough range to be able to make the bias supply negative enough to move from 6L6's to KT88's. 6L6's need bias something in the -40 volt range whereas the KT88's need bias voltage in the -50 volt range. I don't know about your Mesa Boogie Triple Rectifier - some Mesa Boogie's don't have adjustable bias voltage - you will just have to investigate the schematic carefully to determine if it will work and not overheat.

  • ElPaso TubeAmps
    ElPaso TubeAmps 3 года назад

    I can't remember the tube layout for every amp so I make mistakes. I have done mods for clients whereas I have swapped out 6L6's for KT88 and the client is happy just as you are. The potential problem is the 6L6's draw 0.9 amps each for the filaments and the KT88's draw 1.6 amps each so, most likely the filament voltage is low and the power transformer filament winding is overly stressed. It may last years or it may not. If it works and you are happy with it then all is good. Not all amps (next)

  • decadepast decadepast
    decadepast decadepast 3 года назад

    Yea but like I said I replaced the 6L6s in my Marshall JCM 900 with KT88s years ago and it still play perfectly. Do you know anything about guitar amplifiers? When I switched out the 6L6s with KT88s I had the amplifier re-biased and the thing plays better now than it ever has before. I'm just wondering If it is possible to do the same thing with a Mesa Boogie Triple rectifier.

  • ElPaso TubeAmps
    ElPaso TubeAmps 3 года назад

    A pair of 6L6's is good for 50 watts and a pair of KT88's are good to 100 watts - that's just the way it is. You cannot put KT88/6550 in place of 6L6's - It will "work" but not very well and not very long. Hope this helps.

  • decadepast decadepast
    decadepast decadepast 4 года назад

    I use a Marshall JCM 900 with KT88s and it destroys my guitar player's Mesa boogie triple rectifier which has 6L6s in it. We can't put KT88s inside the Mesa so is there anyway that we can get the Mesa as powerful, clear and have anywhere near the same headroom my Marshall has? Please help!

  • thescubacult
    thescubacult 4 года назад

    I love this guy.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    For a RPN calculator it is 1 (enter) 1 (+) to get the display of 2

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    Is that where the stainless McIntosh (repos) are coming from? I haven't seen them on Ebay yet but I have seen the "redone/rechassis" amps that look great. If someone starts making repo Mc transformers I have serious concerns that they will be the quality of a Mc transformer which is the heart and soul of the Mc performance. Thank for bring this up.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    That is correct about the 7027 - pin 1 is tied to pin 4 internally which is the screen - which also means depending on how the socket is wired, if a 6550 with its metal ring is plugged into a 7027 socket you could end up with screen voltage on the metal ring (because of pin 1). Also, pin 5 and 6 are tied together on the 7027 which means on the 6L6, etc group of tubes, often times pin 6 is used as a tie point. Thanks for the post and bringing this to light for all viewers.

  • gbtayc
    gbtayc 4 года назад

    What guitar are you rockin

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    I have only experienced the KT120 in some Dynaco spin-offs - can't remember the brand - impressive tube, very healthy bottle. I think their cathode current is a bit higher than the KT88 and KT90 so. some care should be taken before switching them.

  • ronnieparfait
    ronnieparfait 4 года назад

    Great Video, you must be an O'l TV repair man with all the knowledge you have, thanks for taking the time to educate some of the students of electronics out there., and please keep up the good work, I myself is can't thank you enough for the free teachings that I'm sure you had to pay for through out life. Thanks again and people out there if you don't like the video don't watch it,, it's that simple.

  • Hayves Yong
    Hayves Yong 4 года назад

    Have you try to use KT120 as well?? It seems like this tube relatively quite new to market...

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    I have never personally had any KT77's but they are sold as replacements for EL34. The KT66 is sold as a replacement for the 6L6. KT88's and 6550's are interchangeable. All, of course, with attention to bias and plate current when initially setup.

  • Danyrd P.
    Danyrd P. 4 года назад

    So you can use KT 77s instead of 6LS? KT77 would be fine for 6l6?
    This video kicks ass.

  • Justin Franks
    Justin Franks 4 года назад

    +1 (or should it be 1+ ?) for using an RPN calculator :)

  • littlegoobie
    littlegoobie 4 года назад

    today, all i can say is thank goodness i did not buy one because i i would need to go buy cassette tapes in the clearance shops beside VHS/Beta tape display, which is behind the film camera aisle, around the corner from the wooden wagon wheels. I still like my vinyl but it's pretty much impossible to find new stuff these days and besides that, the analog edge is gone now that mastering is pretty much all digital so tapes and vinyl makes less sense to me every day. I enjoyed the video.thanks

  • littlegoobie
    littlegoobie 4 года назад

    20/20 hindsight. I can only hope my retirement funds start moving soon because, according to their growth the past 2 years, i've been in some sort of time stopping bubble where nothing gains value.

    I also remember nakamichi dragon tape decks in the pawn shops and i was drooling all over those too, given the choice, i could see myself getting that instead of the power amps because i was looking at all kinds of amps. There was so much to choose from compared to top shelf tape decks.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    I wish I had bought every Mac amp I saw even at retail price back in the early 60's - the MC275 was $444.00 new - a NIB one now from that era would be worth an easy $10K+ - better than the stock market. I love this stuff and now that I am retired I get to play with it a lot - I don't have any vids on mods - I have done mods for musicians around my area but it seems many turn out disappointing as they don't get the sound they thought they were going to get - good luck.

  • littlegoobie
    littlegoobie 4 года назад

    jibber jabber volts amps jibber. I wish i had the background to understand what you're doing. looks like really cool stuff. I used to see mcintosh amps in pawn shops 25 yrs ago. I wish i grabbed a few when they were there. Oh well.

    I got here while watching vids on adjusting/modifying tube guitar amps

  • Rocknrollthor
    Rocknrollthor 4 года назад

    Brilliant video man!

  • Lawrence Hudlin
    Lawrence Hudlin 4 года назад

    God I wish I knew valve technology like you do. Fantastic video and commentary.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    6L6's run at about 40 mA each (80 mA per pair) and it is customary to put in a fuse at twice the normal current rating. KT88's are about 70 mA per tube (140 mA per pair) so,l with that approach you would need 280 mA fuses - It would be wise to stick with the 6L6's - there are many brands out there, and 6L6's are about half price of KT88's. Hope this helps.

  • Noillacspart
    Noillacspart 4 года назад

    Well my ENGL's 60 watt's per side and spec's say that each power amp tube gives has a fuse of 160mAM, is that enough to power it? Will i have to look at other 6L6GC's?

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    KT88's draw 1.6 amps per tube filament current to a 6L6 of only 0.9 amps per tube so, that is one concern. The bias on the KT88 will need to be made considerable more negative to reduce the cathode current to an acceptable value. There is likely not enough adjustment in the bias pot to accomplish this. You will need to change bias divider resistors and even then, you may not be able to attain a proper negative voltage setting for your KT88's. You may find more help in some of my other videos.

  • Noillacspart
    Noillacspart 4 года назад

    Hi, i have an Engl 930/60 watt power amp and was looking into changing the 6L6's inside to KT88's. Will i be able to do this or will they have to be biased for them to work?

  • Anthony Brown
    Anthony Brown 4 года назад

    this is so cool. I am a worship pastor and I use tube amps with my electric gutiars and I love how all this works. I hope to have the understanding and some of the testing equipment you have someday. Glad that there are people doing this kind of stuff still, no one my age seems to be (Im in my mid 20's). The only tube thing Ive built so far is a 12ax7 powered overdrive pedal.

  • Doogie Lindsay
    Doogie Lindsay 4 года назад

    Those EL34's are filament pigs too...

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    Thank you for your comments. There are may reasons we can't permamently swap 6550's into a 6L6 amp and one of the most fundamental reasons we can't is because of the filament current required for the 6550's. A pair of 6550's require 3.2 amps to light the filament whereas a pair of 6L6's only require 1.8 amps. The filament winding on the power transformer will be damaged over a period of time if it is overloaded by that amount.

  • fiream773
    fiream773 4 года назад

    I am brand new to the world of tube amps. I have a '55 AMi G-120 Jukebox with a mono 6L6 amp that a knowledgble friend helped me rebuild. I've been bugging him about modifying the amp to clean up the sound and improve the headroom with 6550's. He's an excellent engineer, but not good at explaining why I can't just drop in 6550's and make 1 billion gigawatts. This video showed very clearly the pitfalls of tuning an amp "by ear". Best 15 minutes I've ever spent on YT. Thank you 4 posting!

  • KyleCarrington
    KyleCarrington 4 года назад

    Interesting. Makes perfect sense, really.

  • pseudokowski
    pseudokowski 4 года назад

    Unfortunately it does, on most Audio Amps all the vibration is isolated from the speakers unless it's a console unit.

  • KyleCarrington
    KyleCarrington 4 года назад

    Interesting point. Does that same ideology apply, then, to a combo - wherein the tubes are in the same box, that the speakers rattle?

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    I understand what you are saying but I came to realize that the creation of music and the playback of music is two different worlds and the fact that tubes are electro-mechanical devices and are affected by the vibrations of the cabinets is what allows musicians to create the music we love. Then, we try to playback at ultra-low distortion the music they created a 10% THD and above. Holding the guitar in front of the speakers may create a desirable tube related mechanical feedback that we like.

  • pseudokowski
    pseudokowski 4 года назад

    It's all about the pre amp section, also I don't get why Guitar players insist having the Amp head on top of the speaker stack. Man all the 12AX7's will go microphonic so fast compared to a Audio system that isn't going through a Earth Quake of vibration.. Kills me seeing nice OLD Tubes going into Guitar Amps sitting on the stacks!

  • Bob Hope
    Bob Hope 4 года назад

    Would you build a 6V6 6SL7 stereo amp with good power reserve and clean output in return for a pair of diy 2-way audiophile speakers? You can listen to my audio skills at

    youtube.com/watch?v=UaNeqOT95ck

    I have some other speaker projects going too.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    I have noticed that the vintage amps deliver a lot of power with low distortion and no clipping up to full power - example - Ampeg SVT. And I agree with you on the Fenders (6L6) - usually run at a lower distortion (overdrive) level and cleaner tone and the Marshall's (EL34) run at high power and lots of overdrive/crunch (which is clipping at 10% THD or better) They all have their purpose in making the music we love.

  • Jonathan Justin
    Jonathan Justin 4 года назад

    i have come to realise most amps and most tones require the power section to not clip. ie deriving their tone via the preamp section. the only tone i can recall off hand would be like a marshall crunch a la acdc that would benifit from a clipping power section. namley EL34s. for instance a fender mostly needs the 6l6's to not distort. my engl savage 120 has only 2 kt88 tubes getting about 60 watts per tube or 120watt amp total. all the tone is comming from the 6 12ax7 tubes

  • BixLives32
    BixLives32 4 года назад

    EXCELLENT. Once again a sober, well researched test based on science, repeatable measurement and not some air-head's "killer ears".

    All it takes is some reading and LEARNING from text books. The ARRL handbook is a great primer. The math is simple, but verbal hype from the guy at the bar or the salesman at the snobby hi-fi shop will NOT teach you about this technology. Most people hate having to learn even simple science. Lazy brains I suspect.

  • BarchettaValveAmp
    BarchettaValveAmp 4 года назад

    Always use the tubes recommended for the correct voltage, bias, and primary impedance of the output transformer. Period.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    Speakers being connected is only important if you are driving the amp with a signal. You can safely measure voltages with no speakers attached. High voltage is a rattle snake so connect your leads before you plug it in to the wall. Work with one hand in your pocket and never lean on the equipment and don't use both hands when measuring high voltage as you want to avoid any current flowing from arm to arm through your heart. Getting across the plate voltage is a bad experience :-(

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    If you check plate voltage with no tubes the power supply will be operating with no load and it will be abnormally high and not the voltage that will be on the plates of the tubes when they are plugged in. The bias voltage will not be affected with or without the tubes being in.

  • marmuguk
    marmuguk 4 года назад

    I am a beginner, this is likely based on a marshal major 200, modified at some point with tag strip. possibly modified to drive four EL34/6CA7 as hard as possible. I need to check the voltages delivered to the power valves as safely as possible, if I run the amp to test the plate and bias voltages without tubes installed I am guessing the main danger is the capacitors. Is it safe to run the amp without power valves as long as I connect a speaker? I have a variac and a digital multi meter. thanks

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    Pin 1 on the metal based 6550's is connected to the metal base and should be grounded for safety. Pin 1 on EL34's is the third grid and must be connected to ground or the cathode (pin 8). G2 at 1Kohm sounds like a smaller tube than the 6550. G1 at 55Kohm is pretty high. Glowing hot EL34's - not good - make sure the bias voltage is adjustable to at least -60 and there is no AC ripple on it. My guess at a distance is bias supply issues. Hope this helps.

  • marmuguk
    marmuguk 4 года назад

    Hi, a 1965/7 amplifier 4 octal and five 9 pins, I think it showed 800V on the plates, blew a capacitor and stopped fiddling! I am guessing a KT88 amp. I have EL34's but they quickly glowed very orange! G2's go through 1 KOhm 5W resistors to the PT output, and G1's go through 55KOhm 1W resistors to the preamp section. Kathodes are tied to pin1's (G3) could this be the main cause of the orange glow along with 650V on g2 ?
    Is it safe to check delivered voltage with no power tubes or speaker? cut@1

  • Dean
    Dean 4 года назад

    oh, i missed the point.

    so it's 'safer' to run kt-88's if you are already set up for el-34's? interesting.

  • Dean
    Dean 4 года назад

    the marshall jtm 45 head is basically the same as the fender bassman, which is the original westinghouse design that fender bought the license to back in the 1950's.

    marshall used the kt-88 in this amp for a time.

    all 3 tubes have been used in various models of the same circuit. the typical marshall tube today is assumed the el 34, but the kt-88 was in the marshall amp eric clapton used way back.

    but best to have a pro set it up.

  • Fionn Mac Cumhaill
    Fionn Mac Cumhaill 4 года назад

    This is interesting. Nice Vid!

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    Best to tube it for what it was designed for. KT88 and EL34 draw about the same amount of filament current (1.6A and 1.5A respectfully) 6L6 is only 0.9A so if you put the bigger tubes in a 6L6 amp, you will overload the filament winding of your power transformer. 6L6 and EL34 are close in power output. You can get a little more out of the EL34 as it can take 800 volts on the plate, 500 max for the 6L6. KT88/6550 is twice the power tube of a 6L6.

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    With correct cathode current of 40 mA on the 6L6 raised to 80-100 mA, I would say less than an hour. I used to have a McIntosh MC240 that would ruin 5881's within the day. 6L6GC would last years. If you look up the old specs of tubes you will see that a 5881 has less plate dissipation than the 6L6GC - and plate dissipation matters very much. If you can measure cathode current, just simply multiply that times plate voltage and you will get idling plate dissipation. It will increase under drive.

  • Dean
    Dean 4 года назад

    what does 'not very long' equal???

  • Dean
    Dean 4 года назад

    ok, so all three tubes are pinned the same then? i'm no expert and i suspected as much. but i don't want to blow anything up. 2 6l6's get around 35 watts, i take 'clean' because all class ab amps that are touted as 50 watts use two el34's or 6l6's.

    people tell me i need to tube my 50 w. marshall head with kt 88's. will all i need is have the bias set? btw, i suspect the bias adjust on my marshall head is NOT very wide at all

  • ElPaso TubeAmps
    ElPaso TubeAmps 4 года назад

    Yes, 420 - 450 is a healthy plate voltage for 6L6's although 490 or so is not uncommon and I believe that is what the Dynaco and MA230 cloan is running at in this video.

  • JR
    JR 4 года назад

    how much voltage do these amps run on the plates? I'm guessing around 420-450?

  • ElPaso TubeAmps
    ElPaso TubeAmps 5 лет назад

    Yes, I have always heard that the 6L6 is a better bass amp tube than the EL34. Not sure how to measure that. Again, got to think about it. Thanks for your comments.

  • ElPaso TubeAmps
    ElPaso TubeAmps 5 лет назад

    Fortunately my wife has stayed with me 35 years, so far. The first one didn't though :-)
    As for 3 main types of distortion I assume you mean amplitude, harmonic, and frequency? And the old amps often times specified phase distortion. I will have to think about it and see if I can come up with something real. Thanks for your comments.

  • ElPaso TubeAmps
    ElPaso TubeAmps 5 лет назад

    I will have to think about this one. It will be a challenge. Some of my first videos are comparing KT88/6550's from vintage to modern Chinese and Russian in a Dynaco amp. You might find some of them interesting.

  • BixLives32
    BixLives32 5 лет назад

    The extra power sure helps with bass and transients. Frequencies below 200Hz use most of your current. You speaker is 8 Ohms @ 1000Hz (NOMINAL impedance). At 80 Hz where the deep bass of the kick drum or string bass resides, most speakers go down to 1-2 Ohms -or less! -That's where all that great KT88 current goes! Now they even make a KT120. But if you get up to that power range, Why not experiement with 811s or 572Bs (they're not expensive) -You might like class A at 50 watts!

  • BixLives32
    BixLives32 5 лет назад

    PS: I'd love to see a proper show-down of the 6L6 vs. EL 34 (for hi-fi purposes). Do a sweep through all extreme freqs, IM, HD, clipping, headroom, periodic harmonics, etc. E.g. do the 6L6s really have better bass? Which valve has better transients? @ which freqs? Using the same amp (output & power x-formers) with reasonable bias, cathode current & B+ adjustments makes this a 1st class objective test, Next, I'd love to see tube brands tested with no tears. -GREAT STUFF!

  • BixLives32
    BixLives32 5 лет назад

    Excellent presententation. You, um, have a few more test toys than I do. You are either a serious pro or your wife has left you. Seriously, This shows that there is not much difference in the desgin for push-pull amps using the popular valves. However, it also shows that there is a more than simple grid bias to good tube life, low distortion & high efficiency. Could you do a test on power vs. 3 main types of distortion with the 4-5 main power tubes?

  • rcmoot
    rcmoot 5 лет назад

    i wish i could learn from you. i always wanted to figure out how to build an amp for my stereo. looks as though it would cost alot for equipment .

  • Simeon Petkov
    Simeon Petkov 5 лет назад

    at 11:53 is that an aeroplane

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