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    Can anyone talk about the sound of different tubes in this amp?

    I have 6V6, 6K6, 6L6 and EL34 so I am looking for info on things like the KT series large bottle higher power tubes.

    Is the difference worth the cost of those tubes? How are they better / different?

    Any comments on particular versions or brands would also be appreciated.

    Another question on tube substitutes. On the EL34 pin 1 is the supressor grid and needs to be connected to pin 8 but if you use a 6l6 with metal can this leaves the can connected to the cathode. What to do? Clip the pin 1?


    I’ve never tried a 6L6 with the metal case. Pin 1 on V2 (the power tube) is connected to ground in a Two Stroke so you can use a EL-34 type tube. The 6L6GC and 6V6 don’t use pin one. On the 6L6 (metal case) pin 1 goes to ground so I’m guessing you can use it without issue.

    I’ve tried most of the tubes you listed. Tubes from different manufacturers can sound very different. The KT-66 (Electro-Harmonix) I’ve tried I found to be harsh and not as pleasing as a good 6L6, but they are a little more powerful.

    I talked at length to the JJ factory rep recently and he said their E34L is about 20% more powerful than their EL-34. I did not care for the JJ E34L that much, it was not nearly as sweet as a JJ 6L6GC. I just got a JJ 6550 (big and pricy) and am looking forward to trying it in a Two Stroke. JJs are my favorite new tubes because they tend to be rugged, sound good and are reasonably priced. I think a good JJ 6L6 is hard to beat in a Two Stoke.

    NOS tubes can be great (or not), I especially like a good NOS rectifier tube in the Two Stroke if you are going for a browner tone because the NOS 5Y3 tends to run about 40v less than a new tube (like a Sovtek)


    Thanks Robin.

    The 6l6 is an old GE and sounds very good. I tied pin 1 to pin 8 as it would be in most beam tetrodes. I guess it is not much difference connecting it to ground. Just a little longer wire. Perhaps I will try that. Seems a pretty clean solution.

    I am using an old 5y3 that gives about 400v so I would hate to try one of the current production tubes.

    I have several EL34s that are associated with a couple of Stereo 70s and have so far only used a RCA which sounds a little harsh compared to the 6l6.

    I am tending toward old tubes as there are some bargains out there if you shop carefully.


    Yeah, a 6L6 just sounds more complete, especially the bottom end, over an EL34. If you happen on any spare NOS RCA black plate 6L6s, please send them my way, I’ll “test” them for you. :)


    I use the KT-66 in my Two Strokes. I just love the”beef” I get from them. And I use JJ’s for those. I’ve used the 6550 on some other amps that I’ve built and love that tube. JJ’s there as well.

    On our Renovo Tejas amp, I tested around 15 different 6L6 tubes. Everything from GT to JJ. And I landed on Tube Amp Doctor for our 6L6’s there. They just fit the bill best. There can be a huge swing of difference between tube brands. What I have found, is that JJ seems to be the most consistent and well priced across all their tube offerings.


    It would be easy to just list tubes that I like for various reasons, but that would not be benificial to you. A power tube’s sound depends on a lot of other factors. Even though people are following the same basic schematic, your choice of speaker, power supply, output transformer, and preamp tube will affect the power tube’s sound. It would be troublesome to get to technical in this area, just test out the tubes you can get a hold of easily. Some power tube might sound bad in your amp, but sound great in someone elses due to supply voltage and sag, or due to their choice of preamp tube or speaker. Since adjusting you power supply, or sampling various speakers is quite a hassle, and expensive, then try a couple of different preamp tubes with each power tube. Some might sound horrible with one brand of 12ax7, but sound amazing with another brand due to the fact that companies nowadays will lable almost anything an 12ax7 regardless of their actual tube curves. Same goes for any new power tubes as well, just cause it says 6l6, the curves may be well off the mark. You can also try 12at7, 5751, 12au7, or even a 12dw7 among many other preamp tubes. Also don’t get hung up on price. Just because some tubes cost a fortune, does not mean they sound any better. You can even change the cathode resistor value to adjust your bias point if you want. This will also affect the power tubes tone. It is all about trial and error and most importantly, trying to have fun.


    Thanks for the responses so far.

    Perhaps a little clarification of the question is appropriate at this time. I am not expecting I will necessarily like what someone else likes or that my amp / speaker / guitar combinations will sound exactly the same. What I am looking for is other peoples experience with various tubes but especially some of the more esoteric and expensive versions or any less expensive plumbs out there.

    It might be helpful to understand what speakers / guitars etc are used but I suspect useful insight can come without that information.

    I see no reason to shy away from technical discussion on a technical forum.


    To start off, here is a link to some very useful information.

    All things being equal to someone else’s amp (voltage, bias, load) then a similar tube should sound similar in your amp. The load is your speaker, and every speaker responds differently to different frequencies, meaning the load changes at different frequencies depending on the speaker. Also just because the speakers nominal impedance says 8 ohms, they are never spot on. So now the load gets amplified through the output transformer between say 300 and 1000 times, depending on which tap you choose.

    If you have read the link, then you know that for every tube you would need to set up a load line to see how the tube would respond to an input swing. This is where the plate voltage, screen voltage, and bias point come into play. Adjusting any of these, changes how your tube will sound.

    Now all amplifiers were originally designed off of the tube charts a given company would put out. Over the years people just based their amps off of someone else’s, and now you have people wanting to run 400v across their 6k6’s. Now you can do that, but not without some other modifications to the circuit. You would still have to set up a load line to see how the tube would respond. The two stroke amp was originally designed as a parallel single ended amp for dual 6v6’s hence the name. Over the years the schematic has changed and it is now designed for a single 6L6/EL34 type tube. Even if you change the impedance switch, you are only changing the load that the tube sees, that does not mean the amp is suited for a different tube. The biggest problem is the plate and screen voltages. If you did want to try low power tubes with hopes of a good response, then you would have to bring these voltages down closer to 300v. The link I gave will help to find the right cathode resistor for a given bias voltage. Obviously that link is describing triode curves, but once you learn it, it is easy to figure out pentode curves as well, except with different screen voltages from the ones they recommend, then it gets a little more tricky.

    If you would like to lower your B+ to try that out with different tubes here is a link to a possible solution.

    I have not personally tried this method, but it should work fine. You could even put in a switch so you could change the B+ from 300v to 400v. The things I had mentioned in the earlier post were about the driver tubes. Once you figure out your load line, then you know what type of voltage swing you want. Some output tubes also like a little more driver current. This is where your choice of preamp/driver tube comes into play.

    Like I said it gets troublesome talking too technically, and I have just barely scratched the surface. That being said, sometimes the sweet spot on tubes lies outside the recommended values, but not very likely.



    Thanks for the response. I have no problem understanding what you are saying or the info in the links.

    I also realize that if I use a 6k6 that I am running it beyond its rated plate voltage. That said part of the rational of this design is that it is suited to various output tubes per Dave Hunter. I have not calculated load lines for the various tubes suggested but have tried some substitutes and found the results interesting.

    Perhaps in the future I will take the time to do the calcs and see just what that reveals but in the meantime I am still interested in others experience with alternative tubes in this design.

    For the record I am using the C8r and C10r as recommended in the book. I also have an Eminence Mavrick and a vintage Wilder 12 available. Although speakers do vary a bit I would be surprised if it was apparent in a side by side test. I suspect the amount of break in is probably much more apparent.

    As for the B+ reduction the Zeiner approach is perhaps more elegant, I am ok with using resistors and accepting a bit more sag and resulting compression. Perhaps it would be interesting to try the other approach just to feel the difference. Another option is to use the lower voltage taps in the transformer which is probably a better approach than the Zeiners.


    I’ve tried the Two Stoke circuit with several different transformers, voltages and a bunch of speakers (ceramic and Alnico). They all sound different. Different speakers make a very notable difference. Those ceramic Jensens are my favorites with a Tele. If you have not yet tried the Weber break-in procedure (especially for that C10R), you’ll be surprised how it opens up those Jensens right before your ears in about 10 minutes or so. I suspect the new Jensen Jet 10 would also be a good, low cost speaker for the Two Stroke.



    Are you suggesting the 1/3 power 60hz method? Somehow I got the idea that it took hours of that to break in a speaker.

    While we are on the subject any experience with removing or reducing the doping on the surround?


    Sounds like you read the “Breaking in your speaker” post in the TAN forum topics. I think Andy has had good luck with the 1/3 power process.

    I use Ted Weber’s original formula that can be found here:

    There has been some controversy about how safe these processes are, so be aware that apparently some people have damaged speaker trying to do it. I’ve never had a problem and I’ve done a number of speakers. Some speakers have a more dramatic change (like the C10Q) but most have had a discernible improvement.

    I first tried isopropyl alcohol as a solvent for the edge doping but it had no effect……lacquer thinner on the other hand, works great. Rubber cement thinner and acetone would probably work too, I’ve never tried those.

    I get the speaker going on the Variac and then gently wipe the edge dope a few time around with a paper towel moistened with lacquer thinner. It does not take much and you can hear the change.

    I am not recommending that you do it, some guys think this process is a terrible way to break in a speaker. I’m just saying it was worked well for me. All those chemicals are harmful and depending on how much power your speaker is rated for, it can get loud.


    I used some Acetone to reduce the amount of doping by scrubbing with a toothbrush and wiping with a paper towel.

    I will try that while running on a 60hz signal.

    I tried lacquer thinner also and that may be a little more effective than the acetone.

    I don’t see how you can fry a speaker at 1/3 power.


    Results are in…

    The C10r was subjected to 8 -10 hours of 60 hz at about 1/3 power (8.1v) and the output at that point had increased by something like 8db measured at the face of the speaker. This was done with the bare speaker sitting cone up on my bench.

    I did not start with a virgin speaker as it already had a few hours on it and I had removed most of the doping before I thought to measure it. I used a Radio Shack meter set into a wood strip placed across the face of the speaker.

    I think I got about 6db increase out of the C8r but the measurement was not as clean or complete.

    There is now quite noticeable increase in the bottom and a smoother and cleaner top end. This is apparent with a guitar or playing music through the amp. It may be louder over all as the amp appears to have a bit more head room. I did not measure at any other frequencies.

    All in all a very successful undertaking. Thanks for the suggestions.


    Assuming your C10R speaker is 8 ohm, 8.1 volts would be about right, if it’s a 4 ohm speaker, 8.1 volts would be high, maybe too high. Around 5 volts would be more appropriate. The “1/3 of the speaker power rating”
    is too simple an approach and might explain why some have had problems trying to break-in speakers using it (IMO). I bet the magnet would get pretty warm running 8 volts in a 4 ohm speaker and that leads to the voice coil coming apart.

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