Choosing power tubes?

[Chuck H]

You know, THIS needs to be the banner for this site, and an open declaration at the top of the FAQs.

Justin

Agreed

Incidentally, one of my FAVORITE things is to be at a gig where a good player is using one of my amps. That's when you REALLY get to hear it.

[Justin Thomas]

Update! I still have the amp, I love playing with it, and what I'm going to do (slowly and as time allows) is try out some of the various options in this thread, to learn for myself the various impacts of changes, etc. Maybe in the process, the amp will become more "mainstream" and sound good to a wider audience. And on multiple hearings, it is a little bright. Or, the tone control seems to only have effect between 0-4; more useful range, yay. So for now I'll still call it a work in progress. Will post if anything dramatic happens!

Justin

[Chuck H]

Fine tuning for a more gen pop/commercial aspect is one of the most frustrating things I've had to face. It's also rewarding when it works out that you still like the amp after any redesign. I've never had an amp design make it to market (on a commercial level), but I've dialed one in for that purpose. The things that you begin to consider once your head is in that place will invariably be things you need to learn more about. That's not all bad. It's good to obsess a little in the spirit of perfection. Pride can be hard earned and a just pleasure or presumed and an ugly sin. You don't seem presumptuous

[Justin Thomas]

I'm not "producing" anything more than once, so... yeah, each one has been a request - "hey, can you build me an amp?" Erm, okay... the first was for me without intent to sell, but I sold it anyway. So each of these was for the individual, 3 of them loosely Spitfires, and one "Voxange." This last one was a response to "Here's what I don't like about my boutiqe amp," and partially an "I'll show you!" move on my part.

I saw a comment in another thread about crossover distortion due to cold bias sounding "reedy," and I thought, that could be good, too... I don't know - maybe we measure all the fun out of the amps we build, because it's not "right."

Always room for improvement... the last comment I received was, "it makes my guitar sound tinny..." Well, MY guitar, the only one I have to test with, is not your average Telecaster, and, did you even move the knobs from where I left them, Beavis? But I guess nobody likes to turn the knobs to an extreme appoint, nonmatter how it sounds. But as Enzo says, don't worry about the position of the knob... oh, if only it were that easy!

Justin

[Chuck H]

Getting cut is the $h!ts. The first design I presented to Dean Markley was a TrainWreck-ish type thing that I DESIGNED BEFORE ANYONE KNEW THE TW SCHEM. Anyway... The tone controls work as distortion modulation controls and have very little effect on the final EQ. Dean fricken Markley said it was "lacking" because the EQ controls didn't adjust the final listener image. For us, here in guitar amp land this is common and expected. For Dean it was a flaw. And that's an accurate assessment for the average bone head, bedroom guitar kid. He wasn't looking for "the" amp. He was looking for an amp he could sell. THERE IS A DIFFERENCE !!! Coming to terms with that, and being willing to send and police designs overseas !@#$* has proven to be troublesome for me

[Justin Thomas]

Spent some time on this this weekend; main changes were changing the 500K tone control to a 250K and dumping the bright cap on the Volume, and made both of the caps on the PI .02uF. I didn't realize the one in the actual signal chain already was, but the one to ground on the other side was still a .01. I may revisit that EF86 preamp and twiddle the voltages there, mainly that the screen is pretty high, and I may tweak the 6V6 screens too.
Chuck, wasn't quite sure what you meant in post #38 by "100K pot with a 4.7K to ground." Is that basically wired like a Fender's "Bass" knob, and the 4.7K is the "Mid Control?"
There's a lot of gain here, and it can get kind of ratty at the extremes (as in, everything on 10), but I'll have more chances later to really dial in sounds and see if I like it. But moving in the right direction. Thanks again for all the help!

Justin

[Chuck H]

Without going back and re reading the whole thread...

I was probably outlining a possible way to make the tone control more concise. IIRC you mentioned something about not being able to use it beyond some low setting and not being good for much all the way down. So changing to a smaller pot and adding a resistor to the ground connection should put the useful range of the control across a wider range of motion without the useless extremes at either end.

[Justin Thomas]

Chuck,

Okay, that makes sense. I don't think that's quite what I was going for, because I do like the interaction between the knobs at the extreme settings. I may tweak some tube parameters a bit; it just seems a little thinner than I would like, maybe brittle? . But pulling the Bright cap on the Volume helped quite a bit. I know nothing will blow up, so it's all good. It definitely sounds better with the Master up high, and the Volume up. But, that's why I only build small amps!

Justin

[Chuck H]

No. Nothing will blow up. That's one of the fun things about preamp voicing. You can sort of fly with the "WTF, I'll try it and see." attitude :thumbsup"

As for brittle and bright caps, etc... Keep in mind that it's always possible to "remove" too much top end later in the circuit. Sometimes something that sounds "ugly" voiced early in the amp sounds fine if you mitigate HF in a later circuit and the advantage is that clipped harmonics are being generated, and then "EQ'd" to a more pleasant final tonality. Keeping the HF in early in the clipping stages is usually desirable. Taking too far isn't, but that's where some of the mojo and mind numbing tweaking come in. You could try to keep the HF in the early stages and voice later stages to balance the final EQ and find that you get to keep your harmonics without the harshness.

[Justin Thomas]

The nice thing about tube tech in general is that it's a lot harder to blow up, period. And there's usually a warning sign. The "silent and sudden death" syndrome isn't quite so bad. And I like to actually "see" my guitar going through the tubes, watching glowing screens, changing fluorescence, etc. One of my favorite things I watched was the tube voltage regulator in an old Leslie cabinet, powered with 2 metal-based KT88s... that was pretty damn sweet... lookedlikesomekind of Martian technology! I should go back to that church & leave my number!

Chuck, I like the way you give "non-answers" and generalities, without doing a paint-by-numbers approach. You state the goal, and just say, "try these three things..." I learn a lot by trial and error, especially the error part, and learn something in the process. And the worst thing I've sacrificed so far is an E-cap that I over-voltaged. Well, that and an EH-6V6...

Justin

[Justin Thomas]

Hey again all,

I've done some good bit of reworking on this amp, after another friend tried it and said "it makes my guitar sound brittle." So after much fiddling:

1. Dr. Z style Treble & Bass controls.
2. Ditched the Master Volume. For 15W? Really?
3. Many cap changes, too many to detail. But I took the Bright cap out completely. I may decrease the coupling caps from PI to power tubes...
4. I put in a big fat sag resistor, 250R/20W. Also changed the 6V6 bias R to 500R.

Much better. Using JJ 6V6s, I have the BFR dropping about 20V, and the plates are at a nicer 420V or so. I didn't check other stuff, figuring all dropped accordingly. By my math I think I could get away with a 5W BFR, but even a 10W 500R got too hot to touch, so the big guy stays. The 6V6 plates seem to be throwing about 12.5W, I know some of that is screens too, but I didn't notice any redplating, so... I'll keep a eye on it. No, it WOULD be nice to use a 5W or 3W cathode resistor, but this thing sounds a serious treat with 6L6s, so I think even 5W might be a bit low there, too! Oh well.

In my trials and tribulations, I DID bust my PI tube's socket, so that gets replaced, too (PTP - the horror!). Also need to buy better octal sockets than what I had - with 6L6s, the sockets allow a lot of sideways motion, and due to existing chassis layout, it may be way to easy for the tubes to hit the OT. I <AM> going to try some EL34s in it, too, monitoring the heaters, of course.

Again, thanks to all who've helped on this adventure!

Justin

[Gingertube]

Justin,
Good work, if I could suggest a couple of minor changes for possible reliability improvements:
1) 1K5 grid stops are not enough for a 6V6. I suggest 10K.
2) The JJ 6V6 are fairly tolerant of runing at higher voltages but you may run into problems with other 6V6. I would suggest doubling the 6V6 screen resistors to 1K and using 5W resistors. Actually if you have time/inclination to try an experiment I would add 2 x 1K in series on each 6V6 screen and see if you like the amp better that way or with a short across on of the 1K on each tube (that is do you like it better with 1K rather than 2K), if so then fit 2K2 5W as final value.
Cheers,
Ian

[Justin Thomas]

Hey Ian,

Wow, it's kind of an honor... I'm open to trying stuff until it's JUUUUUUUUUST right, honestly. No buyer yet, so I'll tweak it. I have no inclination to run other than JJs in it, knowing the operating conditions. Right now it has reissue TS5881, and takes 6L6 too. I tried some JJ (GT-branded) EL34, but they were noisy - the internal arcing kind of noisy, so they got pulled out fast.

I really like the Z-style tone stack, and it was funny. The first thing I did before that conversion was to simply disconnect the MV, leaving only the V & T controls. And sure enough, volume would drop out in certain spots - parasitics! Which may explain the thin brittle sound ot the first iteration. Yet that didn't happen with the MV (crossline) installed... oh well, no scope, so it's always an adventure. I've got it running through a Rocket 50 (Celestion) now, in a gutted GU12 cab. So it sounds good, but through the intended buyer's Matchless & my buddy's AC30 reissue, it sounds amazeballs. So, SPEAKERS REALLY MATTER!!!

I'll try your suggestions out when I can & report back, but for the next 10 weeks, I'll be going to Canadia & Ireland, so... Yay? First international travel ever. Amp goes to Canadia with me. Maybe it'll sell.

Justin

[Justin Thomas]

Hi all again,

Foreign travels are currently over (for now), so I'd like to follow up on this...

Currently the amp is as it was in post #56 above, and then I made the changes Ian proposed in post #57. The current power tubes are TS RI 5881s, a JJ 12AX7, & a Seimens EF86. I like this one pretty well now, and this amp teaches me that speakers are also VERY important.

I think it sounds great, but it's not quite MY sound. Which makes sense as I built it for someone else... I've got another potential buyer looking at (and my Prosonic), so here's to crossing fingers. I guess my only remaining question is,

Do we think this thing is safe for use with JJ 6V6s, 5881 reissues, and 6L6GC-styles? I'm not thinking KT66/EL34 in that mix, just "American" tubes... was just wondering if the changes Ian proposed would make any of my proposed tubes explode. I don't see them as that radical of a change, and I also don't feel like or have the luxury of torture-testing the amp, tubes, or my ears right now...

Off to the attic for my next project!

Justin

[Chuck H]

I think you're going to have to plug the different proposed tubes in to see how they all bias with the existing cathode resistance. You may need to add a "big bottle/small bottle" switch so the 5881's (modern?) and 6L6's can be run hotter and avoid excess crossover distortion? 750R is certainly higher (cooler) than I've used with 6L6 types at 420ish volts. That doesn't mean it's a problem. It just means I'd prefer a measurement to know if it were me.