Choosing power tubes?

[Justin Thomas]

Haven't had a chance to blast it yet, but last week the rattling/buzzing quit when I moved it, so I don't think it's entirely oscillations there. Of much more immediate importance, in trying to add some beef, I realized that the final coupling caps were 0.002, not 0.02. So that helps! Now to tweak the Tone circuit for Better range.

Justin

[Justin Thomas]

Here's the final version, electronically, with schematic, and thanks to Chuck H. for the Spitfire tweakage inspiration! The Tone control is a bit interactive, but in something this simple, I'm okay with it. Really liking these simple designs, and the EF86 front ends...

Edits: I just plugged in some 6L6GCs and rocked it out for 30 minutes with my meter on the heaters. Never dropped below 6.95V and the tranny never got above "vaguely warmer than room temperature." I'm calling it 6L6-compatible.

I'm not hearing any artifacts since I found a loose nut, other than at the extremes of knob settings - when the Master is really low. But I'll call that PI imbalance and not care. It's very sensitive to string/pick attack, so I'll have to fine-tune those slippery elements of my playing. Or its (hopefully to me) buyer. Cleans up nicely, except (again) at extreme settings.

I wouldn't mind figuring out how to make the Tone control a bit less interactive, but I'm not willing to change this one because it's so great. I'll have to check my other again to see if I miswired something; I don't think so!

Justin

[Chuck H]

What is the tone control "interacting" with? I only see the one channel in the schematic. I see more in the chassis shot, but I can't be sure what to interpret.

Good luck letting it go. I've made a half dozen amps I liked so well that my brain tried to find a way around turning it over to the customer I've always handed the amp over because I also take pride in having a project I believe in land on the customer too. One guy has two such amps and I tease him that he has nicer amps than I do. It's supposed to be funny because I make them. But somehow that makes it NOT funny. I should stop saying it.

[Justin Thomas]

I'm not sure what the interaction is - it's with the Volume control (not Master). The knobs from L to R are M, T, V. Tone works normal from 0 to 5, then from 5 to 10 it adds some more volume, aside from treble, and adds to the Screaming Banshee factor. It's not a bad thing! I'll check it against my first build (layout and actual sound) to see if I made a mistake, but I really like having the extra POW! factor.

Okay, here's what it's like - an analogy. If you have a Fender with just VTMB, and turn all the tone controls to zero and the volume on ten, nothing happens. But if you slowly turn the tone controls up, it'll start clean, but as you keep going (especially the Treble) it'll add highs AND as more signal gets through, more "fun." While mine doesn't kill all the volume on zero (because it's not enough to load down the entire signal so much, or because it's not such a cap value to send the entire spectrum to ground?), I DID use a 500K & .02 cap because it gave me the best useful range for the Tone control. It's not an awful thing, as long as it's understood that that's just a fun side effect of playing this amp.

Otherwise, I'll be finishing the cabinet this weekend, so I don't have to carry it around by the chassis!

Justin

[Chuck H]

Ok. Got it. What you mean by interaction that is.

It's normal because the "tone" control rolls off HF (and upper mid) signal, and therefor gain. Just as the volume control rolls off the gain of all frequencies. The cap values in your circuit do give the tone pot control over a lot of the available frequency range to the next stage. That's why it has a more than average effect on gain.

If you increased the volume pot bypass cap value from 100p to 330p or 390p, increase the PI grid isolation cap value from .01uf to .02uf and decrease the tone cap value from .02uf to .01uf the tone control interaction would likely be more what you would expect. AND IT WILL CHANGE THE WAY THE AMP SOUNDS NOW! Probably for the worse since you like what it's doing now. Something about what it does now may ultimately be lost and things you don't want it to do may be added. You can always experiment more with the cap values just to see.

As a pseudo guide to the proposed changes:

Increasing the value of the volume pot bypass cap will allow more upper mids through the circuit that would be less effected (but not unaffected) by the tone control. Keeping more gain in the circuit relative to the tone control setting.

Increasing the value of the PI grid coupling cap allows more LF through the circuit that is unaffected by the tone control. So a greater proportion of the available signal would be unaffected by the tone control setting. This would have the side effect of increasing LF from the preamp. For better or worse. The difference between a meaty tone and a farty tone with the addion of LF. You'd need to try it to see how the change affects YOUR circuit.

The affect of decreasing the value of the tone cap is pretty obvious. You'll be sending more unaffected signal through the circuit and trimming up the frequencies affected by the tone control to a higher range. This would somewhat work in tandem with the volume pot bypass cap value with the changes I proposed.

What you would expect for tonal change would be a small increase in LF, More upper mids at low volume settings and more unaffected upper mids at low tone control settings. This could have the affect of making the amp sound less "sweet" for cleaner tones and more focused at higher gain settings. The possibility of the additional LF causing definition problems for low notes exists, but we won't know unless you try it. Soooo...

If any of that sounds like a direction you do or don't want to go is up to you. There's no right or wrong about your circuit either way. Just a bit different.

[Justin Thomas]

Some of that definitely does sound good, especially the "more girth at low volumes" thing... Plus, it's a chance to learn some more! I did try the higher value pF cap, and didn't like it as much as what's there now... But the rest sounds good! Thanks, Chuck!

Justin

[Justin Thomas]

Quick update:

Chuck, I raised the cap to the PI to a .02 & the Volume to a 250pF. I like it, adds some Fendery bottom to it; I'll see if the intended buyer likes it, it's an easy enough swap if he likes it brighter. The Tone control is a BIT less interactive, which is nice.

So, here's the fun part, and a caution to builders: in the process of swapping the caps, I nicked one of the other caps (a Mallory 150) on the underside with my iron and exposed some innards. But I didn't see it, cuz it was upside down. So after doing my cap swaps, I turn it on. It's humming as I raise the master, and the pot is scratchy. One of my 6V6s begins to redplate. Huh. DC on it, but how? I didn't mess with THOSE caps! So I check for DC on the grids of the tubes - 120VDC! Wow!

Looking at the schem, I figure it's only a limited number of ways for DC to get THERE so I started checking for leaky coupling caps - the 2 feeding the power tubes. Both of them with 120V on the grid end, though with one of them, the voltage drops as I turn the pot. So of course, the last cap I look at, I turn it over and see the gaping hole in the side and the aluminum foil showing... replaced and all is good. Also dropped SOMEthing in the Volume pot to make it act up; a simple swap and we rock again!

New filter can (525V) is on order, along with rubber feet & more pF caps. I appreciate the lessons I learned in diagnostics, putting some of Jack Darr's book into practice.

Justin

[Chuck H]

And there's still the tone cap to play with... .015uf or .01uf will trim it's effect and add a little low mids to the overall tone (for better or worse). If you like the affected frequencies now you could also use a smaller tone pot and fixed resistors to trim it's range. Like a 100k pot with a 4.7k resistor to the ground. Lot's of options to trim up the performance to idealize intuitive use.

[Justin Thomas]

Just wanted to say "Thank You All!"
I think this design is a real keeper. The intended buyer described it as "Ferocious!" and my audience of reviewers for a scholarship audition remarked "you MADE that? Wow!" Now I just hope the intended buyer actually goes through with it and keeps his word; 8 weeks is NOT so long a wait for a custom-built amp that you forget your commitment and decide to buy a Kemper instead... > And I don't need another amp, I need cash!

Anyway, great amp, thanks for all the help & tweaking ideas! I just drew out the schem with updates & voltages this morning for reference!

Justin

[nickb]

Just wanted to say "Thank You All!"
I think this design is a real keeper. The intended buyer described it as "Ferocious!" and my audience of reviewers for a scholarship audition remarked "you MADE that? Wow!" Now I just hope the intended buyer actually goes through with it and keeps his word; 8 weeks is NOT so long a wait for a custom-built amp that you forget your commitment and decide to buy a Kemper instead... > And I don't need another amp, I need cash!

Anyway, great amp, thanks for all the help & tweaking ideas! I just drew out the schem with updates & voltages this morning for reference!

Justin

Do take & post some pics before you wave it a fond farewell

[Dave H]

I wouldn't mind figuring out how to make the Tone control a bit less interactive

It's interactive with the vol pot because it's connected to the vol pot wiper and the impedance from the wiper depends on vol pot setting. You could try connecting the tone pot to the cw end of the vol pot where it will be working with a constant impedance and therefor not be interactive.

[Justin Thomas]

Hey Dave,

Aww, dammit! You used the "I" word, which seems to be the main concept in electronics that causes me more "reason to experiment" than just about any other!

I'm happy with it as is, and so is the intended buyer, but I'll try it out anyway just so I know WHAT the effect is, even if I still don't know WHY the effect is... But I think I can mentally get it. An experiment will help.

Justin

[Justin Thomas]

Hi again,

So, the intended buyer has backed out (saving for a Kemper "amp") and I am still looking for a buyer. In the meantime, I'm enjoying playing it myself and am taking it to school Saturday to make glorious noise for all my classmates for the next 9 months. There's one guy there who wants a Maz 18; I think he might like THIS one too, and maybe willing to sell his AC30 to do it.

Anyway, the reason for this post? The original intended buyer said it sounds "as good as my Matchless (Phoenix 35)" and when I let him try it out, he asked what power tubes were in it. I made him guess. He guessed EL34s first, and didn't believe 6V6s could sound that good. Then, I asked him to put it up against his 65Amps Ventura. My amp tore it up, and I told him, "I built my amp using the exact same tubes as your 65, because you said you didn't like 6V6s."

Whad'ya know, maybe circuit DOES matter more than tubes!

Another guy in the band said "don't tell him, but your amp sounds BETTER than his Matchless..." Sorry to brag, but I'm just pretty damn proud to hear those compliments, and to know for sure that I can do this! I can say I am the best amp builder in my town, for people who don't play Jazz. Yes, now I'm just gloating!

Thanks to everyone for your help, I could not have done it without you all!

Justin

[nosaj]

Hi again,

So, the intended buyer has backed out (saving for a Kemper "amp"... needless to say, I'm a little mad) and I am still looking for a buyer. In the meantime, I'm enjoying playing it myself and am taking it to school Saturday to make glorious noise for all my classmates for the next 9 months. There's one guy there who wants a Maz 18; I think he might like THIS one too, and maybe willing to sell his AC30 to do it.

Anyway, the reason for this post? The original intended buyer said it sounds "as good as my Matchless (Phoenix 35)" and when I let him try it out, he asked what power tubes were in it. I made him guess. He guessed EL34s first, and didn't believe 6V6s could sound that good. Then, I asked him to put it up against his 65Amps Ventura. My amp tore it up, and I told him, "I built my amp using the exact same tubes as your 65, because you said you didn't like 6V6s."

Whad'ya know, maybe circuit DOES matter more than tubes!

Another guy in the band said "don't tell him, but your amp sounds BETTER than his $3000 Matchless..." Sorry to brag, but I'm just pretty damn proud to hear those compliments, and to know for sure that I can do this! I can say I am the best amp builder in my town, for people who don't play Jazz. Yes, now I'm just gloating!

Thanks to everyone for your help, I could not have done it without you all!

Justin

With all of the positive help around how can any of us go wrong. This is one Heck of a community here.

nosaj

[Justin Thomas]

With all of the positive help around how can any of us go wrong. This is one Heck of a community here.

nosaj

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

Justin