New amp build problem(part 2)

[dearmond]

Hi!
Awhile ago I posted a problem I was having on a new build of mine which this forum thankfully helped me out. On that post I had my original schematic (see "New amp build problem"). Since then I made a number of changes. After that first problem was fixed, the amp worked great but I knew I would have to do some tweaking. First, I anticipated problems with the El84's so I used sacrificial tubes but eventually solved the issue with zener diodes. Second, distortion issues-either tube pairs. It would brake up too soon and go into a fuzz box-like distortion (which I actually liked but only if it was a side effect). And third, too much low end. I had to have the treble set at around 75% and the bass set no more than 25%. The treble control worked fine in its travel but when turning the bass past 25% it would be way too much. You name it, I've tried it, to fix both problems. Finally, I got rid of the cathode bias system and switched to fix bias. That made a significant improvement in the distortion. I had 5881's for high power tubes and found their tone limiting as far as my guitars are concerned. I primarily use two guitars: a Guild DeArmond M-75T and a Gibson SG with P90's. The 5881's make the DeArmond shine but the SG is another story-mainly on the neck pickup. anything played on the low E (#6) string was too flabby and bassey. I replaced the 5881's with EL34 and that made a big difference. Now both guitars sound good.
However, the bass control is still an issue. I would like to get more control of the bass control and to some extent, the treble also.
I'm posting my revamped schematic. I appreciate your input.
Thanks!

[67plexi]

on your potentiometers try 250k treble. 500k bass.

[dearmond]

Thanks 67plexi. I've had a 250k in the treble position but not a 500k in the bass. Something to try..

[dearmond]

Tried the pot thing. No go. But it did get me thinking. The amp overall has too much low end and it has to be somewhere past the second preamp tube.
I've tried every mod to the preamp section you can think of: From standard setups to unusual to different tone stacks to even doing away with the parallel set up. The preamp set up you see seems the best to me. It has to be downstream...

[Structo]

Try a smaller cathode bypass cap on V2.
Like a 10uF or 5uF.

Also your swamper resistors on the power tubes (10K) are rather large and probably are attenuating some of the highs.
Change those to 5K1 or so.
The screens are a bit on the large side too, try 1K.

[tictac]

Is that EL84 labeling on the lower pair of power tubes a misprint?

That would certainly be a mismatch running side by side with a pair of 6L6 or EL34 output tubes. If you want to mix output tube types there should be one of each on each side of the primary OT. 6L6 and 6V6 mix well but the EL84's can't handle the higher voltages the larger power tubes can; you'd have to run the whole lot 360V or lower for the EL84's to survive.

TT

[dearmond]

No, I'm not mixing the EL84's with the high power tubes. I have an on-off-on switch to switch between the two. Furthermore, when I switch to the EL84's it also puts the zener diodes into play which knocks down the power to an EL84 acceptable level. That works quite nice. Also, I should mention that right now I'm ignoring the EL84's. I have them pulled out and am just concentrating on the El34's. If you look at the cathodes of both power tubes you'll see a switch. That's the on-off-on switch. "off" is standby.
Thanks tictac for your concern. I see where the switch is confusing. But what I said above is what's happening.

[dearmond]

Structo, As far as the grid resistors, I was thinking of that before I read your post as far as grid resistors. I put 1.5K's in for the grids and 1k (at your suggestion) for the the screens. I also put a 10uf on V2's cathode. Sadly, it sounds worse. Very fizzy and still the same low end problem. However, at your suggestion, I'm going to change the grid resistors to 5.6k (that's all I got) after I type this and see what happens. Thanks!

[Bob-I]

I've never seen an architecture like you have on V1. It appears that these stages are driven in parallel then summed. Typically parallel stages share Rp and Rk as well as having the grids tied like you have on V2. I'm not sure that's a problem, just different.

What I do see as a potential problem though is the bias of your preamp stages and the bypass caps. Typically you want to multiply Rk value by 68 to get Rp. This will bias the stage correctly. I'd either increase Rp to 130k or lower Rk to 1.5k. The bypass cap on V1a at 150uF will make the stage push too much bottom end. I'd drop that until it cleans up, starting with about 22uF, possibly even as high as .68uF depending on the tone you want.

I don't think your grid stoppers or screen resistors are causing the flabbyness.

Another suggestion is to change one thing at a time, listen and decide to keep it or discard it. Too many changes make it difficult to determine where the problem is.

All of this JMO.

Good luck

[dearmond]

Bob-I, thanks. I know my V1 set up is different, but it works and I like it.
I learned that my problem doesn't lie in the preamp section. What you suggested I've tried already. Your right about the power tube resistors though. I've just tried the changes that were suggested (sincerely, thank you every one). However, I found the changes to the grid and screen resistors made it worse. I'm going to go back to square one and try again.
Everybody, please keep your ideas coming, weather I've tried or not, something might click!

[dearmond]

I went back to what you see on the schematic and the amp sounds great with the exception of the limited bass and treble control. I jammed for a 1/2 hr unconsciously with my SG. So, I'm back to square one. Trying to broaden my tone controls.