How can this improve

[fossilshark]

This is exactly how i have the amp wired now. It craps out at high volume and the tone is really muddy on its own. I usually play with the cathode cap switch on, and with my treble booster,  but on its own it doesnt sound good. When the cathode cap switch is off the cleans dont sound too bad, but its a little quiet. How can i improve this design?

I can post sound clips if needed, i just dont know how.

[sluckey]

Two things in the power amp jump out at me... The power tube cathodes don't connect to anything! Surely this is just a drawing error because the amp would make no sound if it were wired like the schematic.

Number 2... You have a 100K pot in the cathode of the phase inverter. This is strangling the PI operation! Put a ground clip at the junction of C3 and R3 (top side of that 100K pot) and see if the amp doesn't wake up.

[shooter]

You have a scope to see what stage things "crap out"?
If not, I'd probably play with R11,12,13,19, change the ratio's to get sound/tone that you want, worry about "loud" after.

play with Ck values on all the preamp gain tubes

play with plate volts on all the preamp gain tubes.

[shooter]

Two things

ya, that to
again  I missed the forest

[fossilshark]

Yes i did mean to connect the output tube cathodes to ground. What do you mean by a ground clip? And how do i figure out ratios for preamp tubes?

[sluckey]

What do you mean by a ground clip?

A test lead with gator clips on each end. Everyone needs about a dozen of these. 16" leads is handy for me.

The idea is to ground the junction of R3 and C3, effectively shorting out that 100K pot. That pot is killing you! I assume you were thinking it is a presence control? But you don't have a negative feedback loop connected to the pot and the pot would be on the order of 5K, not 100K.

[Tony Bones]

You need to check the drawings.

In the preamp, the 3rd stage has no DC grid reference.

With SW1 open, the 2nd stage is left with a 100k cathode resistor. That would bias the stage into cutoff. (I've seen things like this used as a mute function in some vintage circuits.)

Also, when SW1 is open, the cathodes of 1st and 3rd stage are cap couple together. Positive feedback?

I suspect all of these are mistakes in the drawing.

[PRR]

> SW1 is open, the cathodes of 1st and 3rd stage are cap couple together. Positive feedback?

That whole switch network is "WTF??"

I don't think it is positive feedback. Maybe selective lame NFB, plus huge DC shift when switched. I wonder how it arose, or if it is mis-copied from another plan.

[Tony Bones]

That whole switch network is "WTF??"

I was trying to be more tactful than that. 

I want to assume that it'll make more sense once the mistakes in the drawing are fixed.

[fossilshark]

Lol that was the result of my quest for too much gain (hence my asking for a better way to get distortion) when sw1 is open the amp goes clean and sounds pretty decent. When its closed it goes crunchy but its also muddy. And i just double checked, thats what is in the amp.

[fossilshark]

Just jumped the presence pot like i was told and it didnt help much.

[sluckey]

Just jumped the presence pot like i was told and it didnt help much.

Really!

[Tony Bones]

And i just double checked, thats what is in the amp.

mmm.... no. The preamp would not function as drawn.

I don't see a happy ending for this thread.

[fossilshark]

Im measuring 423v on the plate resistor of that tube, could that be why?

[fossilshark]

There is a 100k feedback resistor on the output jack to C3 and R3 i forgot to put in the schematic. Im removing the mess of extra cathode caps for now. Is there a rule of thumb for cathode resistor ratios?

[kagliostro]

Is there a rule of thumb for cathode resistor ratios?

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

also this may be of your interest

https://www.ampbooks.com/mobile/amplifier-calculators/bright-boost/

https://www.ampbooks.com/mobile/amplifier-calculators/coupling-capacitor/

Franco

[sluckey]

Im measuring 423v on the plate resistor of that tube, could that be why?

The plate resistor has two ends. One end connects to a relative high B+. The other end connects to the plate of the tube. If you are saying you measure 432v on the end of that resistor that is connected to the plate, then yes, that 100K pot in the cathode circuit is probably why. Put the jumper back across that pot and the voltage will probably drop to a more reasonable value.

[sluckey]

In the preamp, the 3rd stage has no DC grid reference.

The grid has a ground path through R7 and R6 so it sits at zero volts.

[SolidStateCircuitReject]

I might be way off here, but I am not seeing any signal being introduced back into negative feedback from the OPTx - was this already discussed and I missed it? I assume your trying to replace the NFB resistor with the 100k pot, but I would think you should have a minimum setting there, such as 27k.

But I also think if the Presence circuit is setup wrong (as in no nfb), it would (or likely would) squeal.

Just throwing some stuff out there perhaps it will trigger someone else to find something.

[fossilshark]

Heres the updated schematic. I changed the cathode resistor values and it sounds OK but still really muddy. R5 in the power section is supposed to be a presence control, i think i originally has a 3.3k pot there but it didnt change the sound much so i put in that 100k. What needs to be changed to make that a presence control?

[sluckey]

...and the pot would be on the order of 5K, not 100K.

[sluckey]

Your bias pot is dangerous as shown because you can adjust the bias voltage down to zero. I would put a resistor between the pot and ground like a typical Marshall amp. Start with a 47K and reduce the value if you need to shift the adjustable range of bias voltage.

[SolidStateCircuitReject]

What about thoughts on moving volume/gain (R6, 1M) to the v1 position (after .022, before 470k)?

It's too muddy, so how is the level of distortion? If it is in "high gain" territory, I'd bump v1 cathode bypass cap down to something more tamable for high gain such as 1µF or less. Just my thoughts, other than what has been stated about the presence. pot

[Tony Bones]

In the preamp, the 3rd stage has no DC grid reference.

The grid has a ground path through R7 and R6 so it sits at zero volts.

I looked at that a half-dozens times and still didn't see the ground connection. Thank you.

As for the muddiness, I'd try biasing the 2nd stage colder. Change R20 from 820 to 1.5k, 2.2k, ...

[SolidStateCircuitReject]

as stated a few times before, there seems to be no grid leak resistor before stage 3

[sluckey]

as stated a few times before, there seems to be no grid leak resistor before stage 3

As stated before, yes there is. 

[DummyLoad]

stated visually. yes, there is grid leak for U4.


--pete

[PRR]

> 3rd stage has no DC grid reference.
> seems to be no grid leak resistor before stage 3

While the clever electrons do not care, mere human brains DO care how the drawing is laid-out. We "expect" a grid return to run "down to ground". Here it got drawn up, over, to a ground in the middle of the plan. "It's right there", the draftsman knew it was there, but wide-awake minds miss it. So when sharing, it is polite to lay-out as simple as possible.

> not seeing any signal being introduced back into negative feedback from the OPTx

Many fine guitar amps omit NFB from the OT. Of course many others do it. At this point I was thinking it is how he likes it. As it does not seem to be happy-voiced yet, maybe this is something to experiment with.

[Tone Junkie]

Try changing C1 to 4.7 UF that will help with the muddiness.
Bill

[fossilshark]

Sorry about that, i will keep that in mind from now on. Thinking about it, i dont know if the tone is muddy or i just dont have as much gain/distortion as im looking for, resulting in that overdriven but still clean (if that makes sense) sound. Because when i turn the gain down it sounds fine clean. Would sound clips help? I will also edit the schematic to show what changes ive made so far when i get home

[Tony Bones]

Look, that's a complicated system of gain stages with a cathode follower, a tone stack, phase inverter.... If you were to design that amp from scratch there are dozens of decisions that would need to be made as far as component values alone.

It was obviously inspired by a 2204. Have you considered just making it exactly a 2204 and then working from there? You would be starting with a proven circuit that provides known results. If it's not exactly what you want you can change one thing at a time and decide if you're moving closer to your goal or farther away. More importantly, you'd have something that other people have experience with and thus can provide better guidance. Just a thought.

[Tone Junkie]

Sorry about that, i will keep that in mind from now on. Thinking about it, i dont know if the tone is muddy or i just dont have as much gain/distortion as im looking for, resulting in that overdriven but still clean (if that makes sense) sound. Because when i turn the gain down it sounds fine clean. Would sound clips help? I will also edit the schematic to show what changes ive made so far when i get home

MY friend play thru it a bit and decide if its to muddy or not enough gain. Because there are 2 different approaches to fix the problem. 1 rule of thumb the higher the gain your after the less bass you want coming out of V1. Less mud . That's why I suggested the lower cathode cap on V1.
Bill

[SolidStateCircuitReject]

> 3rd stage has no DC grid reference.
> seems to be no grid leak resistor before stage 3

While the clever electrons do not care, mere human brains DO care how the drawing is laid-out. We "expect" a grid return to run "down to ground". Here it got drawn up, over, to a ground in the middle of the plan. "It's right there", the draftsman knew it was there, but wide-awake minds miss it. So when sharing, it is polite to lay-out as simple as possible.

> not seeing any signal being introduced back into negative feedback from the OPTx

Many fine guitar amps omit NFB from the OT. Of course many others do it. At this point I was thinking it is how he likes it. As it does not seem to be happy-voiced yet, maybe this is something to experiment with.

Apologies, Folks, I was looking south for a leak resistor... I suppose I should follow the entire circuit each time.

[SolidStateCircuitReject]

Would sound clips help?

Please. Probably should've included them in the original post, and we should have asked for them in initial replies.