Peavey classic 20 grounding scheme

[Leevi]

It seems that Peavey uses two different ground levels in classic 20. What is the idea behind this and does it have a practical benefit?


/Leevi

[tubeswell]

Split ground for V1a (including the associated filter cap node)- the 47R provides some potential between the two sets of ground returns to reduce interference noise. They do the same thing on other amps from the same era (C30, DB). There’s no advantage in a conventional amp layout. PV only did it because of the way they laid out their PCBs

[Leevi]

I changed the grounding as described in the schematic and the affect was minimal.I have an issue with my build which is a conversion of Classic 20.I have wired it according to the schematic and tested  and it is extreme loud.It starts to break already on volume 2-3. You can already see that in the schematic since there are 3 amplification stages before the PI.Is the amp really that loud and what kind of mods do you recommend?

/Leevi

[kagliostro]

Split Load Resistor (if you want to tame it a bit and have more control)


You can also give a try to a 2.2M VR5

Ciao Risto

Franco

[Leevi]

Split Load Resistor (if you want to tame it a bit and have more control)


You can also give a try to a 2.2M VR5

Ciao Risto

Franco

Thank you for your tips Franco. The problem is in the 1st tube since there is so much gain.
The master (VR5) works fine. I did't split the load resistors yet but I dropped their values to half which already helped.
The question is if I should add an voltage divider or decrease the ground resistor values or take away cathode bypass cap(s).

/Leevi

[Latole]

Too much gain ?

Preamp tubes voltage are too high.
Or
Reduce volumes pot value ; smaller values = less volume. More "sound " are send to the ground.
Or
Replace 12AX7 ( 100% gain ) with 12AY7 ( 45% gain ,no it not to much ' i use often with success)

[kagliostro]

Split Load Resistor is to be used when the tone of the gain stage is what you want but the output swing is too much

Other methods are fine if you want a different tone from the gain stage

Franco

[Merlin]

I changed the grounding as described in the schematic and the affect was minimal.I have an issue with my build which is a conversion of Classic 20.I have wired it according to the schematic and tested  and it is extreme loud.It starts to break already on volume 2-3. You can already see that in the schematic since there are 3 amplification stages before the PI.Is the amp really that loud and what kind of mods do you recommend?

/Leevi

The classic 20 has a master volume right before the (cathodyne) PI. So your problem is really that you don't like the numbers on the knob being 2-3 instead of 5-6 for a comfortable listening volume? I guess you could add a slugging resistor to the wiper of the volume pot, so it provided more attenuation, sooner. (You are using a log pot, right?)

[Latole]

I changed the grounding as described in the schematic and the affect was minimal.I have an issue with my build which is a conversion of Classic 20.I have wired it according to the schematic and tested  and it is extreme loud.It starts to break already on volume 2-3. You can already see that in the schematic since there are 3 amplification stages before the PI.Is the amp really that loud and what kind of mods do you recommend?

/Leevi

(You are using a log pot, right?)

You are right .
It will help a lot . Log pot A / audio work better than a linear one B.
A linear one ( B) bring  full volume at 3 on volume knob

[pdf64]

...
Replace 12AX7 ( 100% gain ) with 12AY7 ( 45% gain ,no it not to much ' i use often with success)

That change won't reduce gain to 45% of its stock level.
Rather voltage gain of the stage will fall from about 58.4 to 33.5.

But dB provide a much better representation of how we actually perceive such changes.

Swap a 12AY7 into a typical 12AX7 gain stage and the gain will fall from 35.3dB to 30.5dB.

Whilst that change should be noticeable / significant, it's far less so that those valve type mu tables that are widely promulagted would indicate.

See https://www.thegearpage.net/board/index.php?threads/updated-tube-comparison-table.2279173/

[Latole]

...
Replace 12AX7 ( 100% gain ) with 12AY7 ( 45% gain ,no it not to much ' i use often with success)

That change won't reduce gain to 45% of its stock level.

I know but it will help.
It is one of the fews mods he can do.

[kagliostro]

On the schematic VR5 is an 1M pot but they further specific 20%A, not an usual tapered pot (at least to me)


Franco

[Leevi]

I changed the grounding as described in the schematic and the affect was minimal.I have an issue with my build which is a conversion of Classic 20.I have wired it according to the schematic and tested  and it is extreme loud.It starts to break already on volume 2-3. You can already see that in the schematic since there are 3 amplification stages before the PI.Is the amp really that loud and what kind of mods do you recommend?

/Leevi

The classic 20 has a master volume right before the (cathodyne) PI. So your problem is really that you don't like the numbers on the knob being 2-3 instead of 5-6 for a comfortable listening volume? I guess you could add a slugging resistor to the wiper of the volume pot, so it provided more attenuation, sooner. (You are using a log pot, right?)

The volume pots are audio (log)./Leevi

[Leevi]

On the schematic VR5 is an 1M pot but they further specific 20%A, not an usual tapered pot (at least to me)


Franco

Maybe that improves the master.  Also Merlin referred to the problem in the master.
I'm using a standard audio pot which is probably not optimal for that.
/Leevi