Weird treble pot JCM800 build

[sonofmickel]

Just did a JCM800 circuit in a donor amp.
The treble pot is weird and I am wondering what might be the cause.
When the treble is turned up the amp volume goes down and gets scooped. When the pot is turned down to "0" the amp gets much LOUDER and thicker?!

Anyone ever come across my problem? I have the tone stack wired in standard JCM800 feeding a Pre PI Master.

Also V2b pin 6 is getting 332v. Seems high as the schematic shows 245v as the standard.

The amp sounds really compressed as well but gets really loud.

[matt h]

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[Stevem]

Make sure none of your tone stack caps are leaky and passing dc voltage for one thing!
What is your plate voltage on both sections of the PI tube ?

Are your voltages on the plates of V1 high?
Is the cathode voltage on V2b way off?

[sonofmickel]

Thanks for the help.
Voltages: not including heater pins 4/5, and 9.
V1 270, 0, 2.75, 214, 0, 1.73
V2 177, 0, 1.13, 321, 176, 185
PI 210, 15, 40, 217, 12, 40

The wiring is PTP and is a mess inside, I will try adding some shielded signal wire and see if that brings back the treble control and/or adds girth.

[JazzGuitarGimp]

With all of the controls turned fully down, are you still getting 332V at V2B?

[sonofmickel]

Yes that is correct. The plate is fed directly from the power rial, no plate resistor in the Marshal JCM800 design. I have seen other posts about this but no solution was offered/found. That is very high for a preamp stage, yes?

[JazzGuitarGimp]

I didn't realize we were talking about a CF stage. The schematic I found shows four voltages for this node ranging from 264V to 290V, depending on whether it's a 50W or 100W, and with or without MV. It does seem high, and it might just require some tweaking of the power supply dropping string values to get it into a better range. But I think I'd make sure it doesn't have any parasitic oscillations before making adjustments to the power supply.

[Reeltarded]

Look around junction of slope and treble cap. Check that wire to the treble pot with a stick to see if it's loose or in a bad place.

[sonofmickel]

I just re-wired the tone stack 3 times with different components, same result. Treble off = Loud mid heavy (decent) tone.
Treble Full On(10) = thin, low-volume tone. I want to add that this is a gradual effect, as I rotate the pot it gets gradually thinner and drops in volume.

I am at my wits end on this one...
How would I go about testing for high frequency oscillation?

Surely someone has encountered this before.

[Stevem]

If you have or can borrow a volt meter with a frequency test function you can hook it up across the speaker output and play a low B note which is 120 hz, if as the note rings off you read other frequency's above what the speaker can produce ( like 5hz to 7hz) then yes the amp us oscillating!

[matt h]

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[sonofmickel]

No go on the snubber caps. It just resulted in only bass getting through the circuit(very valuable information though!). I can see using small value caps to "smooth" out hairy/peaky/bright amps- so thanks for forcing me to do that!

With snubbers at V1 and V2 the treble pot acts the same, it cuts volume and tone as the pot is turned clockwise. Gets louder turning to "0". Hmmm....

[Reeltarded]

It worked before? Let's see the TS wiring, pretty please?

[sonofmickel]

No, it never worked correctly before.
Reeltarded, you have previously commented on the "rats nest" of the amp I am re-doing. It's the Phaez Rooster. I was never happy with the drive channel and had hum and other stuff not to my liking. So I decided to gut the drive channel and try a JCM800 circuit. It basically sounds mostly like a JCM800 but lacks punch and compression characteristics of the Marshall(I have a 2205 in the room that I just fixed, it sounds awesome, punchy etc...). It sounds just like the previous circuit did, a bit lifeless and can get deafeningly loud, and stays quite clean if the gain is turned down. Both circuits have the exact same treble response symptoms= does not work as a normal treble even if wired as one.
The clean channel works fine, sounds fine and shares V1A as well as the loop recovery stage, then to the Master Volume and PI.
I have jumped the Drive channel directly from the channel MV to the PI coupling cap and the amp treble behaves the same. WTH?

I would like to add that I am "breadboarding" the amp, just cutting and trying different values until I find what I like. It will be cleaned up and I will use at least a few strands of color coded wiring.

Attached is an mp3 of an open "g" with me turning only the treble pot from 0-10. I really do not hear any oscillation happening.

[matt h]

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