Prosonic drive channel week and nasty problem

[torp]

Hi
Prosonic combo with really nasty distortion and low volume on the drive channel.
Normal channel works fine, the when switched to drive channel, turn master volume up and first gain knob alway up I get this fuzzy and nasty sound with little volume ''lack of power''. Imagine the sound of, say 20watts amp trough a 5 or so watt speaker, that kinda sound.
I changed all the valves, indeed it make a very small diference in volume specially the preamp tubes. But no significantly.

This is a PCB amp and quite difficult to see were the problem lies.
Any one experienced the some problem, and be kind to share some info?

Thanks

[Jazz P Bass]

It may be best to take a few basic voltage measurements.
V1 & V2 are the gain channel.
Measure & notate the plate (pin 1 & 6) the grid (pin 2 & 7) and the cathode (pin 3 & 8) dc voltages.
Test with no signal applied.
If a tube is not conducting or a capacitor is leaking, these measurements may point to the problem.
Ideally you would want at your disposal a scope and a signal generator.
http://support.fender.com/schematics..._schematic.pdf

[torp]

Hi
I will the measurements after work.
Nop I dont own a scope and a signal generator.
I will se if I can find open source software for that purpose to go with my PC.
Appreciated
TX

[torp]

Hi
Thanks for your input.

These are the actual readings taken using a multimeter.

V1
Plates
Pin 1 - 0.7V - switch on normal position - 0.9V switch on drive position
Pin 6 - 255V - '' '' '' '' - 255V '' '' '' ''

Grid
Pin 2 - 0V- switch on normal position - 0.8V switch on drive position
Pin 7 - 0V- '' '' '' '' - 0V '' '' '' ''

Cathode
Pin 3 - 0.30V- switch on normal position - 0V switch on drive position
Pin 8 - 2V - '' '' '' '' - 2V '' '' '' ''

…………………………….

V2
Plates
Pin 1 - 264V - switch on normal position - 263V switch on drive position
Pin 6 - 264V - '' '' '' '' - 263V '' '' '' ''

Grid
Pin 2 - 0V - switch on normal position - 0V switch on drive position
Pin 7 - 0V - '' '' '' '' - 0V '' '' '' ''

Cathode
Pin 3 - 1.96V - switch on normal position - 1.96V switch on drive position
Pin 8 - 1.96V - '' '' '' '' - 1.97V '' '' '' ''

All voltages measured to ground.

………………………………..

Note. On tube 1 plate and grid are tied together with a 6.8pF cap.

[pontiacpete]

V1 pin 1 and pin 3 do not look good.
there should be high DC voltage on pin 1, 200v or more
and pin 3 should have something on it, like 1 or 2 volts.
Did you try another tube for v1?

[Jazz P Bass]

Yeah, I would look for an open plate resistor on V1, pin 1.
Or a bad copper trace.
Or a bad solder joint therein.

[torp]

Yeah, I replaced the 6.8pF cap on V1 between pins 1 and 2 and it works, I got the drive channel back also measure the voltage on pin 1 as recommended by you but I still not see the voltage increase to the 200V or more, I could only see around 5V.
I will change the 330k plate resistor tomorrow and I will post my findings...not sure ??

[Enzo]

Why not measure the resistor first to see if it is open or WAY off value? No point in replacing a good resistor to find out the problem is a broken solder joint elsewhere.

[torp]

yeah, sure the resistor its dead, after replacement it show a bit more than 70V on v1 plate.
The voltage before 330k resistor is 382V and 70V then after.
It works but I cant compare it with same other amp so I can judge.

cheers