Grid voltage problem.....

[badfan]

Hi everyone, I am building a dual channel amp (for my son) which is effectively based on having a Marshall (2204) and Soldano (lead) channel (25th birthday present for |September this year...)

I have attached the circuit diagrams for reference... as you can see nothing too difficult... (uses mechanical relays)

After building and initial testing etc channel 1 sounds great.... and the amp overall responds well (a bit spongy...) and is relatively no noise.... (from heaters, hum etc)... however the problem I am having is with the 2nd channel.....

When I switch (trigger the relays).... and start playing, it sounds great for a few seconds..... then starts to splutter and signal fades to nothing....

What I have found is that voltage (negative according to my meter) is starting to appear on V3 pin 7 which increases based on gain / volume level set and builds up over a few seconds effectively turning the signal off.... when I use the meter to ground pin 7 it then releases the charge and the sound comes back on again (for a few seconds) and then goes again.

Initially i thought i had a problem with the blocking cap coming off V2 2nd stage.... but have changed it a couple of times and its not that....

I have built many amps in the past and have checked the usual suspects for cold solder joints and potential shorts (which I have not found) so I am a bit stumped....

What I cant understand is why V3 pin7 is going negative voltage, when it should be 0....?

Anyway any help would be greatly appreciated..... thanks

[Bob S]

Looks like there's no ground reference on the grid when the preceding switch is open.

[passfan]

In high gain mode shouldnt your V3 grid have a ground reference resistor.? Whats to stop it from being affected by the plate cathode interaction ? I could be barking up the wrong tree but its just an observation

[passfan]

What Bob said

[badfan]

ahhhh..... I see what you mean, after the V2 2nd stage, before V3 etc pin7, (also noticed the diagram is slightly incorrect, should connect to b1 pole go b3 on DPDT switch)......

Is there a preferred value for the resistor, or just go with the standard 470k?

Thanks again... appreciate it

[badfan]

Guys, added the resistor (1Meg) and works great..... thanks very much.

[Bob S]

woohoo!
Great birthday gift too.
If you ever decide to adopt someone, I volunteer.