Heavy Oscillation problem...I think!

[thomasdj]

Hi,

I am building a clone of the mesa dual rec 3ch version (don't know if I am the first since I can't find anyone who have tried this before). I fired up the amp and got some heavy oscilations..reversed the OT wires and know it don't squeal anymore..! However..I only got an ugly very low distorted signal through.

When I hit the strings on the guitar..and turn up the amp I see sparks from the tube sockets (I know that they are connected rigth! and there are connection!). I have double checked all resistors in the output section and everything is okay..The bias supply is also okay. I have tried replacing the tubes without luck.

Help please

Thanks

[thomasdj]

Forgot to say..that when I hit the strings..the tubes blinks blue! ..

[Alex R]

Blue glow in tubes isn't necessarily a problem.

Sparks from sockets is unusual. Can you describe what happens more precisely? An arc is a function of the width of the gap the spark is bridging, the voltage difference between the conductors (and the conductivity of the air). Which sockets? How far apart are the conducting parts that are arcing? Could they be moved further apart? Is there a whisker of wire there or something?

I take it this amp isn't on a PCB? Hand-wiring a high-gain amp like this is very likely indeed to give oscillation issues - multi-stage high gain designs of the Mesa, Soldano etc type are built on PCBs for many reasons including economy but also including oscillation control. Do you have a scope to check for oscillation?

[thomasdj]

Blue glow in tubes isn't necessarily a problem.

Sparks from sockets is unusual. Can you describe what happens more precisely? An arc is a function of the width of the gap the spark is bridging, the voltage difference between the conductors (and the conductivity of the air). Which sockets? How far apart are the conducting parts that are arcing? Could they be moved further apart? Is there a whisker of wire there or something?

I take it this amp isn't on a PCB? Hand-wiring a high-gain amp like this is very likely indeed to give oscillation issues - multi-stage high gain designs of the Mesa, Soldano etc type are built on PCBs for many reasons including economy but also including oscillation control. Do you have a scope to check for oscillation?

Hi Alex,

Thank you for your reply. The amp is build on a PCB. However the poweramp section are build PTP. The arc seems to happen between pin 3 and 4 of one of the output tubes. I have moved the parts further apart and don't actually think that this is the course. No whiskers.

I know that blue glow isn't unusual but the tubes blink and "ticks" when i hit the strings.. and the sound is very low and distorted. I got alot of wires runing across the pcb but these are only related to the switching circuit which shouldn't cause any problems, I think. I also got wires running from the pcb to the preamp tubes, and wires form the pcb to the pots. I've build the amp like I would build an effect pedal.. I've got three pcb's for the power supply and three pcb's for the preamp each connected via the preamp tubes.
Have a scope but really don't know how to use it..is there another way?

Thanks

Thomas

[Alex/Tubewonder]

Swap tubes. Does it spark on the same socket or does it follow with the tube?

[thomasdj]

I've now grounded pin 1...but when i switched the high power in..the output tubes blinks and a strange noice occured (maybe from the OT but not sure)..

[thomasdj]

Swap tubes. Does it spark on the same socket or does it follow with the tube?

It doesn't follow the tubes...Don't think it spark anymore..but really don't want to test it since the strange noice I described above..

[thomasdj]

I've now grounded pin 1...but when i switched the high power in..the output tubes blinks and a strange noice occured (maybe from the OT but not sure)..

It was a shorted jack that caused this...

I tried to plug in to the FX return..pulled v1-v3 out and fired up the amp...
Output tubes blinks and a motorboating sound comes out the speaker..