DIY tube guitar amp oscillating, please help!

[frickecello]

Originally posted by Rod Coleman
That sounds like a ground problem!

check that ground wires to grid and cathode are really connnected. these should all be conected together and THEN connected to the power supply 0V/GND.

Hi Rod, yes they are connected, I noticed that after turning the stand by switch off (turning the amp on) the funny fart sound is heard but it seems that after the capacitors get charged the fart sound goes away, and if I turn the volume up music gets fart distorted (as capacitors are discharged) so I will try changing C19 and C24.

I'm using cheap chinese Multicomp axial leaded capacitors, they supposed to be designed for testing purposes! Well they aren't doing their job!

[Wavebourn]

Capacitors charge/discharge through a ground wire, so if an input ground is connected in the middle of that current path, no doubt there will be voltage drop on that wire proportional to current and frequency.

[frickecello]

I replaced C19 first and checked, the same thing happened, then since I knew that it was a leaky or defective capacitor in that part of the circuit I just removed C22 (a cheap Multicomp axial leaded electrolytic) turned the amp on and it worked fine!

Thank you very much for all your help guys! I'll connect the first gain stage to see what happen. Thanks again.

[frickecello]

Well thanks to you guys I had the amp running decently for a couple of months... These are the changes I did to the original diagram I posted to lower gain since I think it was the cause of the problem:

1. Removed C27, C22 and C8 to lower gain...
2. Bypassed R8 with a 100uF IC capacitor and taken the output from R6 instead of R8. (Obviously this didn't reduce gain but it was a correction/upgrade I made to the phase inverter)
3. Upgraded C24, C19, C5 and C2 to IC 22uF caps.

These are other mods I made to reduce noise and standby switch noise (and they worked!):

1. Changed C7 and C13 to 220uF IC caps.
2. Bypassed all bridge rectifier diodes with 1KV 10nF Sprague caps.
3. Changed C15 and C16 to 330uF IC caps.

Also I changed tube layout because wires going to the tube sockets were long, and I reduced them to about 1-1/2".
The last thing I added was a pentode/triode switch.

But I'm getting the same fizzy distortion at low volumes (that I got previously if you read my previous posts), the weird thing is that the amp was running OK before the layout change and the pentode/triode switch mod. Also if the amp is running in triode mode this doesn't seem to occur, at least not at low volumes (the amp is really loud so I can't turn the volume up past its mid setting)

I was thinking to replace the 12AT7 for a 12AU7 to further reduce gain, but I'm not sure about that since the amp worked fine before and the thin distortion problem doesn't seem to occur when running triode mode. Again I hope you guys can help me I'll appreciate it a lot!