Super overdrive préampl makes feetback

[zazman]

Hello,

I've got a feetback problem when i use the overdrive channel + boost
circuit, when the drive level is greater than middle gain, and when the bright
selector is ON.

it's like a high larsen.. any idea about this ?

ps : i put the 22k input resistor near the first 12AX7 triode ... anything to do with that ?

Thanks a lot.

[Bob-I]

Do you mean feedback or oscillation?

I'd start by moving some of the wires around using a chopstick. When you find the cause the sound will change. Then you can troubleshoot the problem.

Pictures and layout drawing might help.

[glasman]

Sounds to me like a lead dress issue or funky tube. I had a ECC803 that would take off as I cranked the gain.

[zazman]

Thanks Gary, i will replace the clean / overdrive 12AX7 tubes to see if the
feedback comes again at the same gain level or not...

I made a main preamp PCB with clean/overdrive stages and all the wirings coming and outcoming are shielded... The only thing i changed in this V2 was to use pretty goods components :

- mallory /silver mica for clean stage
- orange drop /silver mica pour OVD stage

in place of old components. In fact, the signal is better with more
high frequence response (especialy when bright is ON).

I think to increase my OVD stage gain ajust pot, it is at the minus
level, to decrease the HUMMM level when the preamp is working
in OVD + Boost . Maybe the clean stage output signal is too high
and makes feedback when coming at this point because the resistance
on the OVD stage input is "too closed" to make all the signal going
properly in the OVD stage ???

What do you think about ?

Thank a lot... and please excuse my english (i'm french).