Orange OR120 build advice.

4114Effects

Hi guys. Strange one for you seasoned amp techs here. Just finished a clone of an orange or120, but using a Marshall 100watt style PT, so 150-0-150 into a capacitive voltage doubler (bridge rectifier).

Powers up fine, all voltages check out, but All I get is very weak distorted sound. However, when I play chord on the guitar then switch off the power, the amp comes to life - strong and clear for 5 seconds or so then fades out.  

I'm guessing the issue must be in the power section for the amp to work properly when the caps are draining? 

Wondered if anyone had had a similar problem on a build to save me some troubleshooting time! 

Cheers. 

ICBM

That's an odd one...

Bias voltage way too high, valves in cut-off until the bias supply voltage drops?

Self-oscillating outside the audio range due to coupling between the OT and the PT (or choke)?

4114Effects

You've nailed it. I'd already figured it out earlier today but you're spot on... valves were biased into cut-off then started to work when the bias voltage dropped on turning off... rookie mistake, no idea why I didn't check the valve bias.

Was also scratching my head as to why the negative feedback didn't do anything, then realised I'd not grounded the output jack. Sigh. I blame lack of sleep. :-)