Troubleshooting a new build amp

[Wardy]

Hi Gang,

I really need some expert help. am reaching the end of my tether with a new build 50W ODS amp.

The situation is that it fires up and all the controls work as expected but the output volume is low. I have traced through the amp with an oscilloscope and narrowed the problem down to V1a. which does amplify but by a factor of 2 when the expected is 45. Its taken me days to get to this stage!

I guess the question is now pretty simple as the problem is specific. What would cause one side of an ecc83 triode (V1a) to output 2x gain when it should output a gain of 45x according to a recent test on another amp thats the same.

I've checked for resistor values. cap values, checked for no dc on wrong side of caps, correct connections, all earths working, tried 3 valves, inspected every joint with a magnifying glass, reflowed some.

What else could possibly cause this? where else to look?

has anyone ever had a similar problem?

[Williamblake]

Measure DC Voltages at tube pins.
Doesn't sound sexy, but excel is of big help. No more notes flying around, you can see your progress afterwards.

[Wardy]

Ok Thanks, Sorry I forgot to attach the voltage chart I made previously.

best

Steve

[dogburn]

Could it be the input jacks and/or their wiring? Could the FET board have anything to do with it? Have you tried wiring in an isolated input jack and playing through that?

[acheld]

IF what you're saying is correct, then consider mis-wiring of the input to V1a. 

Pictures of your build would be really helpful, showing wiring of the inputs (both normal and FET), and V1.  High resolution helps greatly.

[kagliostro]

Just to know

didnyou swapped V1 with One other tubes, the problema is there also if you do that ?

Franco

[Wardy]

Thanks People

I’ll try all your suggestions tomorrow. Can’t do anything this evening.
Best

Steve

[Williamblake]

V3 Pin1 is probably just a typo. Are your grids 0 V DC or did you not measure them?