Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bman on December 21, 2022, 08:01:28 am

Title: AA864 build - Asymmetrical waveform from output when testing with scope
Post by: Bman on December 21, 2022, 08:01:28 am

Hi all, I'm new to the forum so please be kind if I ask any silly questions!

I've just built an aa864 bassman type amp, and upon completion I have checked the voltages which look fine, and then I've hooked up my scope to check how everything looking on there.

I expected to have to adjust the bias to set it correctly and remove any potential crossover distortion, however I'm faced with a different issue. As I turn the volume up, before the waveform clips it distorts asymmetrically. Can any help with any ideas to figure out what's happening here?

One note, is that I just decided to swap out the OT, as I had another one lying around with multiple secondary taps for impedance switching. Hammond 1750L swapped for a 1760L, both brand new. When I tested it wit the 1750L it didn't have an issue and I was able to bias it correctly, but I have checked over and over that the new one is wired in exactly the same. So I'm wondering if I've accidentally knocked something in the process.
- one way to tell would be to swap back the old OT, but I'm wondering if there's anything obvious I should try first.

I have the experience of building 4 tube amps so I'm not a complete beginner, but also not an expert.

I have a pure sine wave going in, and a 4 ohm load plugged into the output.

Picture attached. I can send a video too if helpful

Thanks

Title: Re: AA864 build - Asymmetrical waveform from output when testing with scope
Post by: sluckey on December 21, 2022, 08:08:24 am

Quote

I have a pure sine wave going in

What amplitude?

Title: Re: AA864 build - Asymmetrical waveform from output when testing with scope
Post by: Bman on December 21, 2022, 08:17:51 am

I think I've just figured it out..the OT primary centre tap is supplying the correct voltage around 450v, but the blue plate wire from the OT primary has no voltage at all, where as the brown one has 450v as expected. Measuring to the centre tap the brown one has 40ohm resistance and the Blue has no reading.

I'm guessing this means the transformer is broken...

Title: Re: AA864 build - Asymmetrical waveform from output when testing with scope
Post by: pdf64 on December 21, 2022, 08:22:06 am

Oh dear, yes, bad OT.
The screen grid on that valve may have taken a beating too.
The output valve idle cathode currents should’ve warned you something was wonky.