I thought I had this figured out...

[dorrisant]

Thanks guys.

What percentage of max dissipation should I shoot for? Class A fixed at 90% or class AB fixed 70%?

[martin manning]

I'd stay closer to 70% than 90%, but maybe it depends on what it sounds like. Is your HT-derived bias circuit working now?

[dorrisant]

No.

[Gregwor]

Holy cow!! What is going on with the Fixed/Cathode bias here?

Can someone explain the bias point that this amp should be set to?

How is it calculated?

Maybe that's why the bias reading (thinking fixed bias A/B) was way too high. Maybe the idle would be set higher with the Fixed/Cathode bias setup in this amp. Maybe I can just take my bias board out and let it eat as is. I mean, that is how it was, stock.

I am puzzled by this one.

James talks about this at around 2hr and 40 minute mark here:
https://www.youtube.com/watch?v=PzOhq3-3d90

Greg

[dorrisant]

Thanks for that. Amp is back with the owner now.

When I had it here last, I put know good power tubes in (tested and labeled 1st) and ran it for about 30 hrs straight into an attenuator with a moderate looper signal. Afterwards, I tested and inspected the tubes and found no real change in the values at the tube tester or visually.

I told the owner to let me know if anything seemed at all strange about it. He has reported back to me that it is working just fine after a few months of regular use. The power tubes were JJs and I showed him how the paint on them can indicate overheating, so he checks that from time to time as well.