Fryette Power Amp Bias Help

[saxxamafone]

Does anyone know how to test the bias of a 2/90/2 or 2/50/2?

It's the first amp I've come across where I'm struggling to work out where to read from and how.

I can't even use my Weber Bias Rite in this one due to how the tubes are located.

cheers

 

[mnemonic]

They don’t have bias test points, you need to either use the output transformer resistance method, output transformer shunt method, or find a smaller bias probe.

I biased my 2/50/2 using one of these I got on eBay that interrupts the plate connection so I can read plate current directly using my multimeter (set to milliamps).

Depends on tube size how easy it will fit, I had to remove the fan to get it on the middle two tubes, and the output jacks to get it on the leftmost tube.

 

[RedB4Black]

Weber used to make L shaped adapters for the bias right… those worked well. Though i was able to squeeze in without the adapter i think. I know I did it once with a 2/90 that I was having a problem with a tube that kept red plating

Eurotubes sells a smaller adapter for a multimeter like pictured above

 

[stephen sawall]

I have been trying to find one of the L shaped adapters from Weber. No one makes anything similar I can find.

 

[saxxamafone]

Thanks very much for this info guys. Great starting point

 

[Racerxrated]

They don’t have bias test points, you need to either use the output transformer resistance method, output transformer shunt method, or find a smaller bias probe.

I biased my 2/50/2 using one of these I got on eBay that interrupts the plate connection so I can read plate current directly using my multimeter (set to milliamps).

View attachment 115741

Depends on tube size how easy it will fit, I had to remove the fan to get it on the middle two tubes, and the output jacks to get it on the leftmost tube.

View attachment 115744

I have a similar device I also bought off eBay 10 years ago. Works great.