Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TerryD on March 16, 2022, 05:31:53 pm

Title: How to Bias Fender PA 100 / Twin
Post by: TerryD on March 16, 2022, 05:31:53 pm

I am looking for some mercy.  I havn't worked on an amp in a while (5 years) and am a novice.  I turned this Fender PA into basically a twin reverb.  I recently purchased two pairs of old RCA 6L6's. I'll put the matched pairs in their proper places -two inside and two on the outside of the line of four. What do I do to bias them once they are in?  Thanks Terry

Title: Re: How to Bias Fender PA 100 / Twin
Post by: sluckey on March 16, 2022, 06:10:20 pm

Quick method... Adjust the bias pot for -52V on pin 5 of the 6L6s.

More involved method...   http://www.duncanamps.com/technical/lvbias.html

Title: Re: How to Bias Fender PA 100 / Twin
Post by: pdf64 on March 17, 2022, 07:55:52 am

Quote from: TerryD on March 16, 2022, 05:31:53 pm

...two pairs of old RCA 6L6's. I'll put the matched pairs in their proper places -two inside and two on the outside of the line of four. What do I do to bias them once they are in?

RCA were making 6L6 for a long time, your amp needs them to be 6L6GC / 6L6WGB / 5881. Other, earlier types aren't suitable.

The bias should be adjust such that all the 6L6 are well out of cut off (eg >10mA anode current) and are not dissipating >70% of their design centre anode limit at idle.