Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on August 19, 2022, 05:41:29 pm

Title: Twin Reverb II Biasing Question
Post by: dpm309 on August 19, 2022, 05:41:29 pm

Trying to figure out how to bias a Twin Reverb II. Looking at the schematic I am finding a hum balance pot (for the heaters) and a output tube matching pot. Will be installing a new matched set of 6L6s and want to bias it correctly. I am assuming that the output tube matching pot is for unmatched tube sets. Unless my eyes are failing, I don't see a bias pot anywhere on the schematic. Any suggestions?Thanks

Title: Re: Twin Reverb II Biasing Question
Post by: pdf64 on August 19, 2022, 06:54:05 pm

Yes, it’s best to add 1ohm cathode resistors. Failing that measure the OT red-blue and red-brown resistances and volt drops; derive current for each push pull side from that, assume an even split between each valve.
Bias can be adjusted by altering the value if the 33k resistor connected to the output valve matching pot.
To make it more easily adjustable, replace the 33k by about 22k in series with a 20k-25k trimmer.

Title: Re: Twin Reverb II Biasing Question
Post by: dpm309 on August 19, 2022, 07:12:25 pm

Here is the schematic that I have. Same one as the one on Doug's schematic page. Looks like I should add a bias pot. Will get with the owner to see if that is the way he wants to go.Thanks,

Title: Re: Twin Reverb II Biasing Question
Post by: dpm309 on August 21, 2022, 12:31:44 pm

I have another question about the output tube balance pot. I think Fender replaced this with a bias pot on later models (or vice versa). Was this for unmatched tubes? Does this balance pot have anything to do with biasing?

Title: Re: Twin Reverb II Biasing Question
Post by: pdf64 on August 21, 2022, 01:55:45 pm

Quote from: dpm309 on August 21, 2022, 12:31:44 pm

I have another question about the output tube balance pot … Was this for unmatched tubes? Does this balance pot have anything to do with biasing?

Yes and no.
It would be a good idea if you could explain what exactly you are getting stuck on in my previous post, then we can try to help you to understand.
‘Bias’ refers to the negative voltage  between a valve’s control grid and its cathode at idle.
‘Biasing’ means to check the effect of that voltage on the valve’s anode current and dissipation at idle, ie that they’re at a reasonable level.

Title: Re: Twin Reverb II Biasing Question
Post by: dpm309 on August 21, 2022, 02:11:43 pm

Yes I understand biasing. Was just wondering if I could eliminate the balance pot since I am using a matched quad of 6L6s and replace it with a bias pot per your suggestion.

Title: Re: Twin Reverb II Biasing Question
Post by: sluckey on August 21, 2022, 03:30:12 pm

.

Title: Re: Twin Reverb II Biasing Question
Post by: dpm309 on August 21, 2022, 03:45:12 pm

Thanks Sluckey! Just what I was looking for.