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SF Fender Twin Reverb Bias Mod

6/28/1999 7:22 PM
SF Fender Twin Reverb Bias Mod
Hi Guys,  
I have a '76 SF Twin Reverb and I'd like to install individual bias adjustment pots for each 6L6GC as opposed to the single balance pot in there. Do you think having 2 pots(1 per pair) for bias is better than 4 individual ones? What kind of pot and resistor values should I use for this?  
I'll install the 1 Ohm resistors to make bias measurement easy. Are there any Fender schematics I can copy a bias section off of.  
Thanks for your help  
6/30/1999 1:32 PM
MJ Harnish
Weber's 1st book has an entire chapter than describes the conversion procedure.  
7/5/1999 4:23 AM
Steve Dallman

Leave your balance pot. On the pot you will see a resistor soldered from the center terminal to the body of the pot. Remove this resistor and replace it with a 5K 1/2 watt resistor in series with a 50k cermet, multiturn pot. There's your bias. Balance your tubes for the least amount of hum. Then adjust your bias with your new pot. Repeat if necessary.  
This is easier than changing to 2 pots yet gives you the same results. I rewired my heavily modded Bassman 100 with 4 seperate pots. It works well. I used 4-50k multiturn cermet pots.
7/5/1999 4:31 AM

Thanks for your reply Steve. I think I will give it a try.  
Right now, 2 6L6GC are drawing 35ma at idle and the other pair is only at 18ma. I can't seem to get this pair any higher withoput disturbing the other pair due to the balance pot. Thanks again for the suggestion and I'll let you know how things go.  
7/5/1999 4:39 AM
Steve Dallman

How are you measuring the bias?
7/5/1999 4:59 AM

Using 1 Ohm 1/2W resistors of the cathodes.  
7/5/1999 7:36 AM
Steve Dallman

Great. Then you can see the draw of each tube. You can put the closest matched pairs on each side and balance the 2 pairs. You should be able to get pretty close.

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