Fender Twin Reverb

[pnut5150]

I have a Fender Twin Reverb export model, Silverface, appears to be late 70's early 80's model. Can anybody tell how to mod the Tube Balance pot to a bias pot?

[The Dude]

IMO, I wouldn't bother. I don't see much to be gained. But, if you must:

Ask Amp Man: Better Biasing for Silverface Twin Reverbs | Premier Guitar

[Leo_Gnardo]

I have a Fender Twin Reverb export model, Silverface, appears to be late 70's early 80's model. Can anybody tell how to mod the Tube Balance pot to a bias pot?

Why not both? What I do is put a 10K trim pot in series with 10K half watt resistor in place of the typical 15K bias setting resistor. I like the Bourns "blue cube" pots but you can use whatever you like. If your amp has the balance resistor on the back panel, with a 33K bias set resistor, use a 20K trimpot in series with 22K fixed resistor instead.

[bob p]

I agree with Leo's most excellent recommendation. Don't go yanking out the bias-balance circuit by cutting out resistors and re-wiring it as a BF-style bias pot. Why not have the best of both worlds? Buy a trimpot and use it for setting the bias offset, and continue to use the balance pot to adjust A-B balance.

FYI here's a GW video of how to do the SF->BF mods. Cue the video up to 4:30. But I like Leo's idea better.

[pnut5150]

Thanks for the help guys, I do like the idea of having both. I'll let you know how it goes

[pnut5150]

I have the the balance pot on the back with the 33k resistor, I already have a 10k pot, can I use it with a resistor?

[Leo_Gnardo]

I have the the balance pot on the back with the 33k resistor, I already have a 10k pot, can I use it with a resistor?

I'd use a 27K in series with your 10K pot. That way you can vary the resistance from 27K to 37K. Somewhere in there you should find a comfortable overall bias.

No series resistor? Why? Then you're varying from 0 to 10K, that will pull the bias voltage down quite a bit & send output tubes into red plate doom at the 10K level, and worse as you dial down to zero. You don't want to go there.

[bob p]

Leo brings up an important point -- it's essential to design a circuit to protect the amp from a simple user-error that could result in catastrophic damage. For this reason you never want to use a pot that allows you to dial down to zero, or to dial up too high -- you always want to size the pot for the proper range of adjustments needed, and you want to use scaling resistors to design the circuit such that the desired bias point is in the middle of the control's adjustment range. And you definitely want to put a safety resistor in series with the pot so that even if you inadvertently twist the pot the wrong way, or all of the way in the wrong direction, you won't damage the amp. A good bias circuit design needs to be idiot proof. To me that means the circuit should have a narrow range of adjustment that covers the necessary range, and doesn't go any farther. It's amazing how easy it is for a smart guy to make a bone-headed mistake if the doesn't always have his thinking cap on. To prevent that from happening, it's best to eliminate that kind of error from occurring by designing the circuit to prevent it from ever occurring. You don't want to design a circuit that would allow a user to twist the bias pot to any setting that will blow something up. It's best to design your amp so that the range of adjustments is adequate and nothing more than adequate. That way you can't possibly mess something up by making a simple adjustment error.

Another thing to consider is designing the bias circuit so that the amp will never run into "red plate doom" if the pot should fail open. Pots fail. Honestly speaking, some fender circuits aren't very well designed because they could allow that to happen. Most fenders never suffer this fate because Fender used very high quality bias pots. but those pots are old now. I think it's better to use a Marshall-type bias circuit where the circuit prevents the amp from going into thermal runaway if the pot should fail open.

[pnut5150]

Gotcha

[pnut5150]

Good points

[pnut5150]

Nice and quite, no hum, sounds fabulous! Thanks