View Full Version : Changing Tubes in 70s Fender Super Reverb
11-13-2008, 08:31 AM
I recently bought some NOS tubes for my 70s Super Reverb & want to see how they sound in it. Sorry for my ignorance but do I need to rebias the amp when I put the new tubes in it? What does replacing the tubes involve. By the way the tubes are RCA backplate 6L6s & I also have a Sovtek 5AR4. Any help from Super Reverb owners or techs would be much appreciated. Thanks.
11-13-2008, 08:50 AM
While you don't have to rebias, it's always good practice to do so. That keeps your tubes running at optimum performance levels.
11-13-2008, 09:01 AM
You've made an investment in some nice tubes.. I strongly urge you to re-bias them.. You could cook them... or, they could be overbiased, and sound poor. Either way, it's a waste of good tubes!
Pre-amp tubes (the 6 smaller tubes) can be swapped without any adjustments needed.
11-13-2008, 09:51 AM
If you are changing your power tubes youi will need to rebias as I think this is a fixed bias circuit. When changing preamp tubes there is no need to rebias
11-13-2008, 10:02 AM
What year is it? At one point Fender went to a bias balance pot as opposed to a true bias adjust.
11-13-2008, 10:07 AM
Chances are this amp has the Fender Bias Balance setup. The bias is set by a fixed resistor and the pot is used to equalize the current to the two power tubes.
If so the only way to change the bias is to either mod the amp to Blackface specs, add another pot to be able to change the bias as well as balancing the two tubes, or swap out the bias resistor with another after measuring.
11-17-2008, 06:42 AM
It is a 1970 Fender Super Reverb. It sounds like biasing the tubes can be quite difficult. Is it best to take it to a Amp tech? Do you have to bias rectifier tubes? Thanks for all the advice so far.
11-17-2008, 09:45 AM
You only need to bias the power tubes. You amp has a bias balance pot - a competent tech can convert this quite easily to a bias adjust setup. Have him convert that circuit to Blackface specs while he's at it and it will warm that amp up a lot.
11-17-2008, 11:08 AM
What Mike said.
12-14-2008, 11:25 AM
I have a '73 SF SR, which had the circuits "blackfaced" before I got it. I've had it a couple of years, and it hasn't been serviced for at least a year. Recently, I'm getting the frying bacon sound out of Channel 2, which increases with volume increase. Anyone have a suggestion on which tube(s) to replace (assuming it's a bad tube) and suggestions on what brand/model of tubes? (Also, do I need to bias that?)
12-14-2008, 04:29 PM
If it's a tube, one of the first two 12AX7s - sorry, don't recall off-hand. More likely a plate resistor. No bias change necessary for preamp tube changes.
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