'68 Fender Bassman and Fixed Bias

B

Boots

New member
I bought a 1968 Fender Bassman head the other day with it's original tubes. I went ahead and replaced the 3-7025 and 1-12AT7 preamp tubes and bought a matched pair of 6L6's.

I read where the early Bassman's have Fixed Biasing. I think the name is deceiving - "Fixed Biasing." It sounds like you do not need to adjust the bias when replacing the power tubes. How do I bias a Fixed Biasing amp?

I haven't opened the case up so I'm not sure there is an adjustment pot.

Any hints or tips on adjusting a 1968 50 watt Bassman Silverface Head?

Thanks.
Scott
 
You take it to an amp tech. For real! There is a lot of power in an amp, even one that is turned off and unpluged can still kill you. Tack it to an amp tech!
 
I've got a '69 silverface bassman head, and there is a bias adjustment pot on the bottom of the chassis. It's in between two of the transformers.

I'm not 100% certain that it's original equipment, but you may want to look. It's a recessed knob that you turn with a screwdriver. It looks almost like an input jack.

Aside from that, I totally agree with 64Firebird. Take it to someone that knows Fender amps. These things are dangerous, and someone that knows what they're doing will likely get much better results. It almost certainly will need a cap job in addition to new tubes. I took mine in awhile ago, and it came back a totally different (and much better) amp.

Good luck!
 
You can't adjust the bias unless you have a scope anyway. Also, you need to check the caps as well. Old amps sound great, but they can be very dangerious. For example, if it has bad, leaking caps and you're not grounded the right way that little shock you get from touching a mic could give you the chance to meet Jimi Hendrix in person. In other words, poof your dead.

It might be a good idea to have the tech replace the power cord with one that has a good ground, one with 3 prongs, while it's there. Ventage is cool, but safty is better.
 
haha Our singer was using my Bassman the other day and went to start singing in the mic and a "lightning bolt" came flying onto his lip. Made his knees buckle for a second.

I saw some mods on how to add a three prong power cable/plug. I AM going to do that for sure.

Thanks for all your help.

Scott.
 
biasing is really not for the faint-hearted/unacquainted. it's meant to ensure that your tubes operate at optimum power.

you've got measure the DC voltage on the grid of your output tubes (u need the schematics of your amp and tube). so aside from the obvious danger of electric shock, you might seriously destroy your amp if you are not familiar with reading schematics etcetc. to be on the safe side, send it to a tech.

cheers!
kb
 
I tend to agree with the others that you should take it to an amp tech, unless you have a variac, VTVM, scope, signal generator and know how to safely discharge large electrolytic capacitors. It only takes one mistake.....

Of course if you had that stuff and knew how to do that you probably wouldn't be asking about how to adjust the bias voltage on a '68 Bassman.

If you want to get into amp repair there are a few good books on the subject. The Tube Amp Book by Aspen Pittman comes to mind. To me that is required reading by anyone into tube amps, even if you don't do your own maintenence/repairs.
 
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