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Post new topic How to set bias on a Bassman
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Author Topic:  How to set bias on a Bassman
Tommy Boswell

Virginia, USA
Post Posted 9 Mar 2017 5:50 am     Reply with quote

Can anyone tell me how to bias a new set of power tubes for my Fender Bassman 70? It has a "output tubes matching" control pot on the rear of the chassis. Assuming they put it there to make it easy, how do I know where to set it?

I have biased other amps using a milliamp meter, and instructions from the manufacturer. Some are easier than others, allowing meter probes to be inserted on the back of the chassis. No such luck with the Bassman.

Any help appreciated.
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Bill Hatcher

Atlanta Ga. USA
Post Posted 9 Mar 2017 6:44 am     Reply with quote

those are the ultralinear fender amps. hated by rock players...fine for clean stuff.

there should be two adjustments on the back of the chassis. these are most likely balancing acts. the filament hum adjustment is adjusted by turning the pot slightly and listening for hum...just turn it till you hear the least amount of hum.

the power tubes pot is probably balance also. you have to start out with a nice set of matched tubes and turn that pot till the bias voltage is the same for each side. you can rewire that pot to become a bias pot that adjusts voltage up and down.

go to google and look for mods for that amp bias wise. theres all sorts of stuff out there.

open up the amp. check the filament voltage and the bias voltage as you turn each pot and let us know what values you get for the best sound of the amp.
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Los Angeles
Post Posted 9 Mar 2017 9:32 am     Reply with quote

To maybe restate what was said:
Hum Balance Control: This has nothing to do with biasing. It may help you get less noise/hum though.

Output Tubes Matching: Varies the grid bias voltage to each power tube at the same time in a "balancing act" mode.
Assuming that you have a somewhat matched set of power tubes, the schematic says to set the OTM pot for minimum hum.
This should help to balance the bias voltage on the output tubes so that they will both be drawing the same plate current. This may not be the ideal current, though.
There is no easy way to tell what the bias (plate current) for either tube actually is since there are no test points, etc.
I say "easy" meaning without probing around inside the amp, which has high voltages.

I'd say, based upon my limited knowledge of this amp:
Put the power tubes in.
Set the OTM pot to the middle.
Turn the amp on and let it warm up.
Set the controls until you get some hum/hiss.
Turn the OTM pot to get the least amount of hum/hiss.
Then, adjust the Hum Balance Control to fine tune the hum/hiss.

I hope I got this right.
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Jack Stoner

Inverness, Fl
Post Posted 9 Mar 2017 10:20 am     Reply with quote

Back when I was doing amp repair (and never heard of the "Hum Balance" method) I would measure the fixed voltage side screen and adjust the other side to match the voltage. Whether that was right or not?? But I did that to a lot of Fender amps and no one complained (I worked at Little Roy Wiggins' Lower Broadway music store in Nashville in early 70's).
Franklin D-10, Hilton VP, POD X3, MatchBro, SG Black Box, Carvin BX500, EPS-15C, Sonar DAW but migrating to Studio One 3, MOTU 896mk3 Hybrid
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Bill Hatcher

Atlanta Ga. USA
Post Posted 10 Mar 2017 8:46 am     Reply with quote

its amazing how the fender amps will still work no matter if they are biased correctly or not....they wont sound as good as they can, but they still work.

the balancing act. when leo left there were all sorts of "improvements" made to the amps.

when you set bias, you want to set it for the best current draw on each side....a good place to start is 30MA on each side of a pair of 6l6s and 60MA for 4.
the balance control as is on the bassman 70 does not allow you to do that. it just sets the bias voltage for each side the bringing up one side and bringing down the other or vice versa. then when you get to the same bias voltage on either side, wherever your current draw is...well basically thats what you have! NOT optimum. it may balance at 50MA or 20 or 70 or whatever the tubes balance out at. the old bias on the black face amps brings the bias voltage up and down at the same time, so providing you have a decently matched set of tubes you can set them for the optimum current draw and the best sound from the amp and you can vary that voltage to compensate for your sound if you like and for the tubes if they are too hot, or weak. i like this set up better than the balance.

the other balance control matches the filament voltage on the tubes. if it is not balanced then there will be an audible hum. that balance pot is not on the blackface amps. you correct them by replacing the resistors in the filament voltage area with matched precision resistors and that will solve any filament hum issues in blackface amps.

Last edited by Bill Hatcher on 11 Mar 2017 6:26 pm; edited 9 times in total
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Los Angeles
Post Posted 11 Mar 2017 8:47 am     Reply with quote

Bill: "....a good place to start is 60MA on each side of a pair of 6l6s and 120MA for 4."

60ma PER TUBE? Are you sure about that? I'd think that they wouldn't be long for this world doing that. I'd go with 30 ma per tube, and be very careful once you start to get above 40 ma per tube.

Also, the bias control on this amp: I believe that the intent is to make the plate current draw the same for each side. That means that the grid bias voltage may very well not be the same per side, depending upon how (un)balanced your tubes are. Also, that also means that you may not have the optimum plate current draw either. But, the plate current will be the same, FWIW.

It's not perfect, but neither is the older bias method/circuit. It sets the bias grid voltage the same for all of the power tubes. If your tubes are not matched, they will be drawing different plate currents, even though their bias voltages are all the same.
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Bill Hatcher

Atlanta Ga. USA
Post Posted 11 Mar 2017 6:23 pm     Reply with quote

sorry about that.....doubled it.

yes to 30MA per tube. thanks for catching that.
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