Author Topic: Fender bassman 300 Pro Bias question Need to know if it is safe to use  (Read 232 times)

Nighttrain1127

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The Manual and the back panel say to set the bias at 100mV and the Balance at 0mV. So Go to set the Bias and it comes up -100mV. Is it the same either way? Or Is there a Problem? Also it does not say But I am guessing the center connection that is used for both settings is common?

Any Help would be appreciated

Nighttrain1127

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  So I guess it is just a matter of putting the probes into the opposite socket to get the right answer.

Psycho Bass Guy

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Bias is a negative voltage. You WANT a negative reading.

amptech

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Bias is a negative voltage. You WANT a negative reading.

Does these amps actually have a test/set point where you actually measure the bias voltage?
I would think (not familiar with the amp, though) the test points are cathode current.

-100mV of bias voltage just seems a tad too hot :)
 

Psycho Bass Guy

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Does these amps actually have a test/set point where you actually measure the bias voltage?
I would think (not familiar with the amp, though) the test points are cathode current.

-100mV of bias voltage just seems a tad too hot :)
 
:o Yep!
It's a built-in test point. I completely missed the "mv" and just read "voltage." A cathode reading should be in current, but who knows what amp manuals say nowadays. I had the amp in question's identical-other-than-cosmetics predecessor, the Sunn 300T, and was never impressed with it so I never dug into it, but from what I remember of its bias monitor circuitry, it was a joke and IIRC caused many output stage failures. I do remember having two of them blow up on me when I was trying them out NEW in stores! (Maybe they were biased in mv, LOL)

Nighttrain1127

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 What would you say the bias voltage should be set at .  I don't really want to cook the tubes that I just got. Any suggestions?  It does seem to be running pretty hot right now.

Nighttrain1127

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OK just for general Knowledge Fender Now recommends Biasing the 300PRO the same as the Super Bassman 300 at 80mV.  Thanks for the information that led me to search out the answer Guys.

amptech

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There you go then, your amp is biased by monitoring cathode current. You actually measure current, by taking the read across a resistor between the catode and ground. This resistor 'translates' your mV reading so to speak, it's a terrific and easy way to monitor bias in theory.

Unfortunately, as PBG said, many things can go wrong when you monitor current constantly - amp manufacturers stick too many components between cathode and ground, somewhere on the PCB, and they increase the number of things that can go wrong.
Of course, all sorts of thing can go wrong in an amplifier, but any design feature that can lead to your amp losing bias should be avoided. I've had quite  a few amps on the bench the last years with these kind of issues.   

Nighttrain1127

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You guys are very helpful. It is nice to have a different Forum to ask questions and get straight answers .