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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Tue Oct 01, 2019 9:31 pm
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Retroverbial wrote:
My Bassman Ten schematic shows a traditional bias-supply pot, with 430 VAC to the plates and -45 VDC for the bias.

Arjay


Arjay, you are correct. I found that the AA371 circuit does have a BIAS pot. Not BALANCE. The OP will need to find out which circuit model he has.

http://ampwares.com/schematics/bassman_aa371.pdf


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Wed Oct 02, 2019 8:34 am
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Damn it! Looking more closely at the AA371 schematic --- it's a hum pot.

To OP: check your circuit. Good luck with the fix.

:mrgreen:


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Wed Oct 02, 2019 11:03 am
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BMW2002Ti wrote:
Damn it! Looking more closely at the AA371 schematic --- it's a hum pot.



:mrgreen:


No it is a balance pot . :lol:

Fender Basman amps ;

http://ampwares.com/amps/fender/bassman/


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Wed Oct 02, 2019 6:04 pm
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Isn't a hum pot = balance pot? I just tweak the sucker to the lowest hum, at idle. Measure idle bias current, to be sure the output tubes are within 60-70% of max range.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Wed Oct 02, 2019 7:44 pm
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BMW2002Ti wrote:
Isn't a hum pot = balance pot? I just tweak the sucker to the lowest hum, at idle. Measure idle bias current, to be sure the output tubes are within 60-70% of max range.
+1 biggest factors are how closely matched the tube pair is and how consistent the cathode voltage is.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Thu Oct 03, 2019 2:39 am
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sfceric64 wrote:
BMW2002Ti wrote:
Isn't a hum pot = balance pot? I just tweak the sucker to the lowest hum, at idle. Measure idle bias current, to be sure the output tubes are within 60-70% of max range.
+1 biggest factors are how closely matched the tube pair is and how consistent the cathode voltage is.


Hum balance pot are on the tubes heater; this balance pot are not there.

With bias balance pot, we don't need matched tubes. That is why a balance ; CBS save some money on tubes ....

For bias adjustment ( if needed )you must change some resistor values ; see a amp tech.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Sun Oct 06, 2019 10:09 pm
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Stratele, I don't think the '74 Bassman has that hum pot on the heater lines. In any event, I adjust the balance pot to get the least hum, at idle. That way I know the two output tubes are close to symmetric in push-pull output.

http://ampwares.com/schematics/bassman_aa371.pdf


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Oct 07, 2019 1:37 am
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BMW2002Ti wrote:
In any event, I adjust the balance pot to get the least hum, at idle. That way I know the two output tubes are close to symmetric in push-pull output.

http://ampwares.com/schematics/bassman_aa371.pdf


Doing that, your output tubes ( may?) have same bias ( they are balanced ). Are the bias is the good one ? You don't know.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Oct 07, 2019 2:11 am
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I run a bias meter, after adjusting to lowest hum. With new tubes, it's usually within 60-70% range. Unfortunately, you need to juggle resistors to change the bias --- in these amp with balance pots.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Oct 07, 2019 2:14 am
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BMW2002Ti wrote:
I run a bias meter, after adjusting to lowest hum. With new tubes, it's usually within 60-70% range. Unfortunately, you need to juggle resistors to change the bias --- in these amp with balance pots.



" Unfortunately, you need to juggle resistors to change the bias. "

Right !

Any tech can add a bias pot and keep balance pot.
Or convert balance pot to bias pot.
I'm sure you know.


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