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Post subject: '79 Ultra Linear hum/matching adjustments
Posted: Tue Jan 28, 2014 4:09 pm
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Aspiring Musician
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I need to ask some advice on the hum balance and output tube matching adjustments on this '79 135w Twin. I'm assuming (I hate that word) from what I've read on the forum that the hum balance adjustment helps to minimize the hum between the two parallel sets of power tubes and the the output tube matching sounds like it finds a happy medium between the two parallels.

I just did a cap job on this amp, voltages are fine and was going to use my bias meter on it to balance things out but figured I'd ask on the forum first to make sure I have the right idea. First time for me on an amp without a bias pot.

Thanks for any help.


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Post subject: Re: '79 Ultra Linear hum/matching adjustments
Posted: Tue Jan 28, 2014 6:36 pm
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Rock Star
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Joined: Sat Jul 03, 2010 2:18 pm
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This hum pot works best with a match set of output tubes. Elimination of noise from the signal, with this setup, is based on the function of the push-pull topology.


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Post subject: Re: '79 Ultra Linear hum/matching adjustments
Posted: Wed Jan 29, 2014 5:02 am
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
Hum balance pot is for removing the hum coming from the Power Transformer heater circuit .

Nothing to do with output power tubes . Unmatched power tube may do some hum too


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Post subject: Re: '79 Ultra Linear hum/matching adjustments
Posted: Thu Jan 30, 2014 7:01 am
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Joined: Tue Jun 29, 2010 1:20 pm
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Location: Indiana
This particular amp has a "Output Tubes Matching" pot, for the output tubes, and a hum pot on the heaters' circuit, labeled "Hum Balance". The former is to balance the current draw of the two halves of the push-pull output, and the latter for any hum from the heater circuit.

Leave it to CBS to misname the hum adjust pot on the heater circuit and call it a "Hum Balance" pot, as it has nothing to do with any kind of balance at all, it's just to reduce noise/hum from the AC heater circuit. Typically in Fender amps, the "Hum Balance" pot is to balance the output current draw between halves, and the "Hum Adjust" pot is for the heater circuit.

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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