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Post subject: Hum Balance Pot?
Posted: Thu May 22, 2014 2:56 pm
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I was checking the bias on my '59 Bassman Ltd amp today, and a friend was by while I had the amp opened up, and he asked an interesting question, does your amp have a hum balance pot? I told him that I didn't know so we have a look around, but didn't see anything that could be it.

So my question to you guys is does this amp have a hum balance pot? and if so, where exactly will I find it?

The reason I ask is not so much that my friend brought it up, but mainly because my amp does have a noticeable amount of low hum.

Thanks in advance guys for any help you can give.


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Post subject: Re: Hum Balance Pot?
Posted: Thu May 22, 2014 3:18 pm
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The re-issue 5F6-A tweed Bassman LTD has an idle-bias adjustment pot (something the early non-LTD's lacked). There is no "hum balance" control. These amps are exceptionally quiet so if audible noise is present you may have a tube or filter-cap problem.

Arjay

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Post subject: Re: Hum Balance Pot?
Posted: Thu May 22, 2014 3:25 pm
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Thanks Arjay, would that be the preamp tubes? Right now I have three reissue Mullard 12AX7's in V1, V2, and V3.


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Post subject: Re: Hum Balance Pot?
Posted: Thu May 22, 2014 4:13 pm
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Difficult to diagnose via keyboard. You could try some creative tube swapping using new bottles and see if the symptoms persist. As an aside, you might think about exchanging the 12AX7 in the V1 position for a 5751 or a 12AY7. That will have the effect of smoothing out the first gain stage and fattening up the mids.

Arjay

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Post subject: Re: Hum Balance Pot?
Posted: Thu May 22, 2014 6:42 pm
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Cool, thanks Arjay, I will definitely try these suggestions.


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Post subject: Re: Hum Balance Pot?
Posted: Fri May 23, 2014 12:08 am
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If "no joy" is observed it's probably time to take a look at the filter caps. Use caution and if you're in any way uncomfortable poking around inside a chassis, seek professional counsel.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Hum Balance Pot?
Posted: Fri May 23, 2014 2:31 am
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Relic G Man wrote:

The reason I ask is not so much that my friend brought it up, but mainly because my amp does have a noticeable amount of low hum.



If same hum at any volume ; check ;

Preamp tube, non-matched Output tubes, (one is very weak)
Wire dress ; moving wire may do some hum .
Bias filter capacitor ,
Power Supply filter capacitor

If hum change with volume ; check circuits before volume;
Tubes, filter caps, input jacks ground contact , solder, wire dress


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Post subject: Re: Hum Balance Pot?
Posted: Fri May 23, 2014 8:07 pm
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Glad to report all is good with my Bassman, I put a couple of new 12AX7 tubes in V1 and V2 and it has quieted down significantly.

Thanks for your time and help guys, very much appreciated.


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Post subject: Re: Hum Balance Pot?
Posted: Fri May 23, 2014 9:06 pm
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Smooth!

I had a 12AX7A go south in my '64 Vibroverb today -- an NOS RCA that turned microphonic.

Almost broke my heart to shitcan it!

:cry:

Replaced it with one of those new Gold Lion Genelex tubes that Cedarblues gave me. Sounds mighty tasty.

Rawk on, RGM!

8)

Arjay

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