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Post subject: Fender Twin hum
Posted: Wed Sep 28, 2011 8:15 pm
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I recently developed a hum in my 70`s Twin Reverb. Can anyone tell me if this could be a result of replacing the 6L6 tubes in the incorrect sequence. I removed the tubes to service the chasis,when it was reassembled I have a hum now. Any advice would be appreciated.

Many thanks
John


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Post subject: Re: Fender Twin hum
Posted: Wed Sep 28, 2011 10:36 pm
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Does the amp have a "hum balance" control on the rear panel?

Arjay

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Post subject: Re: Fender Twin hum
Posted: Wed Sep 28, 2011 10:36 pm
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I believe this year Twin Reverb had hum (or balance) pot adjustment instead of bias pot. The idea to to have both top and bottom of the resultant sine wave as symetric as possible. Thus eliminating AC noise riding on top of the signal. You'll need to turn the pot so that you have the least hum, coming from the speakers.

You should have started with a matched quad of 6L6GC. Measure the idle bias. But, to adjust, you need to alter fixed resistors or put a bias pot into the circuit. Here's one way to do it:


http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm


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Post subject: Re: Fender Twin hum
Posted: Wed Sep 28, 2011 10:50 pm
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BMW2002Ti wrote:
Measure the idle bias.


The plate voltage needs to be ascertained as well.

The ultra-linear amps use a fixed bias but with an external "output tubes matching" pot on the rear panel (a la the AB165 Bassman), which works in conjunction with the hum balance control.

Arjay

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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 3:30 am
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Hello
Many thanks gentlemen for the informative lesson, on how to eliminate the hum. When I replaced the 4 6L6 tubes on my amp ,it was with the original ones that were already in there. My question is this, does the replacement of the tubes back in the chasis have to be the way they were installed originally. I know that the tubes are all the same model # , but does changing them around affect the biasing.

John


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 7:27 am
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If you DO NOT want to touch the balance or hum pot... yes try to put the tubes back in the same sockets you removed them. But, even then, you still may need to tweak the pot to get the lowest hum---as it's not unusual for electrical properties to change ever so slightly (due to slightly different contact with the socket, for instance)---everytime you pull and replace a tube.

Good luck, with that!


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 8:05 am
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Hello
Once again thanks for the advice.

John


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 5:50 pm
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BMW2002Ti wrote:
If you DO NOT want to touch the balance or hum pot... yes try to put the tubes back in the same sockets you removed them. But, even then, you still may need to tweak the pot to get the lowest hum---as it's not unusual for electrical properties to change ever so slightly (due to slightly different contact with the socket, for instance)---everytime you pull and replace a tube.


+1

On the rare occasion that a chassis's tubes need to be pulled, I mentally assign each tube and its respective socket an "A", "B", "C", etc and when the bottles are reinstalled, they all go back to precisely whence they came.

Arjay

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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 6:01 pm
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Hello Arjay
You learn something every day.When I removed the small tubes I marked their positions for the eventual replacement. The 4 6L6s were all the same, so I never thought that it would make a differance. Just goes to show you.

John


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 6:06 pm
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Each tube (even those as part of a so-called "matched set") has its own personality. Thus, there are often minor performance-parameter differences. If a given amp was biased with the tubes in a specific sequence, then it only makes sense to ensure they're all reinstalled in that same order.

HTH

Arjay

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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 6:16 pm
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Hello Arjay
Can you re-bias tubes that have been replaced,and would tubes that need biasing cause a hum in the amp.

John


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 6:26 pm
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"Yes" and "maybe".

A new idle-bias adjustment is always possible but the hum may prove more problematic. It could be any number of issues causing the hum.

Arjay

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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 6:31 pm
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Hello Arjay
I am going to hand the amp in to get it serviced. As this is a new aquisition, I figure I better get it checked out, and see if the caps need replaced and a general diagnosis. The amp is from 1976, so I figure it is about due. Thanks for all your advice.

John


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 7:05 pm
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John, just as a reminder, you need to buy a good matched QUAD of power tubes. Whether you keep the hum pot alone or add a single bias pot to the circuit---the balance between the push-pull partners (two pairs of 6L6GC) helps to eliminate power supply hum.

And yes, the power supply and bias supply electrolytic filter caps should prolly be replaced, if they are more than 15 years old. I usually replace the bypass caps in the gain stage and phase inverter stage, too. As they are prolly the same age as the main filter caps. You'll get better headroom and dynamics with new bypass caps.


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Post subject: Re: Fender Twin hum
Posted: Thu Sep 29, 2011 8:40 pm
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I think you've made a wise decision, John. Scrimp on nothing during the overhaul and you'll have an heirloom-quality amp that will last for many more decades.

Rawk on!

8)

Arjay

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