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Post subject: bias or balance pot
Posted: Thu Oct 20, 2011 8:53 am
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I have a aa1069 circuit how do I tell if it has a balance or bias pot. I assume being it is a aa1069 and fender changed back to the bias pot, it would have a bias pot, but assuming is not a good thing


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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 9:19 am
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The AA1069 revision uses a balance pot in the biasing circuit, at least in the Super Reverb and Bandmaster Reverb. If you compare the schematics for this variant of the Super Reverb and the original AB763 layout, the differences will be immediately apparent.

HTH

Arjay

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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 11:31 am
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Balance pot have 2 wires going to the eyelet board. One for each 6L6 . Bias pot only one.


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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 11:55 am
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Thanks I will look to see if I have 1 or 2 wires connected when i get home


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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 12:05 pm
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Like Retroverbial write aa1069 have a balance pot.

You can instal a bias pot just before the balance pot so you will have the best of both world.


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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 1:49 pm
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If you buy balanced output tubes, the balance pot is superfluous and can be altered to a traditional bias-adjust configuration. Thus, no additional holes will need to be drilled into the chassis.

JMOOC

Arjay

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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 2:05 pm
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Right Retroverbial.

That's is the best advice for most of the people.

But It seems that perfectly matched tubes set could be less musical than tubes set with 3 to 8 milliamps difference.

I ordered unmatched tubes sometimes and I find they are "musicaly" matched.

Also with time tubes milliamps could change and with a balance you could readjust them matched.

But I won't suggest to pay to add a bias pot on a balance pot tube amps. And never drill a hole in a vintage amp. Surface mount pot is easy to install.


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Post subject: Re: bias or balance pot
Posted: Thu Oct 20, 2011 4:14 pm
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stratele52 wrote:
Surface mount pot is easy to install.


+1

Arjay

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Post subject: Re: bias or balance pot
Posted: Wed Oct 26, 2011 5:21 pm
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Would any of you guys have a picture showing the difference between the bias and balance pot? Always been kind of curious about the biasing.

Thanks.

T2

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Post subject: Re: bias or balance pot
Posted: Thu Oct 27, 2011 1:11 am
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Look at these layout ;

First one is a 1069 circuit with balance pot;
http://www.el34world.com/charts/Schemat ... AA1069.pdf

This link is for bias pot layout ;
http://www.el34world.com/charts/Schemat ... _schem.pdf
_________________________________________________________________________________________
The idea is to put the bias pot before your balance pot. You tune the right bias voltage with bias pot and send it as tube needed ( balance ) with balance pot.

Wiring ; the single wire from the bias board should go to the "input" of bias pot ( like aa165 ) and the "output" of bias pot goes to the ( single wire ) " input " of your balance pot. The 2 wires at the balance "output" stay as it.

Bias pot is very easy to find, but balance pot , I don't think so. You are lucky to have a balance pot.
You can also connect your balance pot like a bias pot ; put the wire at same location on pot.


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Post subject: Re: bias or balance pot
Posted: Mon Dec 05, 2011 9:57 pm
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Hi,

Sorry to hijack this thread but I have a very similar question regarding a fender bassman 50 (~1977 I think).

It has a bias balance pot which, when turned takes the bias current of one tube down while taking the bias current of the other tube up.

The circuit (from what I can tell) looks like this,

Image

link : http://www.imgplace.com/viewimg819/8338/22img0011.jpg

Which looks very similar to balance pot layout posted above for the pro reverb aa1069 schematic.

I would like to add an additional bias adjust pot.

To do this do I just add an aditional pot between the bias board and the "input" single wire of the existing balance pot, as stated below?
If so what happens to the cap between the bias board and input of the balance pot and ground? and do I keep the 82 k resister in there before the cap?

Is a pot like this suitable?

http://www.ebay.com.au/itm/CTS-10k-Bias ... 43aaee22ca

Also Is there an issue with mounting the pot on the rear panel of the amp (where the ground switch was, now its no longer used)? Just wanted to make sure in terms of hum/noise?

Thanks for any suggestions
Chris


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Post subject: Re: bias or balance pot
Posted: Tue Dec 06, 2011 2:39 am
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Great idea.

Your schematic is this one ;

http://www.el34world.com/charts/Schemat ... MAN_70.pdf

Follow ther schematic for a bias pot layout I write some answer before.
Put this pot between your capacitor and your balance pot.
The resistor ( 82 K ) is very big , not like the schematic Bassman 70 which have a 2.2 K . Are you sure ?.You could need to replace this 82 K by a 2.2 k.

Before the mod ;
1- Mesure how much voltage you have on power tube's pin 5.

After you do your wiring, do a test .
1- Remove all your power tubes and mesure how much negative bias voltage you have on each 6L6 's pin 5 and if voltage change with both pot and if you can have the voltage BEFORE the mod.

Don't put 6L6's if you don't have a negative bias voltage at pins 5 closer than your's before mod.

You can put the pot anywhere you want ; no noise problem.
The pot you want to buy is good. My self I use cernet multitour pot, for easyer adjustment but it do not have a "vintage "look


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Post subject: Re: bias or balance pot
Posted: Tue Dec 06, 2011 4:55 am
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Thanks a lot for that stratele52, very helpful!

Sorry, I checked the resistor value when I got home and it is 3.3k.
A lot closer to what you thought, this should be ok to leave in correct?

I will not be able to do this until the pot arrives in a few days.

Ok I will check the voltage on pin 5 before and after mod to ensure I don't blow anything.

Thanks again
Chris


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Post subject: Re: bias or balance pot
Posted: Wed Dec 07, 2011 1:48 pm
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Stay with the 3.3 K it is good.

Give us some news Chris


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Post subject: Re: bias or balance pot
Posted: Thu Dec 08, 2011 9:51 pm
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ok great. Just waiting for the pot to arrive in the mail. Not sure when this will be but will keep you posted.

Cheers for the help

Chris


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