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Post subject: Bias splitter resistors 1969 Dual Showman
Posted: Wed Nov 23, 2016 2:00 pm
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Hey! I'm overhauling my 1969 Dual Showman at the moment, and of course I want to make the bias balance into bias adjust. I have done this several times on other silverface Fenders, both by modding the bias balance to bias adjust and by adding a bias adjust to the balance.

My question here comes after looking at http://el34world.com/charts/bias_conversions.htm. There it says to change the value of the bias splitter resistors to 220k, 1/2w when doing the bias conversion. My Dual Showman has 68k's, which I gather is more suited to the 12at7 in the PI and the 47k resistor pair there. I know the 220k is usual in Blackface amps, and I have installed these 220ks in the past when blackfacing say a silverface Bassman, but then I also changed the PI resistors to the 82k/100k pair at the same time.

But in this instance I would like to keep the Dual Showman as original as possible, and just convert the bias balance to adjust, including getting rid of the 3,3k resistor and 50/70v cap between the 'back' of the bias pot and the bias board, and of course do the normal work like recap it and such.

And that got me thinking about if I should indeed keep the 68k's as bias splitters, or change them out with the 220k's. Would keeping the 68k's cause a problem with biasing, or other stuff? And if I should swap them out with 220k's, should I also change the PI 47k's with 82k/100k, or 100k/100k. Because shouldn't the bias splitter resistors be about double that of the PI resistors?

The PI resitor values have been discussed a lot, but I have not seen much talk about the values of the bias splitter resistors, 68k - 100k - 200k, and what the changing of these values do to the rest of the circuit. Does anybody have some insight or thoughts about this?

Thanks a lot!
P


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Post subject: Re: Bias splitter resistors 1969 Dual Showman
Posted: Wed Nov 23, 2016 10:02 pm
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If you're dealing with an AC568 or newer circuit revision you'd be well served to ditch the entire mess and convert everything back to the original AB763 specs, with a true bias-adjust pot and the resistor values that belong in that circuit. That's 220K's for the V5 through V8 bias splitters and the asymmetric 82K/100K pair for the phase inverter. While you're at it, deep-six the tone-robbing snubber caps on the power-tube sockets. That's what I did to my late '68 Showman and it resulted in a significant sonic improvement.

Arjay

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Post subject: Re: Bias splitter resistors 1969 Dual Showman
Posted: Thu Nov 24, 2016 1:35 am
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Retroverbial wrote:
If you're dealing with an AC568 or newer circuit revision you'd be well served to ditch the entire mess and convert everything back to the original AB763 specs, with a true bias-adjust pot and the resistor values that belong in that circuit. That's 220K's for the V5 through V8 bias splitters and the asymmetric 82K/100K pair for the phase inverter. While you're at it, deep-six the tone-robbing snubber caps on the power-tube sockets. That's what I did to my late '68 Showman and it resulted in a significant sonic improvement.

Arjay


This is what to do. Go completely back to AB763 BIAS pot circuitry. Do NOT do a hybrid circuit of half BALANCE and half BIAS.

If you want to have the BALANCE (or hum) pot, you need to add another 10k pot to one leg of the BIAS supply to the push-pull pair of 6L6GC tubes. Easier, just to buy matched quads of 6L6GCs.


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Post subject: Re: Bias splitter resistors 1969 Dual Showman
Posted: Thu Nov 24, 2016 1:42 am
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BMW2002Ti wrote:
Easier, just to buy matched quads of 6L6GCs.


+1

I've got a matched quartet of JAN Sylvania's in mine and the amp is dead-quiet at idle.

Arjay

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Post subject: Re: Bias splitter resistors 1969 Dual Showman
Posted: Sun Nov 27, 2016 10:10 am
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One other reason for the 220K is safety.
If a power tube shorts to the grid, it can load down the entire bias circuit, damaging the remaining tubes.
The 220K provides enough isolation to protect the remaining tubes.


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Post subject: Re: Bias splitter resistors 1969 Dual Showman
Posted: Tue Dec 06, 2016 8:14 am
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Thanks a lot for all the input guys! After reading your thoughts about this, I've decided to just convert it all to AB763.. Can't wait to hear what it sounds like when I'm done!


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Post subject: Re: Bias splitter resistors 1969 Dual Showman
Posted: Tue Dec 06, 2016 9:58 am
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A wise course of action.

As for sonic results, expect extremely loud and extremely clean tones that were hallmarks of the Dual Showman (and Twin Reverb). If compression, dirt, and OD are desired, you'll need to augment the amp with pedals.

Arjay

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