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Post subject: Twin Filter Cap Resistors?
Posted: Thu Oct 09, 2014 7:02 pm
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Does anyone have any thoughts on the effect of changing the (voltage divider resistors in the filter cap circuit) of a AB763 twin reverb to 2.2K and 10K as in the AC568, instead of the 1K and 4.7K that's in there.

The reason I ask, is about 15 yrs ago the filter cap circuit had a meltdown and the caps and resistors had to be replaced. I found out years later that the tech replaced the filter cap circuit resistors with the values of the AC568 circuit. I noticed it when I replaced some bad diodes and put the correct values back in. I was just curious what affect it would have had on the amp.
Bob


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Fri Oct 10, 2014 1:46 am
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Short answer;

With bigger resistor values you send less voltage to 12XX7 tubes = less gain = less power , but would you hear it ?


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Fri Oct 10, 2014 8:28 am
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I checked the bias after replacing the resistors & it was set to 22ma for the highest reading.
Assuming the plate voltage was 405V as the AA270 which used the same resistor values,
the output tubes would be at less than 9 watts if I'm calculating that correctly.
Seems way low to me.
I may try those values to see what it sounds like.


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Fri Oct 10, 2014 11:23 am
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Yes too cold at 22 ma . IF 405 V .

Better to take voltage reading on 6L6's tube with a meter than looking on schematic .

Voltage on schematic are for 110 volts wall outlets , today is 120 and more .

http://www.webervst.com/tubes1/calcbias.htm

Those resistors did not change voltage on output tubes ( read my first answer ) only on 12XX7's

You adjust bias with bias pot or by changing bias resistors value on bias power supply ,not main power supply resistors value .


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Fri Oct 10, 2014 6:50 pm
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Is there still a 27K on the 10K bias pot?


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Fri Oct 10, 2014 7:45 pm
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Yeah, the only resistors that were wrong were the 2.2K and 10K in the filter cap circuit.
Not sure why it was done but as stratele52 said less power because of the larger resistors.

I think the tech saw the silver faceplate and just assumed it was a later chassis when he replaced the resistors but it's really a crossover. It has the AB763 chassis. The faceplate has the black lines separating the channels.


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Fri Oct 10, 2014 10:55 pm
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Yeah, the two resistors, changed in the amp, look suspiciously like early blackface era setup for the PI and gain stages. Or maybe, transition or tweed era.


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Post subject: Re: Twin Filter Cap Resistors?
Posted: Sat Oct 11, 2014 2:07 am
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strat98 wrote:
Yeah, the only resistors that were wrong were the 2.2K and 10K in the filter cap circuit.
Not sure why it was done but as stratele52 said less power because of the larger resistors.
In theory only the 12XX7 tubes will have less gain so there will less power at output tubes. But would you hear it ?

I think the tech saw the silver faceplate and just assumed it was a later chassis when he replaced the resistors but it's really a crossover. It has the AB763 chassis. The faceplate has the black lines separating the channels.


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