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Thread: ab763 vs ab868 power transformer question

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    ab763 vs ab868 power transformer question

    Hi, I have a 1967 Blackface deluxe reverb AB763. The power transformer is marked F025130 606-9-15. I assume it is not original. The transformer has only one wire going to ground. I notice in the layout that ab763 power transformers have 2 wires going to ground. I see that ab763 and ab868 use the same transformer number but on the layouts ab763 has 11 wires with 2 wires to ground and ab868 has 10 wires with one going to ground. Are the 11 wire versions of the 025130 really interchangeable with the 10 wire versions? Is there a difference in their function? Is it safe to use the 10 wire version 025130 on the ab763? Does the "F" that is stamped before the 025130 have any meaning? I really appreciate any input on this matter. Thanks, Bob

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    Re: ab763 vs ab868 power transformer question

    It looks like the AB763 revision -- with its GZ34 rectifier -- had a center-tap ground for the heater circuit. This seems to have been unnecessary in the AB868 circuit, with its directly-heated 5U4GB.
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    Re: ab763 vs ab868 power transformer question

    I studied the schematics and learned what many of you already know. The ab763 had a center tap for the 6.3 going to the pilot light. Later they removed the center tap but added 2 100ohm resistors going from the 2 tabs on the pilot light to ground. After adding these resistors the amp is silent as opposed to having the awful hum that it had before I added the 2 resistors.

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    Re: ab763 vs ab868 power transformer question

    In later years, Fender used a pot in place of the 2 resistors, and labeled it a "hum balance".

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    Re: ab763 vs ab868 power transformer question

    Quote Originally Posted by Bill Moore View Post
    In later years, Fender used a pot in place of the 2 resistors, and labeled it a "hum balance".
    Correct, but only in the larger amps.

    The DR was never equipped with the hum-balance pot.
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    Re: ab763 vs ab868 power transformer question

    Quote Originally Posted by phantomman View Post
    Correct, but only in the larger amps.

    The DR was never equipped with the hum-balance pot.
    My weird-ass Super Twin Reverb has both a hum-balance pot and an output tubes matching pot, both accessible on the rear panel. There is no bias adjustment pot inside the chassis.

    I'm not sure whether the pot on my 1969 Super Reverb is a bias adjustment or output tube matching pot?
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    Re: ab763 vs ab868 power transformer question

    The 79 Twin I had was equipped with both as well, I see on the forums that many add a bias pot also.
    The theory for the tube matching adjustment was tubes weren't as close to spec as they had been, and it was a way to get voltages closer.

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    Forum Member Michael Smith's Avatar
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    Re: ab763 vs ab868 power transformer question

    Quote Originally Posted by Bill Moore View Post
    The 79 Twin I had was equipped with both as well, I see on the forums that many add a bias pot also.
    The theory for the tube matching adjustment was tubes weren't as close to spec as they had been, and it was a way to get voltages closer.

    I've seen a few sites that have tutorials on adding a bias pot, and have printed them and saved them on my computer. The next time I have to go into my Super Reverb, Vibro Champ or Bassman for servicing, I may add a bias pot. I haven't seen anyone specify the wattage rating for the bias pot, and the ones I have seen for sale seem to have very low wattage ratings.
    "Do she still have her fightin' gear, just one look is gonna make it clear" ZZ Top

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    Re: ab763 vs ab868 power transformer question

    Quote Originally Posted by Michael Smith View Post
    I haven't seen anyone specify the wattage rating for the bias pot, and the ones I have seen for sale seem to have very low wattage ratings.
    Keep in mind, the bias circuit deals with currents in the milliamp range and relatively low voltage, typically below 55 VDC. The pot's power rating needn't be large.
    "When injustice becomes law then rebellion becomes duty."

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    Forum Member Michael Smith's Avatar
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    Re: ab763 vs ab868 power transformer question

    Quote Originally Posted by phantomman View Post
    Keep in mind, the bias circuit deals with currents in the milliamp range and relatively low voltage, typically below 55 VDC. The pot's power rating needn't be large.
    I think that's true for "fixed" biased amps, but in a cathode biased amp (like the Vibro Champ), it came with a 470 Ohm 2 Watt bias resistor. The 6V6 was running at 130% idle plate dissipation, so I replaced the resistor with a 1k Ohm 5 watt metal film, and my plate dissipation is now around 103%. (I also installed a new bias cap.) The new resistor gets very hot to the touch. I would like to install a 500 ohm bias pot in series with the 1k Ohm resistor to get a little more control, but since the bias resistor gets so hot, that's why I was worried about the power rating of a bias pot.

    "Do she still have her fightin' gear, just one look is gonna make it clear" ZZ Top

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