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FDP Forum / Fender Amps: Vintage (before 1985) / 56K or 100K Resistor Value

roadhog96

USA / Sesame St.

Apr 25th, 2017 08:08 PM   Edit   Profile   Print Topic   Search Topic

Do you guys use a 56K or 100k next to the last single 25uF 25V cathode bypass capacitor on the AB763 circuit?

The AB763 layout for a Deluxe Reverb shows a 56K in that spot. All other Blackface AB763 show a 100K. This 67 SF Deluxe Reverb and it has 100K Carbon Composite and looks identical to all the other CC 100K resistors that were in this amp. So I dont know if its iriginal or someone replaced the 56K with an 100K.

(This message was last edited by roadhog96 at 10:28 PM, Apr 25th, 2017)

amphead4

Cincinnati, USA

Apr 26th, 2017 11:44 AM   Edit   Profile   Print Topic   Search Topic

I just checked a few schematics and 56K was the AA763 value and 100K is the AB763 value. Your 100K is consistent with the AB763 schematic I looked at.

I would just leave it alone.

roadhog96

USA / Sesame St.

Apr 27th, 2017 07:13 AM   Edit   Profile   Print Topic   Search Topic

Amphead4, I'm not doubting what you said but I checked the AA763 myself and your right that Circuit did use a 56K but whats confusing is I have a AB763 layout that shows it with with 56K and I also have one that shows it with a100K. See the attached AB763 layout. I belive the 100K is the correct value, just don't know why this AB763 56K layout exist.

AB763 with 56K

amphead4

Cincinnati, USA

Apr 27th, 2017 08:25 AM   Edit   Profile   Print Topic   Search Topic

Seems they transitioned from 56K to 100K when they went from AA to AB.

100K would set the bias point higher on the stage (makes grid more negative relative to cathode). This stage drives the neon light in the trem bug.

This could have been an effort to make the trem output more of a sine wave instead of a square wave.

I've never bother changing these and always thought the square wave sound of these was due to the inability of the neon light to dim smoothly. As the voltage ramps up, the dark neon will finally fire into life, causing a step change in the resistance of the LDR. Not the smoothest transition.

If you are into this, you could give it a listen and then jumper another 100K in parallel with the first one to see if it sounds better or worse.

But for all we know, it could have been a change to help with tube life and you are better off with 100K because it lowers the plate current through that triode.

FDP Forum / Fender Amps: Vintage (before 1985) / 56K or 100K Resistor Value




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