Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: etneccas on November 27, 2018, 07:35:30 pm
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Looking for clarification on Deluxe Reverb AB763 The phase inverter V5-3 on Fender has
a 47ohm resistor going to ground
Hoffman’s shows a 4.7k in parallel with a .22 electrolytic going to ground
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Not sure i'm following:
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf
and https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf
both show a 47 ohm to ground after the typical identical PI setups. What are you referring to?
~Phil
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The second link you posted shows the 4.7k red and the.22cap ?.....
that’s whats confusing me
Where did you reference the Hoffman schematic with the 47ohm to ground?
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First, V5 is not the PI tube. It is the tremolo tube. And Hoffman's trem circuit is not the same as the AB763 trem circuit. Neither circuit has a 47Ω resistor in it. The AB763 Deluxe has a 2.7K and 25µF from v5-3 to ground. Hoffman's circuit has a 4.7K and 22µF from V5-3 to ground.
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First, V5 is not the PI tube. It is the tremolo tube. And Hoffman's trem circuit is not the same as the AB763 trem circuit. Neither circuit has a 47Ω resistor in it. The AB763 Deluxe has a 2.7K and 25µF from v5-3 to ground. Hoffman's circuit has a 4.7K and 22µF from V5-3 to ground.
Oh yeah didn't even note that, I kept 'Phase inverter' in head and was trying to understand what was the reference. So yes, from what sluckey shows, there's a slightly lower capacitance for the cathode cap, but nothing too major. as for the 4.7k it's probably to bias that tube a bit cooler, so it's got more clean output, but that's just a guess. I'd wager Doug made that change from personal testing and he preferred it that way. You can easily swap back to fender stock for a few dollars if you want.
~Phil
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Read this... https://el34world.com/Hoffman/AB763Notes.htm
Hoffman's AB763 does not use the optocoupler trem circuit found in a real AB763 amp. He uses a bias vary trem circuit from the 6G16 Vibroverb amp.
https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_6g16_schem.pdf
That accounts for all the differences in the V5 circuit.
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My bad ...I could still use some clafication as to why Doug has R45 a 100 ohm resistor instead of a 40 ohm at V6?
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Nevermind I found the answer in the BOM the Deluxe Reverb uses a 47 ohm......
Thanks for the clarification on the tremolo circuit
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Read this... https://el34world.com/Hoffman/AB763Notes.htm
Then look at the BOM.
Nobody uses a 40Ω resistor. Leo used a 47Ω when the NFB came from the 8Ω tap on the OT. He used a 100Ω when the NFB came from a 4Ω or 2Ω OT tap. Hoffman is just following Leo's lead.