Hoffman AB763 vs Leo Fenders

[etneccas]

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

[pompeiisneaks]

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

[etneccas]

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?

[sluckey]

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.

[pompeiisneaks]

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

[sluckey]

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.

[etneccas]

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?

[etneccas]

Nevermind I found the answer in the BOM the Deluxe Reverb uses a 47 ohm......
Thanks for the clarification on the tremolo circuit

[sluckey]

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.