AB763 w/bias shifting tremolo

[aabbs20]

Hey all,

I'm planning a single channel Deluxe Reverb AB763 type amp and I'd like it to have bias-shifting trem instead of optocoupler. I'm planning on using the 5G9 type since I'll have two triodes and it's my favorite trem. Has anyone done this? Is there anything I should be aware of while I'm drawing the layout/schematic?

[vampwizzard]

give this thread a look: https://el34world.com/Forum/index.php?topic=25535.0

[sluckey]

I'm planning a single channel Deluxe Reverb AB763 type amp and I'd like it to have bias-shifting trem instead of LFO.

All tremolo circuits use a LFO. Maybe you meant to say roach or Fender neon optocoupler?

Hoffman has exactly what you are looking for...

     https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf

[aabbs20]

Yes...you're correct, I meant optocoupler.

That looks great, I'll order up the board.

[Joel]

You won't be sorry.  It's a brilliant amp.  Paired up with a WGS ET65 or G12C... magic. 

One day I'll have my workshop built and sorted and I'll have the outdoor garden projects finished for the wife, then I'll be able to look into installing Sluckey's Trem-O-Nator mod.

[vampwizzard]

ET65 is my current favorite speaker. LOVE them.

[aabbs20]

I took a deeper look at the schematics tonight and it looks like there are a number of component differences between the 5G9 and Hoffman trem circuits. Are there advantages/disadvantages to either one? Is the Hoffman version preferable?
 

[sluckey]

Hoffman's tremolo circuit was taken from the 6G16 Vibroverb, not the 5G9.

[vampwizzard]

I took a deeper look at the schematics tonight and it looks like there are a number of component differences between the 5G9 and Hoffman trem circuits. Are there advantages/disadvantages to either one? Is the Hoffman version preferable?

two different beasts. The 5G9 is oscillating off straight B+ while the vibroverb is oscillating off the bias circuit. In an AB763 with a negative bias circuit,Doug's/Vibroverb would be my go-to.

[PRR]

> 6G16 Vibroverb, .... 5G9

[aabbs20]

Thanks guys, this is very helpful. It seems like 6G16 is the preferred method. Why though? More stability? Better sounding trem? It seems like it puts a higher voltage on the plates, could that be an issue?

two different beasts. The 5G9 is oscillating off straight B+ while the vibroverb is oscillating off the bias circuit. In an AB763 with a negative bias circuit,Doug's/Vibroverb would be my go-to.

Can you explain this in more detail? To me it looks like both are connected to the B+ supply and the bias circuit in a similar way.

[vampwizzard]

Rob Robinette explains the difference between the 5E3 and Blackface deluxe a lot better than I could.. find "Differences Between the 1950's Tweed and 1960's Blackface" https://robrobinette.com/How_The_AB763_Deluxe_Reverb_Works.htm

To answer your questions: Yes more stable, yes better sounding trem when youre pushing the output of the PI and output sections, and the higher plate voltage is part of the recipe there. Theres a reason Fender went that direction.

[sluckey]

Can you explain this in more detail? To me it looks like both are connected to the B+ supply and the bias circuit in a similar way.

They are. Both must have B+ to operate. Both modulate (vary) the fixed bias supply to create the effect.

Both circuits are almost identical, but with very slight differences, and some differences in component values. The 6G16 came along several years after the 5G9. Consider the vib circuit to be a slight upgrade. Study the pic that PRR posted to easily see the similarities and differences.

[aabbs20]

Of the bias modulating trems the 5G9 is my favorite so far, so if the 6G16 is a slight upgrade that will be very exciting.

Thanks all, I'll report back on how the build goes.

[aabbs20]

The build is complete and sounding amazing. The 6G16 tremolo is to die for too.

I've got the 47k resistor to ground to compensate for the lack of optocoupler tremolo but it still has a little more gain that I'd like. Is there a preferred way to ditch some of that?

Thanks all.

[sluckey]

I used a 100K master volume pot instead of the 47K. Let's me ditch as much as I want, or even boost the signal if desired.

[vampwizzard]

Sluckey: that’s the one complaint I’ve had with my build.. it got incredibly loud very quickly. Felt it was a feature and not a bug. It’s the loudest 6v6 amp I’ve ever played

[Bieworm]

Changed the opto trem of my vibro King to bias vary. It's perfect now...no trouble at all. And that's a 60W 6L6 amp!!!

[MWaldorf]

about 20 years ago, before I started futzing with amps myself, I had a tech install the 6G16 bias trem in my '67 Twin Reverb.  Not only is the tone great, but it doesn't reduce the average volume the way the shunt-to-ground optoresistor circuit does.