Mod for Hoffman AB763 equivalent to pulling V1 on original AB763

[osing]

Hi,

On the original AB763 circuits, the cathode resistor is shared for V1B and V2B (pins 8 on both V1 and V2 are connected and go through an 820 Ohm resistor). A popular mod on these amps appears to be pulling V1, which supposedly gives the Vibrato channel a stronger signal which will then "push the second gain stage (V4 tube) harder and give you increased sustain, compression and harmomics" (from fenderguru.com). I read on a different forum (http://www.tdpri.com/forum/amp-central-station/638-drri-v1-tube-pullout.html) that effectively, by eliminating V1, the current going through the 820 Ohm resistor is reduced by 50%, and thus the voltage drop over it is effectively reduced by 50% as well. In looking at an original AB763 schematic, the voltage at the cathode of V2B is 2.1V, so presumably it would then drop to around 1V when V1 is pulled?

I'd be interested in adding a switch to my Hoffman AB763 Super Reverb that would allow me to accomplish the same thing as removing V1 on an original AB763. Since the Hoffman circuit has separate cathode resistors for V1B and V2B, my first guess was that I will simply need to add a switch that will add/remove one  resistor from a parallel pair on V2B.

Now, I did some searching and it appears that bnwitt did something similar on his Vibroverb project. From his thread:

http://www.el34world.com/Forum/index.php?topic=3238.0

it appears that his mod affects the cathode resistors on both V2A and V2B, and his mods match those found on the Fender Custom Vibroverb (the one with the Diaz mods). Looking at the schematic for the Custom Vibroverb

http://www.prowessamplifiers.com/schematics/images/64%20Vibroverb%20Custom%20Schematic.pdf)

the mod mode shows a cathode voltage of 2.70V (resistor value 2.7K) versus 2.06V for the stock mode (effective resistor value ~1.5K for the 3.3K and 2.7K parallel resistors).

So, now I am a bit confused (nothing new there).

1) On an original AB763 with V1 pulled, is the current really being reduced by 50% and thus the cathode voltage for V2B is also reduced by 50% to around 1V? If so, is the Custom Vibroverb mod that disables the Normal channel something different altogether since that seems to increase the cathode voltage?

2) If I truly just want a switchable mod that replicates the removal of V1, do I want the 3.3K & 2.7K resistor pair on V2B that nearly doubles the value from the stock setting (i.e. 2.7K vs 1.5K), or do I actually want a pair that would reduce the resistance by ~50% (perhaps two 1.5Ks in parallel)?

3) The Diaz Custom Vibroverb mod seems to also have and added capacitor in parallel to the 2.7K resistor (0.68uF) on V2B, and as bnwitt explains in his thread, in mod mode the effective values are thus 2.7K and 0.68uF versus ~1.5K and 22.68uF in stock. What effect/impact does this added capacitor have?

Thanks!

[EL34]

If you pull V1 in a fender, you turn off the non vibrato channel pre amp tube
It will raised the voltages in the pre-amp a bit because you have removed some of the load through the power supply (Point D).
The shared 820 now has 1/2 the current going through it and so the voltage drop is less across the shared 820 ohm resistor

Not sure it is enough to warrant a switchable mod??
I would probably just goof around with fixed values and keep the ones that I liked, but that's up to you.

I use a 820 in my AB763 boards because there is half the load and so a 820 ohm works out right for normal gain for 1/2 of a preamp tube current flow.

Marshalls also use non shared 820 ohms for the first pre amp tube.

[osing]

Thanks Doug, this is very helpful.

The reason I want this switchable is mainly for learning purposes. I agree that if one prefers one setting versus the other, you might as well just solder in the resistor value of choice. However, I would like to be able to do an A/B test which will be easy if I can just flip the switch back and forth.

Your point about the voltages being raised in the preamp due to V1 being removed in an original AB763 amp suggests that I probably would want to lift the Normal channel at the same time as lowering the cathode resistor for this to resemble the pulling of V1. I see in the Custom Vibroverb schematic (link above) that they simply disconnect the Normal channel circuit at the 220K resistor before the PI. I suppose I could do the same thing.

Would the disconnect of the Normal channel at the 220k resistor and the use of an 820 Ohm cathode resistor on V2B in my Hoffman AB763 SR then essentially be equivalent to removing V1 in an original AB763 SR? The Custom Vibroverb has a 10M resistor to ground after the 0.047 cap - any thoughts on what this accomplishes?

Finally, I am still not understanding why the Custom Vibroverb mod mode was designed to effectively increase the cathode resistors on both V2A and V2B. Doesn't this increase in voltage drop across the cathode resistor make the grid more negative with respect to the cathode, thereby reducing current flow which is the opposite of what pulling V1 would do in an original AB763 circuit? I guess I might be misunderstanding what is going on, but I do believe I have read a few places that the intent of the Custom Vibroverb mod was to "imitate" the removal of V1 in an original AB763, so any insight anyone might have would be much appreciated!

[EL34]

I don't know anything about the vibroverb mod.

Maybe someone else can help.