AB763 trem disconnect

[osing]

Howdy,


Having used old posts as reference, I added a normal channel disconnect switch and trem disconnect switch to my Hoffman AB763 Super Reverb a few months ago. I hardly ever use the trem, and I had read that disconnecting the trem would alter the sound. From what I understood, in the Hoffman AB763 circuit, you can disconnect the trem by cutting the wire as depicted by the red line in the attached file (you'll see the blue arrow pointing towards the red cut line).


Anyway, I put a switch in where the red cut mark is. When I toggle the switch to effectively cut the line, I do get a little boost. However, to my surprise today, several months later, when I plugged my trem on/off pedal into the amp I realized that the trem still works no matter which position the trem disconnect mod switch is in (I might need to do some further listening see if the trem sound sounds different depending on the position of the mod switch, but there is definitely a trem effect in either position). Am I missing something here - shouldn't the trem be completely out of the circuit when I cut the line after the 47k resistor? I will admit that I don't really understand how the trem circuit really works, so that isn't helping....

[sluckey]

From what I understood, in the Hoffman AB763 circuit, you can disconnect the trem by cutting the wire as depicted by the red line in the attached file (you'll see the blue arrow pointing towards the red cut line).

That does not disconnect the trem in the Hoffman variety AB763. It will give you a slight boost as you've seen.

[osing]

Thanks Sluckey,


I was thinking that since no signal would be going through that resistor, there would be no signal going to V5. However, I see now that that resistor is really just sending some of the signal to ground and by disconnecting it, none of the signal coming out of that 0.1uF cap right before the 47k resistor is being sent to ground which thus gives the boost, right?


Is it correct to say that the AC signal from the guitar never enters V5 and that V5 is instead effecting the sound further downstream at the point where the intensity pot is connected at the 220k resistors near the output tubes?

[sluckey]

Is it correct to say that the AC signal from the guitar never enters V5 and that V5 is instead effecting the sound further downstream at the point where the intensity pot is connected at the 220k resistors near the output tubes?

yes

That drawing is old stuff and not always easy to understand how an amp functions. Doug has updated his info on his amps. A new pdf file includes a nice visual layout as well as a schematic. The schematic makes it very easy to understand his amps. Take a look. You'll like the changes.

[HotBluePlates]

Is it correct to say that the AC signal from the guitar never enters V5 and that V5 is instead effecting the sound further downstream at the point where the intensity pot is connected at the 220k resistors near the output tubes?

Yes.

The Hoffman AB763 switches to a trem circuit which injects trem by varying the bias of the output tubes, like the 6G16 Vibrolux and the AA1164 Princeton Reverb. The original trem circuit for the larger blackface amps with 2x or 4x 6L6's (and Deluxe Reverb) used an optoisolator to ground/un-ground the guitar signal, thereby creating trem.

The "trem disconnect gain boost" is relevant to the original larger blackface Fender amps, and doesn't apply to the Hoffman trem circuit. However, Hoffman added the 47kΩ resistor to ground to shave off some gain to more-closely match that of the original amps. By disconnecting it, you still got the gain boost intended by the modification (albeit, not ditching the trem as in the original modification).

You could have always left the 47kΩ resistor out to get a permanent gain boost intended by the mod, but without losing your trem feature.

[osing]

Thanks guys. I will definitely look for the updated schematic and layout.


I need to try the LED suggestions from some previous posts of yours to see if it might reduce the thumping sound of my trem (very noticeable when trem is on but when I am not playing). It sounds like the LED instead of V5a cathode resistor/cap and possibly LED across the intensity pot could make a difference.


Cheers and thanks for your help!