AB763 - Reverb return to PI Pin 7

[RobBozic]

Hi guys,

Can someone tell me how returning the reverb signal to the 100n PI (pin 7) input affects the tone in a typical blackface style topology?

Thanks
Rob

[martin manning]

I think the biggest effect will come from having to remove the global negative feedback loop, which will become positive feedback wrt the reverb signal. With that will come more gain and more distortion in the power stage.

[tubeswell]

Besides what Martin said about removing the NFB,

How are you going to treat the wet recovery signal? the AB763 reverb goes through that first inverting gain stage and then is blended into a further inverting gain stage before being resistively mixed prior to the LTP. You may have to experiment with the recovery side gain and AC loading and interstage attenuation etc to get the reverb level adjustment to a functional blend of wet vs dry.

Also need to consider the dry-side signal. The 'vibrato' channel TS recovery stage is attenuated by the 3M3/220k resistive divider (that has the 470k/100Kn pot in parallel with the 220k) prior to the LTP. Depending on how you mess with the wet side recovery above, you may have to mess with the dry side attenuation as well.

Messy and complicated? Quite possibly.

[jjman]

I don't know which half of the PI uses pin 7. If it's the "bottom" half you need to change the cap on that grid to a resistor to avoid grounding the reverb signal.

If it's the "top" their will be some additional loading of the dry at that point but it should "work." But all of the reverb's signal-boost from the 3rd stage will be gone. So it should be a weak reverb. The 470k/270k could be adjusted to reduce standard attenuation there.

Is there an advantage to this move?

[martin manning]

The side with the 0.1 cap (mentioned above) is the side opposite to the input on the AB 763. The cap would have to be disconnected from the tail (and reduced in value). This makes the inverter function as a mixer, so you don't have the loss of the mix resistors and get a higher impedance input. The AC30 does this as does the 18W Marshall.

[Roe]

the BF and SF fenders really need negative feedback to sound right