Recapping a pull-boost PR. Question about differences from earlier PR

[milocj]

I'm about to do a cap job on a 1979 pull-boost Princeton Reverb for a friend and I noticed a couple of minor differences between this and the earlier SFPR. I'm not looking to argue that the amp sounds any different as I have heard two 1979s side by side with my late '67 drip-edge and they sounded nearly identical with comparable speakers.

I would, however, like it if somebody could explain to me what these different values do to the tone, if anything. I'm not super technical but I do have a little bit of a basic understanding of some parts of the circuit.

The first difference is between Pin 6 of V1 and Pin 7 of V3. There are a cap and resistor in parallel at this point and they are a 1.5M and 22pF on the pull-boost and a 3.3M and 10pF on the early SF and BF amps. Was this changed due to the fact that the boost circuit also feeds V3 right after this point? Does it affect anything and would it make the pull-boost sound worse if they were changed to the earlier spec?

The other difference is on the cathode of the reverb driver. Instead of a 2.2k resistor with the 25uF/25V bypass cap there is just a 680 ohm resistor. Does the pull-boost circuit "steal" from the reverb driver to get the added gain? I'm guessing that this change was made to allow the pull-boost to function better, but what kind of effect does it have on the reverb when the boost isn't being used? Doesn't adding a cathode bypass cap increase the gain of a preamp tube?

Lastly, the schematic shows a footswitch jack for the boost function. There looks to be a plug in the chassis where a footswitch jack could be installed, though I have never seen a footswitch jack on a PR. Did Fender ever offer this or was it only on the schematic?

Thanks.

[laird]

The 3.3M/10pf resistor/cap combination was used to balance out the mix of the reverb and dry signals. By changing it to 1.5M/22pf, more reverb signal could come through. That's a good thing, because the reverb driver's cathode change reduced the gain being pushed into the reverb tank. This was done to keep the reverb from making that crashing sound when the boost is pulled. Overall, these changes pretty much balance out.

I know the footswitch jack was present in most of the larger pull-boost fenders. If I recall it's just one wire, easy to add or remove.

-Laird

[milocj]

Thanks for the info/lesson.
I had more or less assumed that this was the case but I don't have the electronics to training to back up my theory

I have seen the boost footswitch on the larger Fenders, just never on a PR, and was kind of surprised to see it on the schematic and the plugged chassis hole.