Strange non-stock resistor in reverb circuit

[slidincharlie (Carlo P)]

My SF Deluxe Reverb ('77 pull-boost version) has got a non-stock 3.6k resistor installed between one of the filter caps and the reverb tranny's red wire.
What could the reason for this resistor? Is there a special application (a mod or repair peculiar to the reverb circuit maybe?) that might explain this?
One clue: the previous owner had converted the reverb cathode to BF specs (2k2 w/25@25bypass), which I have reverted back to late SF specs.
Photo below.

[Enzo]

Don't know. Maybe they wanted to drop the plate voltage a little at the drive tube

[slidincharlie (Carlo P)]

Enzo,
do you think that it is safe to eliminate it and see (ehm... hear) what happens?

[Enzo]

Probably. Then again, what does it sound like WITH it?

[Tom Phillips]

Carlo,
My 2 cents:

If it sounds OK AND the user is satisfied with the sound I'd let him play the amp as is. However, if time and money allows I'd put the amp back to stock. (In your case I'd go to BF specs) Then sound test. If something isn't sounding right, then I'd investigate and correct to root cause of the problem. Many times I have found that strange mods like the one you have were installed as bandaid approach to correct some real or perceived problem.

It will be interesting to hear what you find out.

Regards,
Tom

[Enzo]

I like the perceived problem idea. Someone sees 410v at the plate on the schematic but measures 422 or something. Then decides he MUST lower the voltage there somehow. Who knows.

[slidincharlie (Carlo P)]

The amp is mine, so there is no other user to satisfy.
I'll snip the resistor and see what happens. If everything goes well, I'll leave it stock. I'll report here.

BTW, the late silverface reverb driver arrangement (plain 680 ohm cathode resistor, as opposed to 2k2 with 25 uF bypass cap of the blackface version) sounds a little less boingy (=better) to me: is it just my impression?
What's your opinion? Should I experiment more?
I ask because I am not really happy with BF/SF reverb, and tend to keep it at around 2 or 3 on the dial.

[Tom Phillips]

...the late silverface reverb driver arrangement (plain 680 ohm cathode resistor, as opposed to 2k2 with 25 uF bypass cap of the blackface version) sounds a little less boingy (=better) to me: is it just my impression? What's your opinion? Should I experiment more?
I ask because I am not really happy with BF/SF reverb, and tend to keep it at around 2 or 3 on the dial.

Carlo,
I don't think it's just your impression. I think the SF configuration is less boingy too if that's what you want. You can experiment a lot with the 12AT7 driver cathode circuit values. You can also replace the 1 Meg resistor at the input of the 12AT7 reverb driver with a pot to adjust the reverb drive. Then you have a reverb dwell control. I've used an internal trim pot for this. You can also put a pot in the rear panel “Reverb Pedal” jack hole. This requires a little re-wiring of the existing wiring to the tube and the “Reverb Output” jack but it's all fully reversible. Most people don't seem to use the reverb footswitch anyway. With that mod you may like the BF cathode values. Lots of combinations to experiment with.
Regards,
Tom

[slidincharlie (Carlo P)]

Tom,
thanks for your reply.
I'd really like to experiment with the reverb driver circuit, because (maybe my opinion is against the current one) I am not really happy with the Fender reverb, unless I keep it low on the dial.
What keeps me from experimenting is messing with the soldering/desoldering job in a very short time while testing. I must find out a way to rapidly swap components to test what sounds better in any part of the circuit board...

[slidincharlie (Carlo P)]

Update: too high plate voltage on the reverb recovery tube

There ya go. Sorry for the delay.

I've measured the plate voltage with that 3k6 non-stock resistor in place:
- 399 VDC on the plate of the reverb driver (V3) instead of 410 on the schematic;
- 237 and 241 VDC on the plates ov the reverb recovery (V4) instead of 170-180 on the schematic.

Now I snipped the 3k6 resistor, and this is what I found:
- 410 VDC on the plate of V3 (right on stock);
- same as above on the plates of V4.

Should I correct the high plate voltage on V4?
If so, how?

As far as sound is concerned, there is apparently no difference with and without that resistor. So I'd leave it out.