NFB/Bypass Cap Switch

[cooltone]

I'm about to build a 5E3 kit and I'm interested in applying the NFB/Bypass Cap mods as suggested by Rob Robinette, but with a footswitch.

I read Angelodp's thread on NFB footswitch with interest and I wondering if I'm going over the top with my design.





I prefer not to pass anything on the signal chain or any DC voltages outside the outside the amp for noise/disruption reasons.

The bypass cap on V2a is simply grounded or not. The R1/100k is leakage resistor to minimise switch pops.

The NFB feedback (R5+R6) 57K is split and grounded or not via C2 (again R2 is to minimise pops). When grounded R5 should have minimal impact on R4 (similarly for OT/R6).

1) Am I going over-the-top with the leakage resistors - does anyone know whether these types of amps thump when NFB is switched in/out with/without leakage resistors? (same for bypass cap).

2) NFB and bypass Cap are switched independently. Do they each have a noticeable or should I switch them at the same time?


Feedback much appreciated,

Cheers, C

[thetragichero]

yes i would certainly expect there to be noise when switching
are you sure you wish to have both footswitchable? one thing you may consider is temporarily wiring them to different poles on a dpdt on-off-on switch and seeing if you'd find them useful changing in between songs (for me the only thing i like footswitchable is tremolo but i might just be too simple)

[cooltone]

I'm the same, I like things simple. Sometimes these mods are quite subtle; good for recording, but not enough difference live.

[thetragichero]

Aug 10, 2023 4:57:32 GMT -5 cooltone said:

Sometimes these mods are quite subtle; good for recording, but not enough difference live.

i mean I've used the nfb switch on my voxy combo (lisan al-gaib) live to change the response, I've just never needed to do it mid song
preamp tube cathode bypass cap might be useful though (blues junior calls it the "fat" switch) and I'd be much more comfortable having that on an external shorting footswitch (if i absolutely needed footswitchable negative feedback is feel much more comfortable having that controlled by a relay)

[cooltone]

Thanks. Difficult one, I think I'll have to suck it and see. I'll make it all reversible.

I'm comfortable without the isolation for the NFB for a couple of reasons:

1) Effects loop send often tap off a cathode resistor without problem

2) Speaker out is low voltage/low impedance so no problem

[reTrEaD]

Aug 9, 2023 9:47:47 GMT -5 cooltone said:

1) Am I going over-the-top with the leakage resistors - does anyone know whether these types of amps thump when NFB is switched in/out with/without leakage resistors? (same for bypass cap).

I think they're prudent. Imagine what would happen if the amp was switched off with the cathode bypass cap in the circuit. That cap quickly discharges to zero volts after current stops flowing in that triode. Now disconnect the cathode bypass cap, then switch on the amp. As current begins to flow in V2a, the voltage at the cathode settles in to roughly 1.5 volts. The voltage at both ends of the cap are at the same 1.5 volts. Now switch in the cathode bypass cap. The voltage at the cathode immediately drops to zero volts and there is a surge of current in V2a. Hugh thump. It won't take long for the cap to charge to 1.5v but the transient was pretty ugly.

Besides, resistors are cheap.

Aug 9, 2023 9:47:47 GMT -5 cooltone said:

2) NFB and bypass Cap are switched independently. Do they each have a noticeable or should I switch them at the same time?

By disconnecting the cathode bypass resistor, you give up a significant amount of gain. Adding NFB sacrifices even more gain, and it also tends to clean up non-linearities occurring from that point until the output. You shouldn't notice much (if any) effect of the NFB switch when the cathode bypass cap is in the circuit. The bypass cap will keep the voltage at the cathode pretty steady.

In my estimation, there isn't a ton of overall gain in this amp in stock form with the cathode bypass cap in the circuit. The 12AY7 for V1 has a lower amplification factor than a 12AX7. So you only have two gain stages before the phase inverter. And the phase inverter is a cathodyne, which has unity gain. I don't want to discourage you from experimenting with this, but I suspect you will be a bit disappointed.

If so, you might try a slightly different approach. Split the 1.5k cathode bypass into two resistors connected in series. Maybe a 1.2k resistor in series with a 330 ohm. Wire a 25µF cap in parallel with the 1.2k resistor. Use a second 25µ resistor to fully bypass the series resistor string and make that switchable. Then scale your NFB resistors down by a factor of roughly 5 and scale your NFB defeat cap up by a factor of 5.

Oh, and be sure to leave your leads from the primary of the OT to the plates of your 6V6s long enough so you can swap them, if necessary. If you get them wrong, you'll have positive feedback and the amp will howl, alarmingly loud.