Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch

[TelRay]

Today I placed the scope at the SPEAKER OUT jack to see what is happening closer to the listeners ears

- generated a 400 Hz 100 mV SINE wave (as before, checked that the wave took about 2 divisions of the scope's screen when set at 50 mV/div)
- connected a 4 Ohm Dummy load
- probe connected to the SPK OUT tip and ground connected to chassis

This is what I've seen:

1) The following settings show a SINE wave similar to the one at the input
VIBRATO CHANNEL VOLUME @ 2
TREB, MID, BASS indistinct
MASTER VOLUME @ 5



2) Now, this is what happens if the BRIGHT SWITCH is turned on (this effect is independent of the presence of V1 or V3 in the circuit or the REVERB IN connector plugged or unplugged). All other settings as before



3) Turning the BRIGHT SWITCH OFF and with all settings as in point 1, I increased the VIBRATO CHANNEL VOLUME from 2 to 10

4) Keeping the VIBRATO CHANNEL at 2 and all other settings as in the first point, changed the MASTER VOLUME from 5 to 10

[g1]

Are you still able to produce the glitch shown in post #145?
If not, have you tried a listening test to see if the unwanted sound artifact is still present?

[TelRay]

Are you still able to produce the glitch shown in post #145?

yepp.... placing the scope at the center tap of the VIBRATO CHANNEL VOLUME I still see that glitch right now

[TelRay]

If not, have you tried a listening test to see if the unwanted sound artifact is still present?

To the second part of Mr G1's question I decided to move on to the "audible" part of the test. I kept the scope's probe at the SPEAKER OUT jack, removed the dummy loads and connected the speakers.
this is how the wave looks like (all tubes in, reverb tank connected, etc. CHANNEL VOLUME @6, MTB at 2.5, 7. 2.5 and MASTER VOLUME @ 2)



If I disconnect the REVERB IN plug



here's a video, I hear added noise at 2-8 KHz.

[TelRay]

Any advice is welcome... at the moment I do not know what else to do (apart from Helmholtz recommendation of adding an Aluminum cover plate)

[Helmholtz]

Did you see this:

https://music-electronics-forum.com/...l=1#post527426 ?

[Chuck H]

Yep. The ugliness in the distortion is due to a parasitic oscillation. And it could be due to lead dress or grounding and that models innards are like a plate of spaghetti. So it really is an "on the bench" thing since A LOT of things need to be checked and tested as they happen. Trying to dictate it a piece at a time would be like cutting grass with scissors. But here's one of my techniques for tracking oscillations...

You'll need two alligator clip leads and a small value capacitor. You basically just set up the amp to initiate the problem, hook up your scope and use the alligator clips to connect the capacitor between different points in the signal path and ground. Start with something like a 220pf capacitor. Increase the value to 470p or 1000p if you're not getting results. If you find a conection that stops the oscillation you can then examine the lead dress an/or grounding for the associated circuitry. Keeping in mind though that the point of oscillation may not be the cause. Rather, some nearby radiant field may be affecting it.

[TelRay]

that's great guys! you have already given me something to do over the weekend.
will keep you guys posted on the process

[TelRay]

Hi all,

I've tried the method suggested by Chuck (thanks man so much!). I've used a 200 pF and a 560 pF cap. They main differences were found with the 560 cap, not completely killing the oscillation but presenting different degrees of reduction depending on where I placed it.

The biggest one (75% reduction of the oscillation on the scope) was at the MASTER VOLUME so i decided to change the wires for new ones (used one of those old RCA cables I had around the house) and replace the potentiometer with the original one (with the 120 pF attached to the 4th lug).
I have additionally put a new tube socket on V6 (phase inverter) for the reasons I will explain later.
I need to investigate more but so far I do not see the REVERB IN having an effect anymore.

There is an old issue (or maybe it's normal) I have detected in the past on the phase inverter circuit, I mentioned in the past that whenever I would place the Multimeter probe to read voltage I would get loud noises on the speaker (at that time speaker and not load was connected to the output jack).

V6 pin 2: extremely noisy (could damage the speakers even with the master volume at 0)
V6 pin 7: very noisy
V6 pins 1, 3, 6: just scratchy when touched with the probe

the problem was always with the Multimeter, no noise with the wooden stick (see video below). Another symptom is that I've never heard a change in sound when lifting the 820 Ohm resistor going to the speaker jack (marked on schematic below). I think there's something wrong in this area. All the components are new (resistors, capacitors) and now the tube socket and the 12AT7 tube itself (the old one broke after messing with it way too many times).

With the following settings I see oscillation (photo below): VIBRATO VOL @ 8, TMB @ 5,5,5, MASTER VOL @ 8



This oscillation goes away if I place the Multimeter probe on anything coming out of V6 pin 2 (i've checked the 330 K resistor, changed the .01 uF cap for an old one, etc) or it is reduced by placing the 560 pF cap (placement marked on schematic below)

I believe the sound has improved and the guitar has more "punch" as now those transients are probably not being clipped by the oscillation so much. The following "crunchy" sound is coming 100% from the amp (no pedals or anything, VIBRATO CH VOL at 6, MASTER at 4). This is finally starting to sound as I expected. The distortion is not entirely clean (if the use of those contradictory terms can be allowed) but it is getting there (mp3 sample below).



what do you guys think?

[Helmholtz]

The scope picture shows no HF oscillation. The multiple trace effect is a lower frequency than the signal and is most probably an injected hum modulation (60Hz or 120Hz). If you "squeeze" the scope's timebase you should be able to see the low frequency modulation on the envelope of the signal.

[eschertron]

Another symptom is that I've never heard a change in sound when lifting the 820 Ohm resistor going to the speaker jack (marked on schematic below). I think there's something wrong in this area.

Did someone replace the external speaker jack with a shorting jack? That would prevent NFB from having an effect.

edit: and the difference between 100R to ground (820R lifted) and 100R|820R = 90R to ground is probably not audible.

[TelRay]

no HF oscillation. The multiple trace effect is a lower frequency than the signal and is most probably an injected hum modulation (60Hz or 120Hz

ahhh... thank you / vielen dank, sir... sooo much to learn!

the photo is not super-clear but what I've seen was something like this:



about 60 or 120 Hz hum, should I be hearing that hum? the amp is dead quiet (only get some hum when I turn the REVERB volume up)

[TelRay]

Did someone replace the external speaker jack with a shorting jack? That would prevent NFB from having an effect

well, well, well... thank you for that, man. I believe someone did do that in the amp's "past life". I highlighted in on post #46 (maybe you could read the reply I got on post #47 and see if you all agree)

the difference between 100R to ground (820R lifted) and 100R|820R = 90R to ground is probably not audible.

interesting statement... I've seen many FENDER TWIN REVERB Mods and this one is one of the popular ones and supposed to change the voicing of the amp.

This is what I've read:
"The NFB switch fundamentally changes the clean and overdrive tone of the amp."
https://robrobinette.com/AB763_Modifications.htm

"If you disconnect the NBF loop you’ll notice that the volume increases and tone gets much more aggressive. More white noise too. unfortunately"
http://fenderguru.com/amps/twin-reverb/

In any case (this could be a later discussion): should I be changing the SPKR Jack back to a standard (non shorting jack)?

I am curious now if this is going to remove some of the aggressiveness / bite and clean up the sound (which I was - both - looking to increase).

[The Dude]

well, well, well... thank you for that, man. I believe someone did do that in the amp's "past life". I highlighted in on post #46 (maybe you could read the reply I got on post #47 and see if you all agree)......

Enzo thought he was wrong once, but he was mistaken.

[eschertron]

well, well, well... thank you for that, man. I believe someone did do that in the amp's "past life". I highlighted in on post #46 (maybe you could read the reply I got on post #47 and see if you all agree)

interesting statement... I've seen many FENDER TWIN REVERB Mods and this one is one of the popular ones and supposed to change the voicing of the amp.

This is what I've read:
"The NFB switch fundamentally changes the clean and overdrive tone of the amp."
https://robrobinette.com/AB763_Modifications.htm

"If you disconnect the NBF loop you’ll notice that the volume increases and tone gets much more aggressive. More white noise too. unfortunately"
http://fenderguru.com/amps/twin-reverb/

In any case (this could be a later discussion): should I be changing the SPKR Jack back to a standard (non shorting jack)?

I am curious now if this is going to remove some of the aggressiveness / bite and clean up the sound (which I was - both - looking to increase).

From the photo it looks like the *normal* spkr jack is the shorting type (OK) and the *external* spkr jack, the one not used, is not. So that's OK too. Follow the wires attached to the unused jack both directions. The two white wires, that is. We know the black wire is ground. One should go to the OT and the other to that 820R resistor.

From the info that the sound "doesn't change" when lifting the resistor makes me think that you're running the amp without NFB all the time. So whatever sonic qualities the corespondent attributes to the non-NFB mod, that's what you've come to view as normal.

edit: and yes, the consensus is that Enzo is 'spot on' about most things, and about all things electrical.