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

[TelRay]

Hello all,

continuing my restoration project, the next hurdle is to have a good overdriven sound.
When I push the Vibrato Channel Volume Past 7-8 I start hearing some high frequency clipping instead of a warm overdriven sound.

Trying to force this failure mode I found the following combination:
- Bright SWITCH on
- Channel VOL 5+ (left it at 10 for a most dramatic effect)
- Treble at 10
- Middle and Bass at minimum

Moving the Reverb potentiometer I only get the "screaming sound" either with Reverb at min or MAX, no noise with values in between.

See video for a better understanding (be careful with the volume, the screaming noise is not nice)

I am suspecting the Bright SW 120 pF cap could have gone bad



Do you agree it might be the 120 pF cap?
Any ideas about why it only "screams" with the Reverb Pot at either end of the track?
In any case, this cap should have no effect when the switch is OFF (and still the overdriven sound has clipping highs. Additionally the 250 uF TREBLE cap is new)

Thank you!

[pdf64]

Why do you suspect the bright cap? I would only think that if the issue didn’t manifest with the bright off; is that the case?
My guess is that it’s due to a bad ecap for the preamp HT node, or a bad 0V return connection to the chassis.
The V4 cathode bypass ecap may also be bad.
These things create positive feedback loops that can cause free running oscillation, given suitable control settings.
What effect does the pullboost have on the issue? How about setting the master vol very low?

[TelRay]

Hey! thank you very much for your reply and advice!

Why do you suspect the bright cap?

Well... I've changed every other cap in the amp (those orange things you see in the video)
The only caps I have not changed are the 120 uF caps on NORMAL and VIBRATO Ch BRIGHT SWs

I would only think that if the issue didn’t manifest with the bright off; is that the case?

Yes, in the particular case of this "loud scream" (what you hear on the video), the failure cannot be reproduced with BRIGHT SW OFF
As I mentioned above, the original problem I am trying to solve is that, when playing my guitar at VIBRATO Ch volumes of 7+, there is some high frequency clipping even when the BRIGHT SWITCH OFF.
I've found this way of "creating a problem" with all the HIGHs frequencies to the MAX (BRIGHT SW ON + TREBLE at 10) looking for reasons for that (there might be not related)

My guess is that it’s due to a bad ecap for the preamp HT node

I'm sorry but (due to a severe case of inexperience) I would need help here. Which one is the HT node?
Schematic Fender 100 Watts RMS AMP w/ Reverb and Vibrato (the white schematic, not the blue one)
Nevertheless, if it's a cap, it has been surely been replaced already

bad 0V return connection to the chassis

that's interesting and something I have definitely not explored. Would it be possible for you to highlight the 0V return in the schematic?
I believe these might be the connections you are referring to:

The V4 cathode bypass ecap may also be bad.

If this is the one, it has been replaced along with the associated resistor

What effect does the pullboost have on the issue?

As I initially thought the distorted high frequency could be leaking from the pull boost feature I have completely removed it from the amp.
(meaning I physically removed the 4 wires coming from the back of the push/pull potentiometer and from the circuit)

How about setting the master vol very low?

the video was made at very low MASTER VOLUME (a bit above 2)

thx!!!

[strato56]

Hi there, with Channel Volume maxed, the bright switch should have no effect. If it does, check the wiring or the lead dress.

[TelRay]

Hi there, with Channel Volume maxed, the bright switch should have no effect. If it does, check the wiring or the lead dress.

good catch man! i was about to post this photo when i read your post

solder has gone bad here

[TelRay]

so... the solder was (partially) bad, the other lead of the cap was soldered to the other lug of the BRIGHT SWITCH and, having a solder iron in my hand, I decided to change the capacitor for a new one.

what has not changed?
the "screaming sound" is still there with all other pots at minimum, TREBLE and CH VOL at MAX and BRIGHT SW ON

what has changed?
now the noise is still present regardless of the REVERB potentiometer position
the only pot that seems to have an effect (make the noise disappear) seems to be the MIDDLE pot.

[Chuck H]

I suspect something is wired incorrectly and/or there is a missing circuit ground. These things can cause a positive feedback loop. I suspect this because in the first video the reverb mitigated the problem at all settings but min and max. Yet the screech was reverberated indicating that it is manifesting before the reverb circuit. I know the reverb knob no longer affects the problem in the same way, but this is still a clue. The mid control includes the ground for the tone stack. There may also be wiring errors there for the control to affect the problem as it does.

Can you provide clear photos of the wiring?

[TelRay]

thx Chuck, you are right. In both videos the noise is reverberated 👍
yes! i was just checking the wiring and I read your reply.
you are right, the MID provides the ground, in this case shared with the VOL pot
I have taken a photo but for the sake of clarity I added a drawing of what I see
NOTE: when moving the pots the wire connecting the MID control to the VOL one and then to ground came loose 🤓 (could be an indication of failure)

[g1]

Well... I've changed every other cap in the amp (those orange things you see in the video)
The only caps I have not changed are the 120 uF caps on NORMAL and VIBRATO Ch BRIGHT SWs
...I'm sorry but (due to a severe case of inexperience) I would need help here. Which one is the HT node?

Just to be sure, have you replaced all the large filter caps in the 'doghouse' on the other side of the chassis?

[TelRay]

Just to be sure, have you replaced all the large filter caps in the 'doghouse' on the other side of the chassis?

yessir, the first thing I’ve done when I got the amp (a few months ago).

[pdf64]

It may be that 22k grid stoppers on V3 solves the oscillation and nasty distortion issues.

[Chuck H]

I dunno.?. It's a pretty profound howling screech to think that a single grid stopper might solve it.?. Nothing to lose I suppose. I still think there's a wiring hitch or a bad ground. Maybe we could get a history of the problem. Did the amp always do this? If no, when did it start? etc. What have you done to this amp and what problems did it have?

The picture I was hoping for would be of the full amp open chassis.

Also, is this the model that sends a parallel signal from the reverb input direct to the summing triode in "boost" mode? That circuit has a hefty .01u cap in series with a 13k resistor to ground when the switch is engaged. It seems stability problems were recognized early on this one. Is the boost switch pulled when the problem is happening?

EDIT: Never mind on whether the boost is engaged. I just caught the info in post #3. There may still be something to report on this. Still... A further history of what's been done to this amp and when the problem started may help.

[TelRay]

Hi guys and thank you again for your input so far.

Regarding the AMP HISTORY:

Bought it a few months ago for a fair price knowing I would need to invest time and money into it.

It had that "broken-fried- high frequency distortion" when pushed beyond half way the channel volume.

I observed the amp had only a couple of components replaced (one of the doghouse big electrolytic capacitor and a couple of resistors).The rest of the components were clearly 45 years old (Mallory paper caps, carbon resistors, etc).

So step by step I started changing the old components by newer ones (with tighter tolerances, etc). There was no concrete change that improved the sound or corrected the previous stated problem, but it started to sound better and tighter (in another post I describe adding the Rob Robinette mod to add a BIAS Voltage regulation by keeping the BALANCE: "Biasing a 1974 Fender SF Twin Reverb Master Volume Push Pull Switch - Reading 13 mA").

I have also added the .022 uF cap tremolo tick fix.

What I have done is to bring the amp back to the Schematic provided earlier (there was a reverb resistor changed to 220K instead of 470 K, the resitor on the rectifier was 470 Ohm instead of 1 KOhm, etc, etc).

Schematic Fender 100 Watts RMS AMP w/ Reverb and Vibrato (the white schematic, not the blue one)

To conclude this section, as stated before, I have removed completely the PULL wires to make sure this "feature" was not leaking any distortion into the clean sound (no real difference or improvement after this last step)

In general I would say that the original problem has shown an improvement of 75% and I am able to use the amp with a fairly clean sound at Channel Volumes below 5.


I hope the description of my history with this amp is what you were looking for.

Here is a view of the WHOLE CIRCUIT BOARD (it's quite a big Pano)



I appreciate the suggestion of adding a screen resistor, however I have avoided all temptations of implementing any of the multiple mods available (blackfacing, removing the NFB, etc) as my primary intention is first to bring the amp to its former glory before deciding to modify anything at all.

UPDATE on the SCREECH SOUND: I have removed the V1 tube some days ago (to see if I could achieve a bit of overdrive at a lower channel volume without the "nasty effect").
A few hours ago I was tryting to see if I could reproduce the "SCREECH" also in the NORMAL CHANNEL, I couldn't. And, moreover, I couldn't make the VIBRATO CHANNEL "SCREECH" again with the V1 tube engaged.
Removing the V1 tube brought the SCREECH to the VIBRATO CHANNEL again.

(as a reminder, I found out about this "SCREECH" sound just a few days ago looking for problems associated with extreme TREBLE settings as my problem was always the distortion on this frequency range)

[Chuck H]

Ok!!! You may have microphonic preamp tubes and/or dirty/oxidized connections in the pin sockets. You should clean, deox, pot cleaner everything and try putting known good tubes in the V1 and V2 positions. That would be my next move fo shizzle.

[TelRay]

Hi!
I did DEOXIT all sockets (including the power tubes) and moved tubes around a couple of days ago.
Yesterday, after finding out that the "SCREECH" cannot be reproduced with the V1 in the circuit, I found that if I move (rocking back and forth) the V1 tube there is a "pop" (like disconnecting and connecting something).
I've tried different tubes in the V1 position and some do this "pop" and some don't, I am guessing it is because of slight differences in the pins geometry (some more open, some closer). Nevertheless, I might replace that socket today.