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

[Dave H]

OPEN LOOP (NO NFB)

MV Pot Wiper = 0.98 V AC (w/multimeter, ground connected to chassis)
FBK V = 0.00 V AC (w/multimeter, ground connected to chassis)
DUMMY = 7.42 V AC (w/multimeter, connected to the output SPKR jack)

Now, if I understood well: Go = 7.42/0.98 => Go=7.57 which is not far from the calculated 7.84

Does this mean that the NFB is working, regardless of what's been said before (no noticeable change in volume, not a big increase in wave's amplitude when seen on the scope)?

Gain without NFB is 7.84, gain with NFB is 4.21.
It looks like the NFB is working OK.

What was the setting of the Master Vol pot? The CCW end of the MV pot is connected to the NFB input. At low MV settings the feedback signal will add to the input signal. I think that is causing there to be no noticeable change in output when the 820R is disconnected. Check to see if the signal on the MV pot wiper changes when disconnecting the 820R.

[Helmholtz]

What was the setting of the Master Vol pot? The CCW end of the MV pot is connected to the NFB input. At low MV settings the feedback signal will add to the input signal. I think that is causing there to be no noticeable change in output when the 820R is disconnected.

Good point! I also think that this MV wiring above the 100R introduces a little positive feedback.

The effect you describe should be augmented if the second PI triode has less gain than the first/direct one.

[Helmholtz]

As Dave H already stated, the measured gains Go=7.57 and Gcl=4.21 show that the NFB is working properly. The values are somewhat low, though, and indicate that your tubes don't have full gain. If I interprete your tube tester results of 80% (power tubes) and 72% (PI) in terms of tube gain, the open loop gain will be 0.8x0.72=0.58 of the theoretical full gain value of 7.57/0.58=13, which is exactly what I calculated/predicted in post #218.

Additionally, I've measured V AC at both sides of the 0.1 uF 200V capacitor and it's 0.75 V AC on one side but 0.58 V AC on the tube side (V6 pin 7).
I've removed the new Orange Drop and put back the old Blue Drop... that values did not change (.0.75 on one side and 0.58 on the other).

The voltage difference must be due to a loading effect by the input resistance of your measuring instrument. What did you use? If you measure across the cap with a DMM, you should get zero voltage.

[Helmholtz]

This is how the signal looks like at V3 pin 6... and, damn! that thing was hot... I had to put the scale at 5 V/DIV (0.5 ms/DIV)

A 400Hz signal has a period(ic time) of 2.5ms. I.e. successive positive (and negative) peaks are separated by 2.5ms. If your horizontal deflection is 0.5ms/DIV, this corresponds to 5 division between two positive (or negative) peaks. Your scope picture shows 10 divisions between same side peaks, so I assume you used 0.25ms/DIV?

I count 7 HF periods per division. This would mean a frequency of 28kHz, using 0.25ms/DIV.

[TelRay]

What was the setting of the Master Vol pot? The CCW end of the MV pot is connected to the NFB input. At low MV settings the feedback signal will add to the input signal. I think that is causing there to be no noticeable change in output when the 820R is disconnected.

MV is set at @ 4, same settings as in post #225
(anyone has the curve for this 1M-JT potentiometer so it can be empirically demonstrated that at that setting the NFB would not be noticeable?)

Check to see if the signal on the MV pot wiper changes when disconnecting the 820R.

by "see" I understand you mean connecting the scope to the MV pot wiper, right?
because electrically we have already measured:
CLOSED LOOP MV Pot Wiper = 1.64 V AC
OPEN LOOP MV Pot Wiper = 0.98 V AC

[Helmholtz]

V3 ANALYSIS

This is how the signal looks like at V3 pin 6... and, damn! that thing was hot... I had to put the scale at 5 V/DIV (0.5 ms/DIV)



same calculation as before: 3,141 px between peaks and 44 between oscillations is about 28.5 KHz

If I disconnect the REVERB INPUT the wave increases in size and the oscillation disappears

Getting back to the oscillation issue:

It shows strongest at the reverb driver V3 plates, so I assume it is generated by V3 (I have been suspecting V3 for a while). Probably a higher harmonic of the plate signal triggers a parasitic resonance of the reverb transformer, cable capacitance, reverb transducer arrangement.

A possible cure would be an RC snubber (resistor in series with cap) across the reverb transformer primary. To find ideal values will require some experimenting. I would start with 5K in series with 220pF. The idea is to use as much resistance and as little capacitance as necessary to damp the resonance. Too little resistance and too high capacitance will degrade the reverb signal.

Remains the question, how the HF couples to the direct signal, as the reverb springs won't transduce such HF. I assume a coupling via the power supply. Could you scope the supply at point A (red wire of reverb transformer primary)?

[Dave H]

MV is set at @ 4, same settings as in post #225
(anyone has the curve for this 1M-JT potentiometer so it can be empirically demonstrated that at that setting the NFB would not be noticeable?)


by "see" I understand you mean connecting the scope to the MV pot wiper, right?
because electrically we have already measured:
CLOSED LOOP MV Pot Wiper = 1.64 V AC
OPEN LOOP MV Pot Wiper = 0.98 V AC

But I wasn't to know the pot or input level hadn't been changed between posts #225 and #238

If nothing has changed -
Closed Loop: 1.64 * 4.21 = 6.9V output
Open Loop: 0.98 * 7.57 = 7.4V output

The output difference is < 1dB at the speaker.

[Helmholtz]

en Loop: 0.98 * 7.84 = 7.7V output

FWIW, he measured Go= 7.57 (post #238).

[Dave H]

FWIW, he measured Go= 7.57 (post #238).

Thanks, I mistakenly used the calculated Go.

[g1]

I will recommend that, whenever possible, use the master up full, reduce channel volume as required.
The amp is based around the non-master version, so full master gives a better indication of 'original' design.
If you need to reduce the master to get the oscillation, that is fine, but for things like NFB test, it should be full up.
I'd really like to see a video like post #208 with the master up full. It would certainly help clear up the NFB questions.

[pdf64]

Yes, otherwise the gain increase when open loop may be getting counteracted by the fall in input impedance (thanks to a midway setting of a 1M master vol).

[Dave H]

I'd really like to see a video like post #208 with the master up full. It would certainly help clear up the NFB questions.

With the master full up I think you'd see the output voltage nearly double when the 820R was disconnected (the same as it would with the master CCW terminal connected to ground) because the signal voltage on the wiper would be constant.

Why did they connect the master CCW terminal to the NFB input?

[TelRay]

I will recommend that, whenever possible, use the master up full, reduce channel volume as required. The amp is based around the non-master version, so full master gives a better indication of 'original' design.
If you need to reduce the master to get the oscillation, that is fine, but for things like NFB test, it should be full up. I'd really like to see a video like post #208 with the master up full. It would certainly help clear up the NFB questions.

Again, completely logic. It makes total sense that the NFB works at its best with the MV at 10

NFB

OSCILLOSCOPE

SCOPE at SPKR OUT
VIBR CH VOL @ 3
MASTER VOL @ 10
820R being disconnected / connected during the video

for the last part of the video I've increased the VIB CH VOL to 3.5

VOLTAGES
MV @ 7, VIBR CH VOL @ 3 (nice clean SINE WAVE)

NFB CLOSED
MV Pot Wiper = 1.90 V AC
FBK V = 0.84 V AC
DUMMY = 7.78 V AC

Gcl = 7.78/0.84 => Gcl=9.26

NFB OPEN
MV Pot Wiper = 1.68 V AC
FBK V = 0.00 V AC
DUMMY = 12.15 V AC

Go = 12.15/1.68 => Go=7.23

is the 12.15 to 7.78 VAC increase translated as about +4dB? now there should be a noticeable difference.

[TelRay]

The voltage difference must be due to a loading effect by the input resistance of your measuring instrument. What did you use? If you measure across the cap with a DMM, you should get zero voltage.

I was measuring the AC Voltages at both sides of the .1 uF / 200V cap with a Fluke 177 DMM with the ground attached to the amp's chassis
You are right, if I put the ground probe on the other side of the cap I get 0 V AC (unless I push both CH VOL and MV to 10 and then I get about 60 mV)

[TelRay]

Could you scope the supply at point A (red wire of reverb transformer primary)?

sure thing! i couldn’t get a clean sone wave here, this is the closest i’ve got