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

[eschertron]

Have you tried a value other than 820R in that NFB loop?

[Helmholtz]

AC MEASUREMENTS
MV Pot Wiper = 1.64 V AC (w/multimeter, ground connected to chassis)
FBK V = 0.75 V AC (w/multimeter, ground connected to chassis)
DUMMY = 6.9 V AC (w/multimeter, connected to the output SPKR jack)

This means that your closed loop gain Gcl = 6.9/1.64 = 4.2 (somewhat low but in the ballpark, I predicted Gcl = 5.35 above).

The feedback signal is perfect, meaning that H = 0.75/6.9 = 0.11 (see post #218).
You should be able to measure the same FB voltage (0.75VAC) at the grid (pin 7) of V6B, provided the 0.1µ cap is ok.

Now we need to know the dummy voltage and MV wiper voltage with the 820 Ohm disconnected to determine the open loop gain Go.

Your multimeter measures AC rms voltage. The scope measures peak-to-peak voltage (Vpp) between top and bottom. Vrms and Vpp are related by Vpp = 2.83 Vrms. So your ouput voltage of 6.9 Vrms should show as 19.5 Vpp on the scope.
Do you use a 10:1 probe? If so you need to multiply your scope reading by a factor of 10. And make sure that the vertical deflection knob is in its "calibrated" position.

Are your tubes new?

[TelRay]

Have you tried a value other than 820R in that NFB loop?

no sir, I havent

[TelRay]

This means that your closed loop gain Gcl = 6.9/1.64 = 4.2 (somewhat low but in the ballpark, I predicted Gcl = 5.35 above).
The feedback signal is perfect, meaning that H = 0.75/6.9 = 0.11 (see post #218).
You should be able to measure the same FB voltage (0.75VAC) at the grid (pin 7) of V6B, provided the 0.1µ cap is ok.
Now we need to know the dummy voltage and MV wiper voltage with the 820 Ohm disconnected to determine the open loop gain Go.
Your multimeter measures AC rms voltage. The scope measures peak-to-peak voltage (Vpp) between top and bottom. Vrms and Vpp are related by Vpp = 2.83 Vrms. So your ouput voltage of 6.9 Vrms should show as 19.5 Vpp on the scope.
Do you use a 10:1 probe? If so you need to multiply your scope reading by a factor of 10. And make sure that the vertical deflection knob is in its "calibrated" position.
Are your tubes new?

Great feedback, man! thank you!
I will re-read post s #218 and 227, disconnect the feedback loop (I don't know how many times that poor resistor has been de-soldered and soldered back in, I measured and it's still alive) and re-measure.

That's right, I am using a 10:1 probe.
If the vertical deflection knob is in the calibrated position? hahahaha... good question, will need to check which one that is (less than 1 month seniority with using the scope)

The tubes: power tubes are maybe 6 months being used. the rest more or less the same. measured them with a tube tester and all above 80% of "life" (to put a unit of measurement to that). with the exception of the 12AT7 in the PHASE INVERTER that I broke after removing it from the newly changed socket recently (those things are tight when new! second tube I break) which is at 72%.

[Dave H]

AC MEASUREMENTS
MV Pot Wiper = 1.64 V AC (w/multimeter, ground connected to chassis)
FBK V = 0.75 V AC (w/multimeter, ground connected to chassis)
DUMMY = 6.9 V AC (w/multimeter, connected to the output SPKR jack)

Additionally, with the scope set at 0.2 mV/DIV I counted 8.25 DIV between the top of the WAVE and the bottom, so it should be 1.65 V (AC? DC?)

From the above measurements -

Closed loop gain is 6.9/1.64 = 4.21
NFB ratio is 0.75/6.9 = 0.11 (as it should be with an 820R, 100R divider)
I calculate the Open loop gain to be 7.84 (if I did it right)
Which means the output voltage should nearly double i.e. x1.86 (7.84/4.21) when the 820R is disconnected.

Should that be 0.2V/DIV?
That 1.65V is peak to peak which is 1.65/2.828 = 0.58V RMS if its a sine wave

Edit:
I see a few posts slipped in while I was thinking about it

[TelRay]

Closed loop gain is 6.9/1.64 = 4.21
NFB ratio is 0.75/6.9 = 0.11 (as it should be with an 820R, 100R divider)
I calculate the Open loop gain to be 7.84 (if I did it right)
Which means the output voltage should nearly double i.e. x1.86 (7.84/4.21) when the 820R is disconnected.

Should that be 0.2V/DIV?
That 1.65V is peak to peak which is 1.65/2.828 = 0.58V RMS if its a sine wave

Edit:
I see a few posts slipped in while I was thinking about it

that's great Dave, thanks a lot!
yes, the mV/DIV is a typo
And yes, posts go fast and I am not learning at the same speed )

[Helmholtz]

Agree to the calculated open loop gain of Go=7.84.

[Helmholtz]

REVERB TANK
- Removed the tank from the amp's cabinet
- Moved cables (from tank to amp) around
- the oscillation is present also using other cables
- the oscillation disappears either when disconnecting the INPUT cable at the tank or at the amp
- the tone generated with the tone generator is heard on the INPUT transducer of the tank (this is probably normal?) - you can hear this in the video below
- the oscillation show up on the scope between (more or less) 300-900 Hz

Can you see the oscillation at the reverb input?

[TelRay]

Can you see the oscillation at the reverb input?

interesting! failed to think of that

I am thinking of different ways to see this:

- connect the scope to the REVERB INPUT jack at the amp with the cable connected to the REVERB tank
- connect the scope to the REVERB INPUT jack at the amp with the cable disconnected from the REVERB tank
- place the scope (if possible, need to check that) right before and after the REVERB INPUT transducer

any other ideas?

[Helmholtz]

- connect the scope to the REVERB INPUT jack at the amp with the cable connected to the REVERB tank
- connect the scope to the REVERB INPUT jack at the amp with the cable disconnected from the REVERB tank

Let's start with this.

- place the scope (if possible, need to change that) right before and after the REVERB INPUT transducer

Not sure what that means.


I would also be interested to see the signal at the plates (pins 1&6) of V3. Reason: This is the signal at the primary of the reverb transformer. The signal at the reverb input is the transformer secondary. If the oscillation is generated by V3, the reverb transformer will attenuate HF, meaning that it should show stronger at the primary.

(To be honest, I am not sure if the faint oscillation showing at the output could have an audible effect, especially as its frequency is 15kHz or higher and won't be reproduced by the speaker. The phenomenon is interesting nevertheless.)

[TelRay]

Not sure what that means.


I would also be interested to see the signal at the plates (pins 1&6) of V3. Reason: This is the signal at the primary of the reverb transformer. The signal at the reverb input is the transformer secondary. If the oscillation is generated by V3, the reverb transformer will attenuate HF, meaning that it should show stronger at the primary.
(To be honest, I am not sure if the faint oscillation showing at the output could have an audible effect, especially as its frequency is 15kHz or higher and won't be reproduced by the speaker. The phenomenon is interesting nevertheless.)

About the SCOPE check at the REVERB INPUT I wanted to write that I need to check if it is possible to measure signal also after the Transducer locate at the Reverb Tank

I’m curious, where does the 15KHz value come from? (willing to learn)

If we are speaking about the same thing, remember post #162 in which the effect of connecting/ disconnecting the REV IN was very noticeable (is not today and can be seen only with the scope or maybe those blips of oscillation in loud passages)

[Helmholtz]

About the SCOPE check at the REVERB INPUT I wanted to write that I need to check if it is possible to measure signal also after the Transducer locate at the Reverb Tank

To me there is only "at" the reverb transducer. If the reverb input cable is connected (and not defective) the signals at the reverb transformer secondary, the reverb input jack and the reverb transducer must be identical.
I don't expect to see the HF at the reverb output as the tank is a low pass filter.

I’m curious, where does the 15KHz value come from? (willing to learn)

A very rough guess by counting (estimating) the HF peaks per division. E.g., if the peaks of your (400Hz) signal are separated by 2.5ms and the HF peaks by 0.0625ms, the HF frequency is 2.5/0.625 = 40 times the signal frequency or 16kHz.
The separation time between 2 peaks is the period T and the frequency is 1/T. So 1/0.0625ms = 16kHz.

(is not today and can be seen only with the scope or maybe those blips of oscillation in loud passages)

I think this confirms my opinion/assessment about the audibility of the HF. I may be wrong, though. But you achieved some improvement in the meantime.

[TelRay]

A very rough guess by counting (estimating) the HF peaks per division. E.g., if the peaks of your (400Hz) signal are separated by 2.5ms and the HF peaks by 0.0625ms, the HF frequency is 2.5/0.625 = 40 times the signal frequency or 16kHz.
The separation time between 2 peaks is the period T and the frequency is 1/T. So 1/0.0625ms = 16kHz.

great!

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)?

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 were did not change (.0.75 on one side and 0.58 on the other).

[TelRay]

REVERB TANK ANALYSIS w/OSCILLOSCOPE

- generated 400 Hz Sine WAVE
- placed the scope at the end of the REVERB INPUT jack in the amp (0.2 V/DIV and 0.2 ms/DIV) [NOTE: also tried at the end of the cable and the signal was clean]
- got the cleanest SINE WAVE at the maximum possible VIBRATO CHANNEL VOL (5.5) and TMB (7.5, 6.5, 7.5)



- plugged the REVERB TANK
- the waveform changes (still no oscillation)



- oscillation shows at VIBRATO CHANNEL VOL > 5.5



- zoomed in the image and measured 3,664 pixels between "sine" wave peaks and 50 pixels between peaks on the oscillation, which is about... 25 Khz (?!?!?!)

[TelRay]

I would also be interested to see the signal at the plates (pins 1&6) of V3. Reason: This is the signal at the primary of the reverb transformer. The signal at the reverb input is the transformer secondary. If the oscillation is generated by V3, the reverb transformer will attenuate HF, meaning that it should show stronger at the primary.

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