Fender SF Twin Reverb footswitch

[Sk10]

Hey,

I have a mid seventies non-UL MV Twin Reverb. It works perfectly (recent recap....), apart from an irritating POP when engaging the vibrato via the footswitch.
This POP only occurs when hitting the footswitch, it's not the infamous 'ticking-vibrato'.
I did try another footswitch, so I suspect that the problem is caused by the amp itself.
Any suggestions how I can solve this?

[rudy kenyon]

Sk10... I had a similar one of these, a 100W Twin Special w/ a 15 inch.
From the schematic I have, the only thing that could cause the POP would be the 80uF/75V
capacitor in the negative grid bias. Since this cap's positive lead is at chassis ground, like the
vibrato pedal ground, this could be a culprit. If it were one of the vibrato's .01uF oscillator
caps, you'd have no pulse.
I cannot recall if mine popped when I engaged the vibrato. It's been 18 years!
But I hope this helps. See Ya

[52 Bill]

I would start by changing the vibrato tube.

[Tom Phillips]

The popping you describe is not a usual problem with that circuit. Has your amp always popped when you use the foot switch to turn on the tremolo or did the problem start on it's own or after someone did work on the amp?

+1 to 52 Bill's suggestion. If you don't have a spare tube then just swap it with another 12AX7 in the amp, such as the Ch 1 preamp tube and determine if the pop changes. Looking into the back of the amp the tremolo tube is the 5th one in line counting from the right side. The Ch 1 preamp tube is the first one on the right.

Cheers,
Tom

[Jazz P Bass]

There is a 50uf cap over on the right, at the voltage divider for the bias.
It may be a dud.

Link: http://www.davidsonamp.com/sf/images/twn_rev_ab763.jpeg

[Sk10]

Changing tubes didn't help. It also pops without V5.
Bias caps are OK.
I only started using the footswitch recently because I started to use the Twin at gigs. At home/rehearsal I don't need a footswitch....
What voltage should I measure at the footswitch? I currently measure -37V.

[52 Bill]

What voltage should I measure at the footswitch? I currently measure -37V.

I don't remember what the voltage at the footswitch jack is, but it must be negative to turn off the oscillator tube control grid. The switch shorts out the negative voltage through a 2M2 resistor that will normally keep the switching noise to a minimum.

If the bias supply filter caps are okay, then perhaps check the 2M2 resistor and the related wiring. Also check to see that the wire going to the footswitch jack is away from audio lines.

[Tom Phillips]

...It also pops without V5...

In that case my guess is that the POP cold be making it's way into the signal chain through the bias supply. Is the 2.2 Meg resistor correct value and are all ground points solid? Does it still pop if V5 AND the phase inverter 12AT7 are removed? If so, I would replace the 50uF bias filter cap previously mentioned.

...What voltage should I measure at the footswitch? I currently measure -37V.

That sounds about right. Theoretically the voltage would be greater amplitude (closer to the raw bias supply voltage). However, when you try to measure such a high impedance point your voltmeter loads down the voltage. (The input impedance of the meter and the 2.2M resistor forms a voltage divider)

[Sk10]

Does it still pop if V5 AND the phase inverter 12AT7 are removed? If so, I would replace the 50uF bias filter cap previously mentioned.

No more popping when the phase inverter is removed....

[Tom Phillips]

No more popping when the phase inverter is removed....

It seems that we have ruled out the bias supply path as a route for the pop. Since this is not a known common problem with the Fender tremolo circuit we are back to checking wiring dress as 52 Bill suggested and making sure the rest of the circuit connections are good especially the grounds. I'll let you know if I think of anything else.

[Sk10]

thx!
Tomorrow, I'll check the lead dress once again.....

[Tom Phillips]

thx!
Tomorrow, I'll check the lead dress once again.....

Lead dress has a big effect on the common ticking problem. However, this foot switch popping issue is confusing because I don't see where the current pulse is flowing from/to when the foot switch is closed. It will be interesting to know the results you get by moving the wires. Can you post photos of that area inside your amp?

[g1]

If you just use a jumper wire between the jack and chassis ground (no footswitch), does it still pop?
Is the rev. footswitch jack ground connection clean where it meets the chassis? Sometimes corrosion at mechanical ground connections will create strange problems. The vib. switching relies on the ground of the rev. footswitch jack as the rev. lead is shielded but the vib. lead is not.

[Pedro Vecino]

I used sometimes a 1M resistor between footswitch pins with good results.

[pdf64]

How about trying the reverb plug into the trem back panel socket, leaving the reverb socket empty?
That should identify / eliminate coupling along the footswitch cable, eg due to degraded screening.
Pete