High voltage on pin 1 of V5 in '67 Super Reverb

[Don]

I just did some repair work on my '67 Super Reverb. The amp sounds good, all the voltages are within 10% of the schematic except for pin 1 of V5. According to the schematic it should be 280v. I'm reading 435v with my DMM.

The voltage is 447v on the other side of the 220k resistor where the schematic calls for 460v.

[JHow]

If I read you correctly, you say you are dropping 12 volts (447 - 435) over a 220K resistor, but the schematic suggests you should be dropping 180 volts (460-280) over the same resistor? That seems to suggest a difference in current from .054 mA to .82 mA respectively. Seems like you have lower than expected current. Are you sure about the value of 220,000 ohms for that resistor? Could it be a lot lower? Is that what the ohmeter says? I don't know what "V5 pin 1" is, not having schematic, are we talking a 12ax7 plate? Edit: is this the tremelo oscillator? Does the trem work?

[Don]

Pin 1 is the plate. It's the tube for the tremolo.

I tested the old resistor, it read about 250k. I replaced it anyway with a new one that read 220k with similar results.

[JHow]

Does trem function correctly? Any leakage across the .02 cap off that same plate?

[Enzo]

Plug in a footswitch and turn on the trem, see if that old voltage drops right into place.

When the trem is off, the grid is held down so the tube can;t conduct. No conduction means no current. No current means no voltage drop across the plate resistor.

http://www.schematicheaven.com/fende...b763_schem.pdf

[JHow]

Enzo, is open footswitch in "on" condition?

[acorkos]

closed is "on"....a shorted rca plug works, too.

[Enzo]

Shorted RCA plug works great, and if you don;t have a footswitch, you can leave one plugged in for trem always available.

On the bench I usually just run a clip wire fron the jack to the chassis to turn the trem on.

[Don]

With the tremolo footswitch plugged in and turned on the voltage fluctuates from about 200-360v. I didn't realize that would make a difference.

The tremolo works fine though it ticks louder than the tremolo on my Vibrolux Reverb.

[Enzo]

Look at the circuit. The tube grid is held negative by a line from the bias supply - that is pretty darn negative for a tube like that. That cuts the tube off so it cannot conduct. hence the B+ reading.

High plate voltage is the classic symptom of a tube not conducting - that and zero on the cathode. That generally means a loss of heater, or a defective tube. Or this.

The footswitch grounds out that bias supply line and also references the grid to ground so the circuit can come alive. Now the tube conducts and the plate voltage will drop into place. Of course now the stage is oscillating so the reading will not be stable.

[Don]

If I set the tremolo speed to 10 the voltage only fluctuates between 270 and 300v.

[acorkos]

The tremolo works fine though it ticks louder than the tremolo on my Vibrolux Reverb.

move the wires around on v5 and see if the ticking decreases. i had a 74 twin reverb that ticked until i zip-tied some of the wires together....they liked being that close together

[Jazz P Bass]

Fender Tremelo

No Vibrato
Vibrato tube going bad, either the oscillator section or the driver section; swap it and see.
Neon/LDR module going bad (Some Fenders)
Broken/open resistor, capacitor, or wiring
Faulty speed or intensity control
Faulty vibrato footswitch , jack, or wiring
"Ticking" Vibrato
Fiberboard contamination: Dust, dirt, and junk can let the LFO signal leak into the audio path. Vacuum the dust and dirt away, and if it still persists, remelt the wax top and bottom with a hair dryer.
Solder blobs from eyelets touching insulating board: Sometimes excess solder drips out the bottom of an eyelet and can intermittetly contact the insulating board, can cause ticking. Remelt the eyelets and examine the board underneath for any blobs dripped down.
Funny ground on some SF Fenders; On one of the signal tubes, the cathode cap was placed on the tube socket, and wired to a ground lug on the vibrato cancel jack instead of across the resistor on the fiberboard. The vibrato shares this ground line, and can the vibrato current can cause audible ticking in the audio path. Rewire the cap to another ground or relocate it to the board.
Poor Signal wire layout: signal wires run too close to vibrato leads can pick up the LFO signal. Move them around and see if the ticking goes away.
Bad repair/replacement foot switch cable: the Fender footswitch cable is not two conductor; it's single conductor shielded, plus single conductor. The reverb wire is shielded, vibrato wire is not. This keeps vibrato out of reverb. If you retrofit with two conductor shielded, you get vibrato ticking onto reverb audio.
Sharp tick in vibrato oscillator: On neon/LDR Fenders, on the neon bulb side of the module there is a 10M to one side of bulb, 100K to the bulb; from the 10M straight across the board is the gnd point of the LDR. Put a 0.02 cap from 10M/bulb to the ground point; this works by filtering the output of the oscillator.

[Don]

Thanks!