Tremolo stopped working in old Silvertone 1482 amp

[slidincharlie (Carlo P)]

Hi all.
The tremolo in my '67 Silvertone 1482 amp (schematic attached) stopped working. It used to work fine until a few months ago.
Whatever the intensity and speed settings, with or without the footswitch inserted, there is simply no tremolo effect.
In this amp model the tremolo is on when the footswitch is unplugged.
I have tried two different tremolo (6AU6) tubes to no avail. On both of them the voltage on the screen grid (pin 6) is 144 V instead of 75 V per the schematic, and stays stable at any intensity or speed setting.
The 6AU6 tube is COLD and apparently with no lit wires inside when the amp is ON.
The AC and DC voltages are about 10% lower than specs in this amp (except on the two 12AX7's where they are correct), but in a previous thread some of you Ampagers stated that those voltages were not of great concern.
What happened to the tremolo and why is that screen grid voltage in the 6AU6 so high?
Thanks in advance.
--Carlo

[Jon Snell]

The screen grid voltage will be roughly half HT voltage so 1/2 X 330 = approximately 150volts.
Check you have heaters lit between pins 3 & 4, nominally 6.3volts AC.
Is the anode at 330volts? If it is, it is not conducting.
Ensure the 3 x 0.02 (20nF) capacitors are not leaky electrically.

[Helmholtz]

The screen grid voltage will be roughly half HT voltage so 1/2 X 330 = approximately 150volts.

That would ignore the additional voltage drop by the screen current.
I think the 75V in the schematic make sense.
I assume the tube isn't conducting.
Possible reasons:
1) Missing heater supply,
2) Bad heater socket contacts,
3) Open tube heater.

[Mick Bailey]

If the tube isn't conducting, remove the tube and check the voltage at the heater pins, as I've had situations where the contact has fractured internally with some tube bases. If you're getting a good reading it's worthwhile discharging the amp and re-tensioning the base contacts with a smallest size jewellers screwdriver or fine dental pick. Check the resistance between the tube's heater pins and make sure they're not corroded.

[slidincharlie (Carlo P)]

I checked all the suggested points with three different 6AU6 tubes:
- I have re-tensioned the base contacts;
- the heater voltage is 5.9 VAC between haeter pins, with or without the tube in its socket. I's low-ish, but as I said all voltages are rather lower than schematics in this amp;
- the resistance between the heater pins in all three tubes is about 3.6-3.9 ohm. The pins look nice and shiny. What is the expected resistance?
- and yes, I have HT (about 300 VDC) on the plate of the 6AU6.
Does it still seems to be tube's fault?
What points in the circuit should I check to see if and where there is an interruption?

[Helmholtz]

Make sure the cathode measures zero Ohm to ground and check the 2M grid leak resistor.

[Chuck H]

The 6AU6 tube is COLD and apparently with no lit wires inside when the amp is ON.

Do the 6au6 filaments seem to be working now? Or is the tube still cold and dark?

[Helmholtz]

What is the expected resistance?

Around 4R, so fine here.

[slidincharlie (Carlo P)]

Is the anode at 330volts? If it is, it is not conducting.

I have 300 VDC on the anode.
The grid leak resistor reads fine in the circuit.
The cathode measures 0 ohm to ground
The tube is still cold and dark although the filament voltage is 5.9 VAC with or without the tube in place.
Again, I am getting these results with three different tubes.
I haven't yet checked or replaced the three .02 caps - should I?

[g1]

Have you measured the heater voltage on both sides of the socket? (at tube side with tube removed and at lug side with tube installed)

[slidincharlie (Carlo P)]

Have you measured the heater voltage on both sides of the socket? (at tube side with tube removed and at lug side with tube installed)

I just did it and it measures the same on both sides

[Helmholtz]

I still suspect a bad (loose) heater socket contact.
Does the tube sometimes light up when you wiggle the tube in the socket?

[Chuck H]

I still suspect a bad (loose) heater socket contact.
Does the tube sometimes light up when you wiggle the tube in the socket?

The only other possibility would seem to be three 6au6's with dead filaments. Though it seems unlikely and you have continuity between the filament pins.

[g1]

Definitely pins 3 and 4?
Agree with Helmholtz must be bad connection to the tube. Can you pull the tube out only a little but enough to get your probes right onto the tube pins?

[Mick Bailey]

The three oscillator caps won't be responsible for the tube not conducting, but leaky caps will prevent it from oscillating.

Be sure to get a good look at the tube - sometimes the heater can be so far inside the cathode that it's not easy to see. Another thing to check is that there's no voltage between cathode and chassis; whilst the continuity reading looks fine, under operating conditions this may not be the case.

Is there any voltage on the grid?