What makes a preamp tube bias go crazy???

[slidincharlie (Carlo P)]

The reverb stopped working in my '79 Fender sf Vibrolux Reverb.
While troubleshooting, I saw that the12AT7 reverb driver (V3) has glowing plates. Is this a symptom of bias gone nuts, I suppose?
Furthermore, the cathode voltage is around 30V instead of 6V shown in the original schematic.
This amp has the stock late-70's arrangement with a 680 ohm cathode resistor and no bypass cap. Earlier versions of this amp had a larger cathode resistor with 25/25 bypass cap.
My questions:
- what makes the plates glow?
- should I use the earlier bf-style arrangement, or am I better off leaving the stock plain 680 ohm therein?

[Gtr_tech]

replace the 680r cathode resistor.

[tubeswell]

Or would a kaput reverb tranny take down the reverb driver (apart from a dodgy cathode resistor)?

[Gtr_tech]

I've never seen it happen....s'pose its possible tho. I have seen the exact situation in question (no rev, redplating driver tube) and the K resistor had drifted way off value. I replaced it with a 1W part. The tube survived.....quite a sight it was tho. Nothing like a redplating 12AT7 to get your attention.

[martin manning]

30V across 680 Ohms is 44ma... probably ~2x the max plate dissipation... something is letting it pass far too much current.

I still consider myself a rank amateur, but here goes:

I guess I'd pull the tube and check the value of the cathode resistor just to be sure that it is still within tolerance, and then check the primary resistance of the reverb driver transformer to be sure it isn't shorted.

Then maybe suspect that the grid has lost it's ground reference and is not being seen as a negative voltage with respect to the cathode (bias gone nuts as you said). Pins 2 and 7 should measure something less than 1 Meg to ground.

The socket could be bad, i.e. not making solid contact with 2 and/or 7, so maybe clean and bend the contacts together.

The tube itself could be bad, but I'd check all the above before frying up another one.

The bypass cap on the older circuit would increase the gain of the driver. I'm guessing other differences in the circuit probably make it unnecessary, and you might get too much reverb if you add one.

MPM

[slidincharlie (Carlo P)]

Thank you guys.
1) The cathode resistor is okay.
2) The resistance between the primary leads (blue and red, right?) is 1023 ohms. Sounds in specs, doesn't it?
3) Pin 2 and pin 7 read a little less than 1 Mohm to ground.
4) Contacts 2 and 7 in the socket were retensioned.
5) I tried three different 12at7 tubes.

All the above did not change the result: the plates still glow, and at times the glow flashes along with loud noises from the reverb.

What else should I check?

[pdf64]

What makes you think that the cathode resistor is ok? The only sure test is to sub in another one.
Best to replace it with 2k2 whatever, the various SF values of 470-1k are way too low and run the 12AT7 plates around or above their max dissipation. Peter.

[slidincharlie (Carlo P)]

What makes you think that the cathode resistor is ok? The only sure test is to sub in another one.

Peter,
actually I did not mention a few details. Prior to noting the glowing plates in the reverb driver there was a 1k5 resistor on the cathode. Then I subbed in a 680 ohm new resistor, just to stick to the original schematic provided with the amp.
I might put a 2k2 resistor there, but do you think this would change anything?
If so, I'll try a 2k2.

[martin manning]

One other thing to check for is DC voltage on the grid with the reverb driver tube removed, indicating a leaking or shorted coupling cap.

In theory theory that 680 Rk will work, at idle anyway, but a larger Rk will surely add some margin. Assuming 400V supply and the 1023 Ohm DC resistance of the driver trans primary; I get about 1.7W against 2.5W max.

MPM

[pdf64]

79 is in the era of the conductive circuit board phenomina, so the caps might be fine but dc is managing to track to the eyelet with the 1M grid leak resistor. So if you do find dc there, before you replace the caps, lift one of their legs and check the voltage again.
Another possibility is oscillation - the wiring loom to the pull boost on the master vol can cause this, so try removing at the reverb end. Also has it got a ceramic cap across the reverb transformer primary? That creates a resonant peak which can tip things into oscillation, so try removing that. Peter.

[slidincharlie (Carlo P)]

Fixed!

I've found the problem
There was a bad joint at pin 7 of the reverb driver tube.
Recently I have partially rerouted wires all over the chassis in an attempt to enhance the chassis layout (which is particularly messy in late sf Fender amps). In the process I forgot to solder the wire on pin 7, so there was intermittent contact.
Now all voltages are in specs.

There is another different problem now: the reverb is always ON irrespective of the footswitch position and of the footswitch being connected or not. But this is another issue...
Thanks,

[tubeswell]

If the reverb is on irrespective of the footswitch, it sounds like the footswitch is the problem (broken ft/sw cable?) because it is not switching the reverb signal path to ground

[martin manning]

I've found the problem
There was a bad joint at pin 7 of the reverb driver tube.

Good to hear you've got it working! ing! ing!

There is another different problem now: the reverb is always ON irrespective of the footswitch position and of the footswitch being connected or not. But this is another issue...
Thanks,

Can't tell for sure from the schematic, but it looks like the reverb foot switch jack is a switching type that is supposed to ground the recovery stage grid to the reverb tank output cable shield, unless the foot switch is plugged in. If the reverb is on regardless of the foot switch being plugged in or not, I'd suspect an open connection in the wiring to the foot switch jack or it's path to ground.

MPM

[slidincharlie (Carlo P)]

...If the reverb is on regardless of the foot switch being plugged in or not, I'd suspect an open connection in the wiring to the foot switch jack or it's path to ground.

Thank you guys.
I'll check the reverb/footswitch area as soon as I get home this afternoon.

[slidincharlie (Carlo P)]

FIXED #2 !
The central wire in the reverb footswitch cable had come loose at the plug tip.