Volume pots as grid-leak resistors: workarounds?

[Rhodesplyr]

A 1950s amp I'm working on uses two 1Meg volume pots with the pot wipers connected to the control grids of a 6SC7, and these pots are the only ground-reference that the control grids have. I found that the 6SC7 in this position in the amp had no emission left, suggesting to me that it may have been damaged by excessive current draw. (The filament works; it gets warm.)

I was wondering if there are any ways to keep the same basic circuit, since a complete redesign is out of the question, while protecting the tube from a condition where the control grids are completely disconnected from ground. I know you could increase the pot value and use a parallel resistor, but that would also increase series resistance into the triode, increasing the Miller effect.

I suppose that one way of approaching it might be to use modern higher-quality pots like PEC. The 1950s originals are pretty bad; rotating them with an Ohmmeter attached gives readings that jump all over the place, and they have no openings for spraying contact cleaner inside.

Other ideas?

[jrfrond]

One way around this is to reverse the hot and wiper leads of the pot. In this case, if the wiper does open up, there's no issue except for signal loss. It's also a constant load. As a tangent, this is the preferred way of wiring a PPIMV, especially if a treble cut ("Briiliance" ala Vox) control is employed. If you want redundancy, you could try and find a dual 2M pot and wire both sides in parallel, but good luck with trying to find one.

[Rhodesplyr]

Thanks, John.

In this circuit, the hot lead of the pot connects directly to the guitar input, so, if you swapped them, you'd be varying the input impedance of the amp as you changed the volume control. I'm not sure how well that would work. Since this amp has two of these input pots connected to the same 6SC7, and since it's unlikely that the 2nd input will ever be used, perhaps I could wire the 2nd input with a fixed resistor rather than a pot. Since the 6SC7 has a common cathode, one grid would tend to put on the brakes even if the other one lost its ground connection. That may be what they were counting on when they built it, not thinking about what might happen >50 years later when both pots had seen better days.

6SC7.pdf

[Enzo]

I think you are worried too much. SO what if the impedance to the guitar changes? The input will still work, the control will still adjust the level. That arrangement has been done before.

As to burning out the tube, perhaps age was a factor? COnsider an open grid. Let us thing about what would happen. What is the plate resistor? And the cathode resistor? Add them up. What is the B+ at that node. For example, if B+ was 250v, and the plate resistor was 100k - ignore the 1.5k cathode of a typical circuit. Then the tube as a dead short would only have to conduct 2.5ma. So what are the numbers in this amp? Looking at the 6SC7 sheet, I don't think 2.5ma would destroy the tube very soon.

This is not a power tube.


Cleaning pots? Replacement is not expebnsive, but many pots are assembled with little tabs folded over to keeop the cover n. Unbend them and the cover comes off for cleaning, then put it back and rebend them.

[imaradiostar]

You could always add a high value grid leak between the wiper and ground. Use a high value like 2.2 or 3.3 meg and it's unlikely to cause an audible difference but it should be enough to keep all but the gassiest of preamp tubes from running away. Lots of commercial applications (maybe not lots, but some) have an added resistor like this by design in case a pot opens up.

I like what Enzo said- even if the tube is a short, how much damage can it really do?

jamie

[Chuck H]

The standard amplifier volume control as used on Fenders, Marshalls and pretty much every other amp has the following grids 0V reference on the wiper of a pot. Why do we need to fix something that's been working so well for so long? I think the best answer so far is to add large fixed grid leak resistors. You won't find this on manufactured amps because in complicates the design and adds to parts count. There's nothing to stop the home builder from being extra careful. I don't do it though.