DC voltage on input mystery

[TigerG]

I’m building a circuit that for the most part is a clone of a late 50’s Airline I’ve had for about 35 years. The output and the PI are pretty well in place, and the preamp is built to the point the attached schematic shows. This part of the preamp is schematically exactly the same as the original. The original does not have DC at the input, but for some reason my build does. It’s not the tube—I’ve traded the tubes out between the original and the clone and the original still does not cause scratchy guitar-pot syndrome, while the copy does. With both parallel input stages in place, input section B in the bottom part of the schematic measures +.767vdc on the grid, -.490 at the 100k/.01 junction, and -.494 vdc at the input. This is whether or not input section A, which occupies the other triode of the same tube, is connected to the B+. Now here’s the interesting part: if input section A is disconnected from the input jack, the DC voltage at the input goes to over +6vdc, while dropping to -.88 on the grid.
Many thanks to anyone with suggestions as to where this voltage at the input is coming from.

[Enzo]

The grid of section B SHOULD have a negative voltage. That super high 6.8meg resistor to ground from the grid and the grounded cathode make this a grid-leak biased stage. Negative charge accumulats on the grid to bias the tube.



"If section A is disconnected from the input jack..."

OK, how did you do that? Pull the wire off the tube grid socket pin? Removed the 100k between the jack and pin 7 grid? Lifted some other wire?

Having +6v on the input jack doesn;t make sense in the schematic, so if it is there, it is coming from somewhere. Find out where.

How is this circuit laid out? Eyelet board? Terminal strips? There are two 100k resistors radiating from the jack, where are they? On the jack or on a board or where?

Leave the section disconnected so that 6v stays there. Now there is 6v at - I assume - the tip contact of the jack. Is there 6v on the near end of both 100k resistors? Or if the upper one was removed, is the 6v on the top end of the lower one? According to the schematic if there is 6v on the jack, it ought to travel down the wire to the resistor. So if ther is 6v there, how about the far end of the resistor. You measured -.49 there before, still get that?

Either that 6v is coming through that resistor (or the other one if it is still there), or it is on the jack for some reason. If the far end of the 100k reads higher than 6v then we are closer to the source. If the voltage down there is lower, that is not the path.

I asked about an eyelet board and layout because ssurfaces if those can get a conductive film. SO the B+ a half inch away can leak across the surface of the board causing stray voltage. You can ground your volt meter, and stick the reb probe on the eyelet board right NEXT to a wire eleyet and read voltage. Possibility.

In any case, you have two things wired to your jack, and if I can believe your schematic ONLY two things. Remove the upper 100k if not already done. 6v still there? Now disconnect the other 100k. Still have 6v? if you do, then what are the wires to the jack touching?

[loudthud]

If the plate voltage of a triode is too low, grid current can flow which would tend to put a negative voltage on the grid. But when there is a cathode resistor, the voltage can be positive with respect to ground but it will be negative with respect to the cathode. Make sense? What are the plate and cathode voltages. Verify that those plate and cathode resistors are the correct value with a meter. However, Enzo's comments about leakages could also be the cause.

[TigerG]

Problem solved!

Enzo, thanks for piping up, I was hoping you'd offer your insight. As usual, you have led the way to a solution.

The amp is built using a combination of eyelet board and terminal strips. The input end of both of the 100k resistors were soldered to an eyelet. A check of empty eyelets in that section of the board revealed around +2vdc. So I lifted the 100k's up off the board voila! no DC on the input, no scratchy guitar pot.

Thanks, man, have a great All Saints Day!

[TigerG]

Thanks!

[Enzo]

I recently cleared a conductive eyelet board by blowing a heat gun on it. Cooked the moisture content right out of it. Not always that lucky, but...

[Chuck H]

Is the board one of those damnable black fiber turds. I've seen so many of those go conductive that I would actually refuse to build an amp on one for a paying customer that requested it.

I personally owned two amps with that board material. One was rebuilt on a proper gG10 board after it went conductive and is now under new ownership. The other is gathering dust awaiting a new board and rebuild (about 5 years now) because it is also conductive. Not less than two out of three for any other amps I see built with this stuff that I test show significant conductivity.

Chuck