Bassman 100T

[LarBal]

Owner reports that any tube he puts in one of the 6L6 sockets redplates.
I found that the affected socket's 22r 2W grid resistor showed no reading, and the accompanying 470r Rs read 400r or so ...
I'm looking for some insight on why the grids are setup that way.
Why not short across the diodes and the 22r resistors and call it a day?
Outputs are top of pg1

bassman 100t.pdf

[Enzo]

By :no reading" I hope you mean open, rather than your meter stops working.

Why not short across the diodes and the 22r resistors and call it a day?

Because that wouldn't be fixing it. Besides if you shorted the diode, it would ground your grid. That would ensure red plating and kill any signal.

In any red plating situation, first thing we do is measure voltages on the plates and grids. You need good B+ on pins 3 and 4. And good bias voltage on pin 5. And with an open 22 ohm resistor you could not have bias at the tube.

Why would that resistor burn open? A shorted 6L6. Note the 22 ohm resistors are listed as "flame resistant".

[LarBal]

Sorry, I meant short the 22r and lift the diode off the cathode. I thought perhaps a more traditional setup might be a better a solution.
So what is the purpose of the 22r and diode grid setup?

[Enzo]

Just sitting here, I'd say the diodes protect against the grids going more positive than a half a volt or so. The 22 ohm is fusible. I'd wager the purpose is to protect the rest of the amp from tube failure. Looks like yours did exactly that. Positive on the grid being from internal short to an element with B+ on it.

[g1]

Either I'm not seeing it, or the schematic is not complete, but look at the right side of the main schematic, there is a connector P23.
That has to do with the biasing based on the power level setting. It ties in via those points at the 22R/Diode networks.

[Enzo]

Oops, I am totally stupid. I misread the diode symbol as ground, when clearly it goes to the bias detect circuit.

Look also left center, those diodes feed that as well, which is driving the warning lights.

[g1]

Yeah, I took that as a ground symbol as well. Then I looked all over for any reference to the names it was calling out, and only found that connector.

[LarBal]

There are waaay too many varieties of arrow symbols on that schematic!

[Enzo]

Well, I blame myself. I often tell people to hold a schematic at arms length and look at the whole circuit rather than some tiny part of it. I did not take my own advice. There is clearly a common usual ground symbol in use, even labelled PA_GND. The arrows point to or from, the V6_CATHODE to something and BIAS_V6 from something. That implies elsewhere on the schematic we seek those. Notice C-_RAW. That is old school all the way. C was standard notation for a negative (usually bias) supply, and the arrow points down for negative. Note the arrows for B+, Y+, and +Z all point up for positive. This is all common notation. If I had looked at the overall schematic instead of only that diode and resistor I would have seen that. No one to blame but myself, this ain't my first rodeo.