VHT Special 12/20 RT Driver circuitry

[g1]

So your amp does not correspond to the schematic?
How is standby (silencing) realized with your amp?
How is the standby switch wired?

From my understanding of his description, part S3A of the switch is always on, and only the screen supply is switched (S3B, S3C), with center off used as mute.

[Helmholtz]

From my understanding of his description, part S3A of the switch is always on, and only the screen supply is switched (S3B, S3C), with center off used as mute.

Could be, but would mean a PS wiring different from the schematic.
That's why I asked how the standby switch is wired.

OTOH, a bad switch (sticking contacts) might explain the whole problem.

[g1]

OTOH, a bad switch (sticking contacts) might explain the whole problem.

That would explain the S3A part. But what I understand is the Fet gives proper action only when the screens are disconnected, so can't deliver current. When amp is in play mode (pentode or triode) it doesn't work right. In that case I would think the Fet and zener should be replaced.

[Helmholtz]

In that case I would think the Fet and zener should be replaced.

Yes, but Fet and zener were aready replaced because original parts had failed.
I think a bad switch (intermittent contact resistance, partially shorting) might be the root cause for the failures.
Not even sure if switch and wiring are original.
Need some answers from the OP.

[sparkies]

Hi,

I do have an VHT Special 12/20 RT open at my desk. The problem is that it does not sound very good. The Watts-potmeter does function kind-of-ok at pentode-modus (it lowers the volume, but introduces distortion), and hardly does anything in triode-modus. Then, when I use the tremolo as well the tremolo introduces distortion to the sound every time it lowers the tremolo volume. I have started looking into the schematics and did some research on this forum and came to the conclusion that the schematics-testing-points-voltages are not in line with my findings, but that is a well known issue with this amps-schematics. Then, I measure a great difference in voltages (compared to the given test voltages in the schematic) at the driver stage. The driver stage is a cathode-following phase inverter, and according to the schematics I should measure about 121V at the grid (TP20), where I measure just about 11V. Then the cathodes of the Power tubes, should measure 29V, where I measure a maximum of 18V. i tried to include the original schematic as well as a redrawn part by hand with is slightly different to the original schematic and included the measured voltages.
Any help would be appreciated!

The 470K gridstop is causing over biasing at the concertina phase splitter and probably damaged the tube. replace the 470K gidstop with a piece of wire and replace the tube.
Now the voltage from cathode to grid should be reading the normal ~1.5V unless the cathode resistors burned and went up in value.

[Helmholtz]

The 470K gridstop is causing over biasing at the concertina phase splitter and probably damaged the tube. replace the 470K gidstop with a piece of wire and replace the tube.
Now the voltage from cathode to grid should be reading the normal ~1.5V unless the cathode resistors burned and went up in value.

You replied to the original thread from 2017.
We're discussing a different amp now.
BTW, grid stoppers do not drop voltage, so do not change bias - as long as there's no grid current.

[sparkies]

You replied to the original thread from 2017.
We're discussing a different amp now.
BTW, grid stoppers do not drop voltage, so do not change bias - as long as there's no grid current.

But there is something about going over 1M in the grid circuiit with self bias. Even though the 1st cathode resistor is the bias resistor. The failure would be induced by signal and lack of dissipation of the current.