Originally posted by Helmholtz
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VHT Special 12/20 RT Driver circuitry
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Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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Originally posted by g1 View PostFrom 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.
That's why I asked how the standby switch is wired.
OTOH, a bad switch (sticking contacts) might explain the whole problem.- Own Opinions Only -
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Originally posted by Helmholtz View PostOTOH, a bad switch (sticking contacts) might explain the whole problem.
Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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Originally posted by g1 View PostIn that case I would think the Fet and zener should be replaced.
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.
- Own Opinions Only -
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Originally posted by x11 View PostHi,
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!
Now the voltage from cathode to grid should be reading the normal ~1.5V unless the cathode resistors burned and went up in value.
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Originally posted by sparkies View Post
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.
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.
- Own Opinions Only -
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Originally posted by Helmholtz View Post
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.
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