VHT Special 12/20 RT Driver circuitry

[x11]

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!

[octal]

Hi,

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!

I think the voltages on that factory schematic are completely wrong. For example, B+3 at the power supply is shown as 218V, yet the plate/anode of V5A (in the factory schematic) is shown at 294V. It's not possible for the plate to be higher than the supply node feeding it. In particular, the grid voltage of at TP 20 of "121" is clearly wrong. There is no way it would be higher than the voltage at TP13.

I think your problem is more likely to be in the output stage. A sine wave generator and an oscilloscope would be your friend here.

[x11]

Thanks for the reply. I do have a sine wave oscillator and a oscilloscope set up. The thing is I do not know how the amp should sound and as far as I can see the 'principle' of the Watts and tremolo work fine. But then, the amp does not sound good when I use the Watts-function, especially in combination of the tremolo. So I was very happy to find a schematic with test points, so I had something to start from.
When I use a pure sine input signal of 0,5Vtt and set the Volume 1/2 and the Tone 1/2 the signal is nice and clean up to the part where it runs into the power tubes (Grid (point 6 in stead of 5)). Note: With the Watts potmeter fully open. When I turn down the Watts-pot the signal is a little topped of at the upper part.
When I measure the output (anode, point 3) or at the speaker output, the signal is topped of at both sides (most likely caused by clipping), when I turn down the Watts potmeter, the amplitude goes down and the clipping increases (!), the sine becomes a square. But testing it like this, the Watts potmeter seems to do its job well (except introducing these harmonics by clipping the signal.) So I lower my input signal so that the output signal is not clipped. Now, in triode mode the Watts-potmeter does hardly react, most likely because the input signal is to little. I asked VHT for some advise as well, but so far they did not reply.

[octal]

I suspect it's working as designed, especially if it will put out its rated power into a load with the watts knob up. It's not obvious from the schematic where the cathodes of the power tubes are connected to, but this looks like a cathode biased amp. It also uses "bias vary" tremolo. I suspect that as you lower the B+ using the "watts" knob (This is a "VVR" type circuit) the bias for the tubes is too low. (Not enough current.) It seems like a marginal design & or your tone expectations for the amp are unrealistic. You might be able to add a switch to change the value of the cathode resistors to make it more appropriate for a lower B+ setting, but you'd have to be very cautious to not forget the switch in the wrong setting with the "watts" knob up.

[x11]

Great Octal. Thanks for your reply, and you are probably right. Reducing the power by lowering the voltage and reducing the 'headroom' will lead to distortion. I find this is kind of strange way to lower the outputs power. The cathodes of the power tubes go directly to the depth 2P-6T rotary switch that will change the sound by changing the frequency response at the bias circuitry. Is the tremolo bias-vary? It is connected to the grid (point 6) of the power tubes. I assume it is pulling doen the input signal instead of the bias. Anyway thanks for pulling me through, I'll lower my expectations for this one. When not using the Watts knob, the amp sounds brilliant!

[Mick Bailey]

I've repaired a few of these amps and modded one for harp use straight from the box. All of them distorted when the power was lowered using the watts control. I haven't taken the trouble to look at load lines, but lowering the plate and screen voltage probably shifts operation of the output tubes into non-linearity (hence more distortion).

Interestingly all the amps suffered a blown control MOSFET (Q1) and the associated diode ZD1, resulting in the amp working but no output power control.

[J M Fahey]

when I turn down the Watts potmeter, the amplitude goes down and the clipping increases (!), the sine becomes a square.

Reducing the power by lowering the voltage and reducing the 'headroom' will lead to distortion.

That´s exactly what it should do,lower the maximum power so it clips earlier.
If you want to reduce olume without adding clipping yu just use the normal volume control.

As of TP20 voltages: that schematic is mess , and you simply do NOT measure grid voltage to ground in such inverters, in fact similar Fender amps carry a small label or note: "do not measure" for the very good reason that it is a high voltage ultra high impedance point and meter/scope internal resistance messes with measurement big way.
What you do is measure cathode voltage and assume grid is 1 or 2V lower than that.

You can never ever have 60V cathode grid voltage difference in a 12AX7, even less grid more positive than cathode.
And as mentioned above, plate voltage will never ever be higher than supply voltage.

Whatb you measured on cathode and plate sound very reasonable.

From what I see, I don´t like that amp design, very kludgy and contradicting , but, hey, we are not here to "redesign" but bto "repair", so that´s beyond our scope.

Just make it certain it puts out roughly what it states, meaning reaching 80% of advertised power is fine, Marketing Dept. always stretches things a bit and send it back to customer.

Tremolo cutting off part of the cycle means tremolo signal is too strong, bias tremolo does exactly that: attenuate by driving power tubes into cutoff or near that, just don´t drive them beyond cutoff or signal will be chopped, of course, set depth a little lower or, worst case if customer complains "it should not do that" add some resistance in series with depth pot, try 47k, it might be enough .

[ulval]

Interestingly all the amps suffered a blown control MOSFET (Q1) and the associated diode ZD1, resulting in the amp working but no output power control.

Hi, I have a VHT Special 12/20 (NOT RT) with blown Q1 and ZD1,

I changed Q1 and ZD1, when I put the power attenuator pot at max, the amplifier works (30Vdc at TP11 on the power tubes cathodes, PIN 8)

But when I decrease the power attenuator pot, i have bias drift, I read a value between 1 and 6V on TP11

I checked every resistors on power supply and power tubes, everything is ok

I replaced C47,C48,C49,C50 but the issue is still here

Thank you for your help

Best Regards,

[Helmholtz]

But when I decrease the power attenuator pot, i have bias drift, I read a value between 1 and 6V on TP11

TP11 voltages are given by idle (bias) current times the 470R cathode resistor.
The power attenuator lowers plate and screen voltages causing the bias current to decrease.
So TP11 voltages will drop with lower power settings.

[ulval]

Thank you for your reply,

I understand that TP11 voltage decrease, but the amplifier doesn't work when I decrease the power attenuator

I have voltage dropping from 30V to 6 V suddently, loud hum and Q1 is getting really hot

[Helmholtz]

Do TP15/TP16 voltages vary as per schematic values?

[ulval]

With standby OFF, Voltage Varies slowly at TP15/16 from 357V to 50V when I move the power attenuator (anti clockwise)

With Pentode or Triode mode Voltage drops suddenly from 328V to 5V when I move the power attenuator (anti clockwise)

After the voltage drops I can't reach the 328V with power attenuator at max (clockwise),

I have to put the amp in standby mode with power attenuator clockwise for having 328V and amp working

[Helmholtz]

With standby OFF,...

Not sure what you mean here.
The amp is either ON (B+ connected, ready to play) or in STANDBY (B+ disconnected).

When the amp is in Standby, filter caps C49, C50 etc. are disconnected from the power supply and TP15/16 voltages should slowly drop as the caps discharge.
The power attenuator can only be tested in each ON position.

I suggest to repeat voltage measurements with power tubes pulled.

[ulval]

On my amp, Standby (B+) is always connected to R69,R64,Watts Pot (pin3) and Q1's Drain that's why I can do measurement on B+1, B+2 with Standy switch in middle position (not triode or pentode)

I Tried with brand new 6L6 and same issue

[Helmholtz]

On my amp, Standby (B+) is always connected to R69,R64,Watts Pot (pin3) and Q1's Drain that's why I can do measurement on B+1, B+2 with Standy switch in middle position (not triode or pentode)

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

Maybe your standby switch is bad?