Preamp cathode bias mystery

[barbapapa]

Dear Garage,

I have a question about my friend's bass amp that puzzles me (not hard as I'm rusty in the amp DIY department). I was hoping a better trained eye would more easily spot the fault.

The amp is a SVT-2 Pro and it has lost volume dramatically. The speakers peak at a few watts instead of over 200W.

DC voltages on the preamp tubes show everything nominal except in the very first stage (schematic attached); the V1B cathode DC voltage should be 1.5V with a 1.5K resistor, but it is at 0.3V. Plate should be 200V, but it is at 298V (much too close to HT 322V). It seems only 0.2mA idle current flows through. Grid seems fine at ground potential, the cathode and plate resistors measure correctly, the cathode bypass cap shows reasonable 27uF when isolated from the ground. Swaping the preamp tubes doesn't change the readings much.

Can the cathode bypass cap read correct capacitance and still malfunction?
Can the mute switch circuit have anything to do with this?

I'm probably not even asking the right questions, but thank you very much for any suggestion and for reading.

[pompeiisneaks]

I'm not a trained eye, but that sounds like one of 4 items that could be bad (maybe more). I am assuming you've swapped the tube and have a known good tube still doing this.

1. the anode resistor is much higher resistance than it should be so it's blocking c urrent flow
2. the cathode resistor is much higher resistance and blocking curren flow
3. The C1 Cap is 'bad' and passing all current like an open short (I'm not sure if it's reading as 'good' in the capacitance rating, but it may at low voltages with a test device but arc with a higher voltage?)
4. the cathode C2 cap is 'bad' and shunting power off the side instead of through the cathode resistor like above in 3.

~Phil

[sluckey]

What resistance do you measure from V1 pin 8 (probe directly on the socket) to chassis?

[barbapapa]

I'm not a trained eye, but that sounds like one of 4 items that could be bad (maybe more). I am assuming you've swapped the tube and have a known good tube still doing this.

1. the anode resistor is much higher resistance than it should be so it's blocking c urrent flow
2. the cathode resistor is much higher resistance and blocking curren flow
3. The C1 Cap is 'bad' and passing all current like an open short (I'm not sure if it's reading as 'good' in the capacitance rating, but it may at low voltages with a test device but arc with a higher voltage?)
4. the cathode C2 cap is 'bad' and shunting power off the side instead of through the cathode resistor like above in 3.

~Phil

Thanks, Phil,
The anode resistor measures according to the schematic, ~100K,
the cathode resistor is also in line with the schematic, ~1.5K.
If C1 cap shorted, wouldn't the plate voltage then drop to zero instead of being at 298V?
The C2 was my first suspect, but it measures good capacitance (when isolated from the ground by the -15dB attenuator switch). I was reluctant to remove it because of the good capacitance reading...

[sluckey]

What resistance do you measure from V1 pin 8 (probe directly on the socket) to chassis?

You say the resistor is good but until you make the check I suggested you don't really know if it is connected properly to the tube socket or chassis ground.

[barbapapa]

What resistance do you measure from V1 pin 8 (probe directly on the socket) to chassis?

You say the resistor is good but until you make the check I suggested you don't really know if it is connected properly to the tube socket or chassis ground.

Thanks, sluckey,
Between V1 pin 8 directly where the socket is soldered to the PCB and the chassis it was 1.48K, seemed OK...
I see what you mean, to be sure that there's a proper connection I should measure from the upper side of the PCB directly where the cathode pin meets the socket. But if there's no connection of the cathode to the circuit, wouldn't the amp be silent, i.e. only amplify whatever noise got into the circuit?

[sluckey]

I wasn't expecting an totally open circuit. Just a high resistance.

The attenuator switch may not be switching properly. Check resistance between C2 negative lead and chassis. Should be zero or 100KΩ, depending on switch setting. Also check resistance of R2 right side to chassis. Should be zero only when attenuator switch is on.

[barbapapa]

I wasn't expecting an totally open circuit. Just a high resistance.

Ah, yes, that's also a possibility; so I'm guessing that would mean that where I was measuring DC voltage, the reading is correct for the voltage across the resistor but not yet the total cathode to ground voltage across the entire resistance, so the stage might well be biased near or into cut-off territory?... I can check next time I visit there.

[barbapapa]

Yes, I was thinking about the switch too, but it seems to be intact, the negative leg of C2 measured zero ohm to ground in off mode.
The resistor measured 100K, although not right away; it dropped steadily from a high value to ~100K.

[barbapapa]

I wasn't expecting an totally open circuit. Just a high resistance.

Ah, yes, that's also a possibility; so I'm guessing that would mean that where I was measuring DC voltage, the reading is correct for the voltage across the resistor but not yet the total cathode to ground voltage across the entire resistance, so the stage might well be biased near or into cut-off territory?... I can check next time I visit there.

I measured V1 pin 8 between socket and chassis and got again ~1.5Kohm...

So now I'm totally confused.. everything around this stage works. Except it doesn't.

I measured also the resistance between the grid on pin 7 and the chassis and it shows ~170 ohm (and drops slightly when attenuator switch is on). I guess this has to do with the J176 switch in Q3. I'm not very familiar with that mute switch part of the circuit, but I'm assuming it's not affecting the strange V1B behaviour (low DC voltage on the cathode and high DC on the anode, low current at the same time).

[pompeiisneaks]

I don't think you'd want the screen to have that low of resistance that means anything that attempts to 'conduct' through the tube has a very clear path to ground and would go that way. Seems to me that something there isn't good. Normally a mute that 'dumps to ground' takes the output and dumps to ground, not the grid itself. Maybe some device in that path has gone bad?

~Phil

[barbapapa]

Thanks, Phil, I was worried about that too, although I think that's normal with the amp off; the Q3 switch works when voltage is applied to the gate and no current then flows through it.
I really can't understand how the V1B cathode voltage is at +0.3V with a 1.5K resistor while the voltage across the plate resistor shows 0.2mA idle current... We-eird...

[barbapapa]

Is it OK that there's a very loud bang from the speaker when taking the DC measurement on the grid of V5A (it looks like the grid is supposed to be at 290VDC)?

Follow-up on V1B- strangely enough it now shows normal readings, which could possibly be due to swapping the tubes around...

[Stevem]

If I am not mistaken the amp has a 1/4" TRS pre out/ power amp in jack , so by means of feeding a signal in I would first prove out that the output stage is up to snuff!

[barbapapa]

Correct and I had checked that, poweramp works perfectly, so it was then down to the preamp.