Peavey VB-2 Heater Voltage is High

[bobloblaws]

Hi everybody, I have a Peavey VB-2 bass amp here that was brought to me with a blown F1 fuse and I found that the tubes appeared to have been running hot despite B+, screen voltage etc. looking normal. One indication was the red lettering on the JJ power tubes being prematurely faded despite limited use and limeted amp volume according to the owner. when I replaced the fuse and played on it for a few minutes I noticed a couple of the power tubes suddenly red plate (very red!) and the fuse pop again. I put in another set of tubes and it seemed to stabilize it. But I just checked the heater voltages. I have 7.6VDC on pins 2 and 7 on the power tubes, and on the preamp tubes I measure 13.1VDC on pin 4 and 6.9VDC on pin 9. Obviously those should be much closer to 6.3VDC. Anyway, I'm not sure what I can do about this, can someone advise me? Thanks.

- B

Here's the schematic, which was kindly provided to me by Enzo a couple of weeks ago. He will be missed.

PV VB-2.pdf

[Mick Bailey]

V1 and V2 don't have pin 9 connected - the heaters of each tube are in series and the voltage should be checked between pins 4 and 5. Are you sure the power tubes are on DC? The schematic implies they're AC. Whilst 7.6v is high, many amps I see run high heater voltages (Fender Devilles as an example often read 7.6v) and they have done for years without causing problems. I don't think the heater voltage is the reason for red-plating. Monitor the bias voltage and see if it's constant - I'm thinking the bias voltage may be dropping.

[Helmholtz]

Preamp heater voltage is ok, power tube and PI heater voltage is high.
But as heater voltage is fixed by the PT (and mains voltage) it has been high since long.

Redplating is caused by high plate current not heater voltage.
So check idle currents.

Power tube heater voltage could be lowered by wiring a 0.47R/10W resistor in series with the heater winding.

[Mick Bailey]

Note that if the wiper of the bias pot becomes disconnected for any reason you'll lose the bias voltage - it isn't wired fail-safe.

[bobloblaws]

Thanks, guys. To be clear, I didn't think the high heater voltage caused the red plating directly, but I thought it had prematurely worn out the tubes.

Once I had the "new" tubes in I monitored B+ and bias voltage and all seemed stable.

For what it's worth I'm measuring 13VAC from the PT for the heater circuit.

[Helmholtz]

For what it's worth I'm measuring 13VAC from the PT for the heater circuit.

Which of the 2 heater supplies?

If you have 7.6V across power tube heaters, the winding voltage should be 15.2V.

[bobloblaws]

Which of the 2 heater supplies?

If you have 7.6V across power tube heaters, the winding voltage should be 15.2V.

The 7.6V I mentioned was DC to ground. I guess I was looking for a shortcut hoping the circuit was centre tapped.

Anyway, measuring across pin 2 to pin 7 on the 6 power tubes it ranges between 5,95VAC and 6.92VAC.

[Helmholtz]

The 7.6V I mentioned was DC to ground. I guess I was looking for a shortcut hoping the circuit was centre tapped.

Power tubes heater circuit is pure AC as far as I can see.

Anyway, measuring across pin 2 to pin 7 on the 6 power tubes it ranges between 5,95VAC and 6.92VAC.

So everything is fine now?

[bobloblaws]

So everything is fine now?

Yeah, those look like the right ballpark. I guess it was a false alarm, just measuring incorrectly and juming to conclusions about what might have caused tube failure previously.

Thanks all, I appreciate the help!

[loudthud]

Power tube and PI heater wiring is shown in the lower left hand corner of page 2. Preamp tube heaters are shown on page 1 about half way up on the right. The schematic doesn't show the power transformer or much of the wiring between PCBs so for the fine details, we just have to guess. I think it goes something like this: transformer connects to power amp board at P28 and P29 (page 2). It leaves the power amp board as AC on P43 and P44, then connects to preamp PCB at P41 and P42 (page 1).

Because the transformer winding goes through a full wave bridge rectifier (D121-D124) (page 1) and the minus side output of the bridge is grounded, there will appear to be a DC bias on either side of the winding when measured to ground. This is the same reason that there would be half the output Voltage of the bridge output on the center tap of the winding if the transformer had a center tap. If you look at either side of the transformer winding with a scope, sometimes you see a really bizarre waveform due to capacitive coupling between windings within the transformer. Don't worry, this is normal behavior your tech school instructor didn't tell you about.

If you swap power tubes around, you may be able to equal out some of the differences in heater Voltage between power tubes, but it could be just losses of the PCB traces.

[Mick Bailey]

It's always best to measure heater voltages directly on the tube pins - measurements to chassis/ground can be misleading.

Edit: Without finding out what caused the old tubes to suddenly red plate, I'm thinking that this is a problem that's going to reappear. Tubes don't red plate on their own - something caused that to happen.

[bobloblaws]

It's always best to measure heater voltages directly on the tube pins - measurements to chassis/ground can be misleading.

Edit: Without finding out what caused the old tubes to suddenly red plate, I'm thinking that this is a problem that's going to reappear. Tubes don't red plate on their own - something caused that to happen.

Thanks, good to know.

[The Dude]

......Without finding out what caused the old tubes to suddenly red plate, I'm thinking that this is a problem that's going to reappear. Tubes don't red plate on their own - something caused that to happen.

^^^Yes, THAT. Have you checked bias current, yet?

[bobloblaws]

^^^Yes, THAT. Have you checked bias current, yet?

I checked the bias voltage after putting in a different set of tubes, appeared to be stable at around -48V.

[The Dude]

Good that bias voltage stable, but bias voltage does not give any clue about bias current, which is what's important.