5E7 Bandmaster

[Helmholtz]

Didn't measure but should still be -44V. Will check with the probe.

That would mean that your bias probe measures 20% too low.
Actually an idle current of 32mA is a good value for 6L6s.

[Fletcher Munson]

What was the grid voltage? How do the results compare with the bias probe?

Didn't measure but should still be -44V. Will check with the probe.

[Helmholtz]

Brown (V4) to Red
R = 105.9
V = 3.4
I = .032106 or 32.1mA

Blue (V5) to Red
R = 130
V = 4.2
I = .032308 or 32.3mA

What was the grid voltage? How do the results compare with the bias probe?

[Fletcher Munson]

I say do like g1 suggests and take a current reading without the bias probe for a second opinion.

Brown (V4) to Red
R = 105.9
V = 3.4
I = .032106 or 32.1mA

Blue (V5) to Red
R = 130
V = 4.2
I = .032308 or 32.3mA

[Helmholtz]

That's an interesting question. Are you thinking there's a possibility that the amp was unstable with the previous wiring and switching them made the oscillation ultrasonic because of PFB? I might expect a decrease in audible volume in that case also though.

No clear idea yet. Just trying to get more information. NFB decreases gain and PFB increases gain. There must be an noticeable difference.
The whistling was intermittent, so results may not be reliable.

If voltages were taken with the bulb limiter, they would be low and power supply would be very saggy.

Some amps start ultrasonic oscillation when a probe is connected to a power tube plate. For this reason I don't trust bias probes that read plate voltage and plate current instead of cathode current.

[Chuck H]

A question: When you reversed the OT primary leads, did you notice a change of gain? As reversing the leads turns positive feedback into negative feedback and vice versa, there should be a marked difference in power amp gain. The correct wiring is the one with the lower gain, meaning negative feedback.

That's an interesting question. Are you thinking there's a possibility that the amp was unstable with the previous wiring and switching them made the oscillation ultrasonic because of PFB? I might expect a decrease in audible volume in that case also though.

[Helmholtz]

I adjusted the bias supply voltage to vintage spec -40:

V4
Pin 3: 379V
Pin 4: 381V
Pin 5: -40V

Then adjusted some more:

V4
Pin 3: 397
Pin 4: 399
Pin 5: -44

OK, plate voltage is looking better and bulb is getting dimmer. Then I checked the cathode current using a bias probe: 25.8mA or about 10W plate dissipation. What am I missing here?

These changes are no surprise. More negative grid voltage means less tube current and this reduces sag/increases B+.
I assume that the voltage readings were taken without the bulb limiter?

A question: When you reversed the OT primary leads, did you notice a change of gain? As reversing the leads turns positive feedback into negative feedback and vice versa, there should be a marked difference in power amp gain. The correct wiring is the one with the lower gain, meaning negative feedback.

[Enzo]

Actually, with the tubes off, there is no current path through the transformer, so other than for safety, discharging B+ caps ought not make a difference. PLEASE DO NOT DO THIS, but if you were to pull the power tubes out, a resistance reading across the transformer primary would be possible even with B+ present.

[Chuck H]

I adjusted the bias supply voltage to vintage spec -40:

V4
Pin 3: 379V
Pin 4: 381V
Pin 5: -40V

Then adjusted some more:

V4
Pin 3: 397
Pin 4: 399
Pin 5: -44

OK, plate voltage is looking better and bulb is getting dimmer. Then I checked the cathode current using a bias probe: 25.8mA or about 10W plate dissipation. What am I missing here?

I say do like g1 suggests and take a current reading without the bias probe for a second opinion.

[Chuck H]

just unplug the amp and give the caps a chance to drain.

I'm impatient with this because it seems like there's always some residual voltage fouling my readings. Even with the usual bleed resistors the caps can get down to a trickle and do that for a long time. So I let them bleed down to under 10V and then clip lead the OT CT to the chassis. Not sure if that's bad for the caps or not, to just snap them short like that. But I've done it many times.

[Fletcher Munson]

I adjusted the bias supply voltage to vintage spec -40:

V4
Pin 3: 379V
Pin 4: 381V
Pin 5: -40V

Then adjusted some more:

V4
Pin 3: 397
Pin 4: 399
Pin 5: -44

OK, plate voltage is looking better and bulb is getting dimmer. Then I checked the cathode current using a bias probe: 25.8mA or about 10W plate dissipation. What am I missing here?

[g1]

Agree with Chuck, set your bias voltage to -40V as shown on schematic. Now what is the pin3 voltage at power tubes? Now try the bulb, is it dimmer?
You don't have to disconnect the OT wires to check the resistance, just unplug the amp and give the caps a chance to drain.

[Chuck H]

It's really all about the current, but -30V seems very low. You're going to need more like -40V or -45V


Note: I felt a little silly just now calling a number that is less negative "lower" than a number that is more negative. I hope there's no confusion

[Fletcher Munson]

Apologies for the delayed response. No time to work on the amp yesterday.

What is your grid voltage (pin 5) at the power tubes?

All measurements to ground (V4 and V5 were identical):

Pin 3: 333V
Pin 4: 337V
Pin 5: -30V

You could double check idle current by the method where you record the resistance values of the OT primary halves, then measure the idle voltage across same points and calculate current.

Not familiar with this. Do I need to lift the wires to take the resistance measurements?

I checked my Compu-Bias probe against another amp. Everything looks to be working correctly. I also plugged the Bandmaster into a dim bulb limiter and it's making the bulb glow more than I'd expect. Not a precise measurement but it supports the idea that something is drawing too much current.

[g1]

What is your grid voltage (pin 5) at the power tubes?
You could double check idle current by the method where you record the resistance values of the OT primary halves, then measure the idle voltage across same points and calculate current.