5E7 Bandmaster

[Fletcher Munson]

Also, do you have a way of verifying your compubias unit, like another amp to check?
If power tubes are running way hotter than it shows, that would explain your low B+ voltages.

Yes, that's worth a try. The Compu-Bias was working as expected just a couple weeks ago. But you never know.

[Helmholtz]

The pulled cap was tested and found good.

Thanks. What matters is that B+ didn't change after replacing. An ecap can show full capacitance and still be bad (= leaky) especially when measured with a multimeter.

[Fletcher Munson]

Well, there are only 2 possible explanations for the low B+: Either something draws alot of current and power (and heats up after some time) - or the power supply is way too saggy.
So another idea: Are you sure that both rectifier diodes are ok and are wired correctly? If only one is operative this could explain the sagginess (does this term exist?).

The 5U4GB rectifier socket is wired correctly, and I've tried two different, brand new, tubes in there.

I've been reluctant to run the amp for long periods of time and, therefore haven't noticed anything getting especially hot. Maybe I should just let it run for awhile and do a "heat check?"

[Helmholtz]

The 5U4GB rectifier socket is wired correctly, and I've tried two different, brand new, tubes in there.

Please also measure AC voltages from 5U4 pins 4 and 6 to ground with standby on and off.

[Fletcher Munson]

Please also measure AC voltages from 5U4 pins 4 and 6 to ground with standby on and off.

Pin 4 - 370VAC (stdby) 314VAC (play)
Pin 6 - 369VAC (stdby) 315VAC (play)

[Helmholtz]

Pin 4 - 370VAC (stdby) 314VAC (play)
Pin 6 - 369VAC (stdby) 315VAC (play)

Thanks, rectifier works as supposed but voltage drop between standby and idle (15%) is very high.

[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.

[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.

[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

[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.

[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?

[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.

[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.

[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.

[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.