5E7 Bandmaster

[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]

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.

[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]

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]

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]

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.