5E7 Bandmaster

[Fletcher Munson]

A couple data points with my morning coffee:

I measure 63V drop across the 10k resistor between the 2nd and 3rd filter caps. So 6.3 mA at this point. Unfortunately, I don't know the DCR on the choke.

I measure 0.1VAC at the OT center-tap. The PT is a Mercury FTWB-P.

A small mod that I failed to mention: 470 ohm / 3W screen resistors on the 6L6's.

[Helmholtz]

Instead, test voltage drop across the power rail series resistors. If a cap is shorted it'll show itself with a higher than normal current across one.

I was mainly thinking of the first 2 caps (reservoir and screen node), which don't have a dropping resistor. If one of the preamp/PI filter cap was leaky enough to significantly drop B+, the preamp supply voltage would be much lower than the measured 241V.
Also if a cap is leaky it will heat up and leakage will increase, so voltage drops may not be stable.

[g1]

Also check your AC voltage at the OT center-tap with no signal applied (looking for excess ripple).
The voltage is definitely dropping with idle current increase, so I'm more suspicious of open caps rather than leaky/shorted.

What PT are you using by the way?

[Chuck H]

Lifting the NFB did eliminate the squealing, which had been somewhat intermittent, but nevertheless it did seem to be improved. So I went ahead and swapped the OT primaries. Again, seems to be an improvement. No squealing, and the tap-test sensitivity seems diminished, though not entirely eliminated.

The low B+ voltage problem remains. So the next step is to start replacing filter caps one by one to see if I can find a problem there.

Well... One less thing Glad my hunch worked out. But I wouldn't start replacing filter caps. Assuming they're new that might just be an expensive and unfulfilling exercise. Instead, test voltage drop across the power rail series resistors. If a cap is shorted it'll show itself with a higher than normal current across one. You could do this with the choke too if you know it's DCR.

[Fletcher Munson]

Lifting the NFB did eliminate the squealing, which had been somewhat intermittent, but nevertheless it did seem to be improved. So I went ahead and swapped the OT primaries. Again, seems to be an improvement. No squealing, and the tap-test sensitivity seems diminished, though not entirely eliminated.

The low B+ voltage problem remains. So the next step is to start replacing filter caps one by one to see if I can find a problem there.

[Chuck H]

Well you only acknowledged part of what I wrote. If pulling the PI didn't increase voltages, and it didn't really, then it's less likely that the OT leads need swapping. But I suppose it's easy enough to lift the feedback loop and see.

As far as what to expect from incorrect OT lead orientation I can agree, the near ultrasonic thing hasn't happened to me either. But I have seen it manifest in many ways ranging from motor boating to intermittent frequency dependent crackling, instant whistling or a steady screech. So it wouldn't surprise me if it could also cause an ultrasonic oscillation.

[Fletcher Munson]

I tried pulling the phase inverter (V3) and saw a slight increase in plate voltage on the 6L6's... from about 335V to 341V.

So, if the OT primaries are reversed I could be getting a high frequency oscillation that I can't (or can barely) hear? In the past when I've gotten the OT primaries reversed, the noise was quite loud and unmistakable.

The easiest way to lift one end of the NFB circuit is to disconnect from the output jack. Will try that now. If the oscillation stops, I'll swap the OT leads.

[Chuck H]

Yup. Oscillation. Please try what Helmholtz suggested. And if your voltages DO increase then it may be that your OT leads need to be reversed. You can check for this a bit more easily than swapping the leads by simply lifting one end of the NFB circuit. If that stops the oscillation then you should reverse the OT leads and reconnect your NFB loop.

[Helmholtz]

Sounds like an oscillation. Do your voltages increase if you pull the PI tube?

[Fletcher Munson]

While making this video, I found sensitivity in V1 (12AY7) that is either new or I somehow didn't notice before. There is also a high pitched intermittent squealing sound. Tried swapping in an old-but-good 5751. No change. Is something mis-wired? Lead dress looks good and is typical Fender style.

[Helmholtz]

How best to test the filter caps?

Probably easiest to replace one after the other with new good ones.
Or insert a 1R resistor between negative lead and ground and measure DCV across the resistor.
Does any of the caps get hot after some time?

[Fletcher Munson]

I'm using .09" fiberboard material form Mojotone with .06" fiberboard backing. I fabricated the board myself.

I swapped in a brand new JJ 5U4G rectifier tube. No change.

How best to test the filter caps?

[Helmholtz]

I replaced the 10M with 4.7M because that's the value I see in almost every vintage Bandmaster.

As this is a feedback resistor, lower value means lower gain.

You might have a conductive/microphonic board. What is board material?

B+ in standby mode is 538V. I'm reading 368VAC coming off the transformer (rectifier pin 4).

That excludes the PT, but leaves rectifier tube and filter caps as possible culprits, as obviously something is loading down B+.

[Fletcher Munson]

I replaced the 10M with 4.7M because that's the value I see in almost every vintage Bandmaster.

I've tried three different tubes in the V2 position.

B+ in standby mode is 538V. I'm reading 368VAC coming off the transformer (rectifier pin 4).

[Helmholtz]

The junction of the two 270k and 4.7M resistors

The schematic shows a 10M resistor there.

Did you try a different tube in V2 position?

Your low B+ may indicate a weak rectifier tube, a leaky filter cap and low PT voltage. What is B+ in standby mode?