5E7 Bandmaster

[Fletcher Munson]

I have a new Bandmaster (5E7) build with some issues I can't figure out. The amp is a pretty straight forward clone. I added a bias trim pot and beefed up the filter caps slightly (22uf instead of 16). The layout is vintage Fender.

The problem is noise and sensitivity in the V2 area. If I probe pin 2 I get a loud hum. If I tap on the point where pin 2 connects to the circuit board (at the junction of the two 270k and 4.7M resistors) I get a very loud thump. Just holding a chopstick to this point produces a loud hum.

I've tried swapping tubes. No change. I re-flowed all the solder joints on V2 and changed out the 100k resistor that sits between pins 1 and 6. No change. I measured plate and cathode voltages for both triodes. Pin 3 seems low:

Pin - Measured voltage - Vintage value
Pin 1 - 153V - 140V
Pin 3 - 1.3V - 2.2V
Pin 6 - 241V - 280V
Pin 8 - 152 - 140V

Could it be a bad component? One of those three resistors on the board? Something mis-wired?

[Chuck H]

Typo on the 2.2V spec for the cathode in the schematic. I'd bet on it. Should have said 1.2V. Notice that the following stage has a higher supply voltage, the same plate resistor value and the same cathode resistor value but only shows 1.7V for the cathode voltage.

Is the sensitivity on this V2 lead causing some instability in the amp in operation? Hum when touching a lead is normal. Even with a chopstick sometimes, as recently noted by one of our other posters.

Assuming your cathode voltage is correct (which I believe it is) what problem are you having with the amp in operation? I suppose with that V2 lead being so sensitive I might use a shielded cable for it. Otherwise, it's a guitar amp. It's amplifying a (roughly) 100mV signal up to over 40W!!! Even vintage amps could be considered very high gain by other types of amplifier standards and that can make some very high sensitivity. And...

The loud thump could be a microphonic preamp tube. Have you tried tapping on the tubes to see if they are just as sensitive (or more so) than that junction?

[Enzo]

I see 280v B+ and 140v at the plate, so 140v across 100k. 0.0014A Two plates 0.0028A. but common cathode resistor 820 ohm. So cathode voltage expected: 0.0028 x 820 = 2.29v

So if it is a typo then so are the plate voltages.

The second stage has the same plate stuff, so 1..4ma and 1500 ohms instead of 820, so 0.0014 x 1500 = 2.1v



Third tube has 332v and 213v, so 119v across 100k, so 0.0012A. And that across 1500 ohms = 1.78v Seems close to me on all. Or did I miss something?


His 280 is only 240 so only 88v across the 100k, and that makes 1.3v cathode like he has.

[Chuck H]

I believe the voltages posted are for what is believed to be the troubled circuit, which is V2, because the pin #6 & 8 voltages seem to represent a cathode follower. I also assume V2 is the second preamp tube though there are no V# indications on the schematic. The second preamp tube does not have a shared cathode resistor and I think the information I posted should be representative if this is the case.

[Enzo]

I know the discussion surrounds V2, I just went through the stages to find that all of them seemed to work out on paper. IOW I didn't see anything as a typo. That's all.

[Fletcher Munson]

Regarding sensitivity: the tube does not appear to be microphonic. Tapping V2 does not induce much noise; tapping pin 2 directly does. I've also tried swapping V2 for a new 12AX7. Nothing changes. The junction of the two 270k and 4.7M resistors seems the most sensitive (it's very loud) but pin 2 and the wire joining those two points all produce a lot of noise when touched.

Regarding amp operation: the thing that's puzzling me is that the amp doesn't seem to want to bias correctly. Apologies for not mentioning this sooner. I wanted to solve the noise issue first, but maybe these are related.

The amp is currently biased very cold. I see 348V on the plates (should be closer to 415V) and my bias probe shows ~40 mA of current, resulting in 13-14 watts plate dissipation. If I adjust the bias pot all the way down (minimum resistance) I get ~52 mA but the plate voltage drops down to 315V, way below where it ought to be. The tubes are still biased cold and I'd need to replace my 3.3k bias range resistor to get more adjustment.

So something's wrong with the power supply or the bias circuit. I've attached a couple pictures of my bias probe readings as well as the schematic and layout for my bias circuit.

[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?

[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]

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'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]

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]

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]

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

[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.

[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.