Cathode follower without elevated heater voltage

[Dark Mavis]

Okay, just gotta pop out for a bit. Here's what I got so far.
Filter caps:

Starting at the first cap -
#1 482v
#2 452v
#3 396v
#4 380v
#5 (the one over by the input jacks) 379v (not sure what's up with that. Need to check that the 10k resistor really is 10k, I guess)

Preamp tubes

First tube

Pin #1 377v
pin #6 381v
Cathodes and grids all at 0v

Second tube

Pin #1 362v
Pin #6 381v
Cathodes and grids at 0v

Third tube (assuming this one of the ones wired as cathode followers)

Pin #1 372v
Pin #6 380v
pin #7 380v
All other pins 0v

Fourth tube

Pin #1 378v
Pin #2 0v
Pin #3 0v
Pin #6 376v
Pin #7 0v
Pin #8 0v

Fifth tube

Pin #1 395v
Pin #2 0v
Pin #3 0v
Pin #6 244v
Pin #7 0v
Pin #8 2v

Sixth tube

Pin #1 208v
Pin #2 62v
Pin #3 90v
Pin #6 222v
Pin #7 60v
Pin #8 90v (jumpered to pin 3)



Edit: The 10k resistor IS 10k Unless the colour band has faded. I dunno why it's not dropping more than one volt

Okay. Power tubes:

All pin #3's 481v
All pin #4's 445v, except tube #3, which was 447
All pin #5's -40v
all pin #6's 448v

[Chuck H]

I have to assume that these measurements were taken with no tubes in?

[Dark Mavis]

I have to assume that these measurements were taken with no tubes in?

Umm...nope. Should they have been?

Edit. I mean nope the measurements were not taken with no tubes in. All the tubes were in place.

[Chuck H]

Not for initial testing to see that there aren't any obvious problems. But the only useful voltages for any operational diagnosis would need to have the tubes in place. The reason you're not seeing much voltage drop across those resistors is that there are no tubes drawing current though them. My concern right now is that you DO show some considerable voltage drop between some nodes. This could be indicative of shorted or partially shorted power filter caps. You would do well to make yourself a light bulb current limiter to use for the initial power ups between efforts. It could save you some troubles. It's just a 100W incandescent bulb in series (rather than parallel) with the wall AC.

[Dark Mavis]

Not for initial testing to see that there aren't any obvious problems. But the only useful voltages for any operational diagnosis would need to have the tubes in place. The reason you're not seeing much voltage drop across those resistors is that there are no tubes drawing current though them. My concern right now is that you DO show some considerable voltage drop between some nodes. This could be indicative of shorted or partially shorted power filter caps. You would do well to make yourself a light bulb current limiter to use for the initial power ups between efforts. It could save you some troubles. It's just a 100W incandescent bulb in series (rather than parallel) with the wall AC.

The tubes were all in place. I was asking if I should have tested with them out. All these measurements were taken with the original filter caps. I have replacements, but I wanted to get measurements either side of the swap in case there was anything to learn from it. I think I have some light fittings I can rewire to use as a current limiter.
so...the problem may have been the filter caps all along? I guess I'll replace them all right now and see what's what after that

[Chuck H]

Well if the tubes were in place then there are problems other than filter caps. And the filters may not be part of what I'm seeing with those voltages. From the first to the fourth tube and half of the fifth tube you have no cathode voltage. With tubes in place you should have cathode voltage. I think you must have a ground fault for (at least) the cathode circuits for the first four and half of the fifth tubes.

[Dark Mavis]

That would be a new issue, right? I mean, if the amp at least partially worked before, this must be something that's happened while it's been inactive over the years? Oh...I should mention: My dmm isn't automatic. It was manually set on 1000 volts DC the whole time, and doesn't measure fractions of volts at that setting. Maybe that's why it read zero.

Edit. Nah. Turned down to 200v range, where it measures to a tenth of a volt, and nothing on the cathode of the first stage. On 20v range and 2v still nothing (with 2 and 3 decimal places). I'm assuming I'm supposed to be measuring relative to ground, right? IE, the chassis?

[Chuck H]

It was manually set on 1000 volts DC the whole time. And all measurements were in reference to ground, where the common probe was clamped the whole time.

Reference to ground is fine since the chassis is being used for all grounds in this amp with no apparently isolated circuits. But the manual 1000V range could be responsible for the zero (or virtually so) cathode readings. But you do show cathode voltage for half of tubes five and tube six.?. Still, maybe retest cathode pins on the preamp tubes with the meter range set lower and see if that doesn't change anything, AND/BUT, you're voltage drops are such that I still suspect a ground fault.

[Dark Mavis]

Reference to ground is fine since the chassis is being used for all grounds in this amp with no apparently isolated circuits. But the manual 1000V range could be responsible for the zero (or virtually so) cathode readings. But you do show cathode voltage for half of tubes five and tube six.?. Still, maybe retest cathode pins on the preamp tubes with the meter range set lower and see if that doesn't change anything, AND/BUT, you're voltage drops are such that I still suspect a ground fault.

Already tried it. At least on the first stage. I'll check the others just to be thourough. Grids too?

[Chuck H]

Don't trouble with grids before we get cathode voltage. With no current through the tube there should be no voltage on the grid.

With the amp off and voltage drained, please test resistance from the preamp tube cathode pins to ground (chassis).

[Dark Mavis]

Don't trouble with grids before we get cathode voltage. With no current through the tube there should be no voltage on the grid.

With the amp off and voltage drained, please test resistance from the preamp tube cathode pins to ground (chassis).

Okay.

First tube
Pin #3 806 ohms
Pin #8 99.8k

Second tube
Pin #3 5.5k
Pin #8 5.5k

Third tube
Pin #3 2.6k
Pin #8 95.5k

Fourth tube
Pin #3 986 ohms
Pin #8 1.47k

Fifth tube
Pin #3 1.47k
Pin #8 1.47k.

Sixth tube
Both 22.5k (jumpered)

Also, it's the first and third tubes that are wired as cathode followers. The first stage in the signal path is in the second tube. But that arrangement seems to make sense in terms of where they connect to the board. When I say "first tube", I mean the one physically farthest from the power supply side of the amp, and not neccessarily where they are in the signal path

[eschertron]

If we take from your measurements that the cathode resistors are in place and wired as they should, then the reason for zero volts at the cathodes - and the reason for the high node and plate voltages - is that the tubes are not conducting. A fault in the heater wiring? If Chuck concurs, I'd suggest measuring heater pin voltages if not done already.

[Chuck H]

If we take from your measurements that the cathode resistors are in place and wired as they should, then the reason for zero volts at the cathodes - and the reason for the high node and plate voltages - is that the tubes are not conducting. A fault in the heater wiring? If Chuck concurs, I'd suggest measuring heater pin voltages if not done already.

Concur. I hadn't thought of it before you mentioned it though. I thought it would be obvious if the tubes weren't lighting so I was looking for something else.

[Dark Mavis]

Concur. I hadn't thought of it before you mentioned it though. I thought it would be obvious if the tubes weren't lighting so I was looking for something else.

They're under covers, so, I haven't looked. And the last thing I tried on this amp all those years ago was elevating the filaments on the preamp tubes. I may simply have forgotten to reconnect something. I'll take the covers off and see if the heaters are glowing, then check the voltage

[Dark Mavis]

So...across the filament windings on the mains transformer, that powers the output tubes filaments - 6.3v. Across the separate transformer that powers the preamp tubes filaments - 7.2v. Also, the first four tubes ain't glowing. I dunno if there's a little short somewhere. The tube sockets are really badly wired, so maybe. I think I']m going to rewire the filaments anyways. The wire is in the way when I'm trying to measure stuff.
Maybe the filament transformer is screwed, and that screwed the filaments? Or the other way around?
And I really don't like that little circuit board with the channel switching relay on it, that's kind of hovering right over the fifth preamp tube. It's sitting right on the filament wire. In fact...looking at it now, it looks like the filament wire isn't even connected there, but the board hid that from me

Edit. Nah. It's connected