Hammond AO-43 Conversion

[Pdavis68]

Thanks. That makes a lot of sense. I'll try that tonight. I'd also like to keep the good one if it's only one that's bad. I generally prefer SE amps anyway, so I'll find a home for it eventually.

[Pdavis68]

Okay, well it's probably not the tubes, it doesn't look like. The problem appears to be isolated to V9 (one of the two power tubes, V10 being the other). If I stick a tube in V9, but not V10, then the light on the current limiter comes on. If I instead put one in V10 but not V9, the light doesn't come on.

So I guess I need to look for shorts in that socket maybe... Anything else that comes to mind?

[Pdavis68]

And this seems odd. I was checking to see if any pins in the socket were shorted to ground and the heaters (both sides) seem to be shorted to ground. But the heaters in the tubes seem to work. When I turn it on, the heaters definitely fire up.

I'm showing about .6 ohms between the two heater wires.

[DRH1958]

You are probably reading through the 6.3v PT winding to ground since the CT of that winding is grounded. That winding will have lower DC resistance since it is a higher current winding- it has larger wire. And also, through the 12AX7s heaters if they are installed. I get the same reading on mine and it is a working amp.
How about a shorted C314, the cap in series with R335? Unsolder the leg of that cap that goes to that terminal strip.
Also, I wonder if it will help to take resistance readings to the pins with the tube installed but no power on.

[Pdavis68]

Ah, yeah, that makes sense on the 6.3V...

The readings across the primary of the OPT as I mentioned before is exactly 400 ohms. I also did it from each side to the CT and it's 213 ohms for one and 187 ohms for the other.

C314 isn't shorted, but it's only measuring 40pF. Isn't is supposed to be 1nF? Could that be the problem?

I don't have any 2KV ceramics... Best I can tell, I don't have anything I can replace it with, not even someting in the 400V range. Does it really need to be 2KV or will 400V be okay?

[eschertron]

Okay, well it's probably not the tubes, it doesn't look like. The problem appears to be isolated to V9 (one of the two power tubes, V10 being the other). If I stick a tube in V9, but not V10, then the light on the current limiter comes on. If I instead put one in V10 but not V9, the light doesn't come on.

So I guess I need to look for shorts in that socket maybe... Anything else that comes to mind?

Just to be sure: it doesn't matter which tube you stick in sockets V9 and V10, the problem follows the socket and not the tube, yes?

edit: also have you taken voltage reading at each pin with power on and no tubes installed? The current limiter is not bright under this condition?

[DRH1958]

Ah, yeah, that makes sense on the 6.3V...

The readings across the primary of the OPT as I mentioned before is exactly 400 ohms. I also did it from each side to the CT and it's 213 ohms for one and 187 ohms for the other.

C314 isn't shorted, but it's only measuring 40pF. Isn't is supposed to be 1nF? Could that be the problem?

I don't have any 2KV ceramics... Best I can tell, I don't have anything I can replace it with, not even someting in the 400V range. Does it really need to be 2KV or will 400V be okay?

That RC combination probably suppresses some high frequency problem they had. The cap being a different value will just change the frequency. I just unsoldered it on one of my amps and never got around to replacing it. No problems. I would use a higher voltage than 400 there. This probably isn't the problem if it isn't shorted but with it that far out of spec, I would just clip it off that terminal strip and see what happens.
Also, I wonder if installing the tube with no power and measuring resistance to ground from each pin and between pins. If the tube is good and the OT is good, this might be worthwhile to check just like powering the amp with no tube installed.

[Pdavis68]

Looks like the ground wire for the OPT secondary got disconnected somehow. I was just checking some stuff and the wire just popped up, so I shut everything down. I want to verify, though. There are 2 wires coming out of the secondary. A black one that goes to a terminal strip with a resistor, speaker wire and then goes to the NFB pot. The green one is ground, right? Where is it soldered in yours? I can't really tell what it was soldered to. It looks like it was around V10 somewhere.

I'll clip C314.

[DRH1958]

Yes, the green is ground and it is soldered to the ground lug on V10 in the same spot that the 130 ohm cathode bias resistor is soldered to.

[Pdavis68]

Okay. Got it resoldered a bit better this time. Decided to retube it and see if that was the issue, but no luck. :-(

I've got to go make dinner. Later tonight I'll power it up with just the rectifier tube and get some voltages and see where we stand with that.

[Pdavis68]

So with only the rectifier tube in, I'm seeing 450V on the plates and 448V and 451V on the screen grids of V9 and V10, respectively. That seems awfully high, even without the tubes in. That's over 100V above what the loaded values should be. They should be 335V at the plates and 320V at the screen grids...

[Enzo]

Well, stick ALL the tubes in, then report how high or not the voltage is. With no tubes, the B+ will not drop along the B+ string.

[Pdavis68]

How long is it safe for me to keep it on with the current draw I'm getting? I mean, I can do it with everything but V9 and it'd probably be okay, but then I don't know how valid the voltages would be on V10.

[eschertron]

I think leaving a tube out of V9 is a good idea for this test. You'll be able to estimate (ball park) the plate and screen voltages with two power tubes based on twice the drop of the one power tube in. Be sure to probe around V9 and V10 thoroughly to see if there are voltages that are oddly different between the two tubes, to get data about what may be wrong.

edit: have we discussed the possibility of C311 leaking and pulling the grid high on V9? two tests:
1. with power off and tubes pulled, measure the resistance from ground to pin 2 of each power tube socket V9, V10
2. with power on, voltage on pin 2 of each socket V9 and V10 should be zero regardless of whether a tube is in or out.

[Pdavis68]

Sweet. I'll check that tonight too. Yeah, the possibility of a leaky coupling cap came up in an e-mail with someone and so that's certainly worth checking. I hope it's not the OPT, though it occurred to me that I have a PA (a Davis 201A) that is an EL84 20 watt push pull with, I believe, similar voltages, and the OPT from that would probably be a good match for this, in that case. I'm totally cool with scrapping that PA and reusing its chassis & PT for something else (already have something in mind for it). So that makes me feel like I've still got options without spending any more money yet...