Hammond AO-43 Conversion

[DRH1958]

Yes, that OT should be fine. It's P-P impedance should be the same at 8k and the voltages should be close to the same- in the low 300s.

[Pdavis68]

Okay, I don't know if something has changed or if I just didn't leave it on long enough before, but with all the tubes in except V9, the current limiter light is coming back on. I tried swapping tubes to see if maybe that was it.

I checked pin 2 of both tubes and it's zero, so I think the coupling caps are okay.

It's hard to say what the plate and anode voltages are because once the tubes heat up the voltage just starts dropping from over 400 to well below 300 (I try not to let it keep running very long to avoid doing any more damage than I have to).

So with all the tubes in except the power tubes, I've got the following off the 4 B+ lines: 375V, 365V, 338V, and 311V (obviously high for lack of power tubes). And so I'm seeing 365V on the power tube screens and 375V on the plates (no drop across the OT, it seems). 0V at the cathodes, of course.

The OT seems to be okay based on the measurements I've made. There doesn't appear to be any shorts from primary to secondary. There's 400 ohms from the 2 sides of the primary (and roughly half that to the center tap from the 2 sides, and they add up to 400). The OT is giving about 0.6ohms.

I've tried my new set of tubes, same problem. So the problem is not the power tubes. I don't see anything amiss in the power tube sockets and soldering.

I disconnected the volume pot I added on the off chance it was causing a problem (it's a POT with terminal 2 going between C311 and R333 and terminal 3 going between C312 and R334, doing a phase cancellation). No difference.

The only other changes I made were in the power supply (recapping the filter caps and adding a power switch), replacing the input RCA jack with a 1/4" jack, added a 39K grid stopper to the input at the preamp grid, and I replaced the reverb 50uF cap. I can't see how any of that would be making the current draw shoot up when the power tubes heat up.

Even though the OT seems okay based on the measurements, and as reluctant as I am to say that's it, I'm starting to think there aren't many other possibilities. If nobody else has any new ideas, I guess I'll yank the OT from my Davis PA tomorrow and replace the Hammond OT with that.

[g1]

What is the wattage of the bulb being lit, and how bright is it? And with the power tubes removed it goes dim? (try right after it was lit)
The 0 volts at the grids of the power tubes should be checked when the lamp is lit up, does it stay at zero?

[Pdavis68]

Yes, the grids are 0V with the lamp lit up. I used a 200W bulb. It wasn't 200W bright, but it was well lit up with the power tubes in. Without the power tubes, it doesn't even glow.

As a comparison, I fired up my Davis 201A (a 20W PA amp, so similar power to the Hammond) and going through the 200W bulb, after the tubes are warmed up, it has a very, very dim orange glow on the filament comparable in brightness to a tube filament)..

[g1]

What kind of voltage change are you seeing on the cathode resistor when the lamp lights?
You could try a neon tester on the OT, or swap in another, as you mentioned.

[eschertron]

Remember that the primary benefit of the dim bulb tester is to get a rough gauge of the current through the PT. A secondary benefit is to allow the amp to remain powered on long enough (without blowing fuses or stuff even more expensive) to perform diagnostics. there could be something as weird going on as HF oscillation causing the tubes to conduct as heavily as they seem to be. But start with basics; voltage drop across the power tube cathodes, voltages at plate and screen once the tube reaches equilibrium, etc.

Did you confirm that there was a problem with just one tube, or just one socket? I recall reading about V9 above.

[Pdavis68]

I'll have to make another speaker cable and then I'll check this stuff out (I cut the Hammond plug so that I could use it on the Davis, so I'll need to make another one for the Hammond). Thanks for giving me some new angles to look at.

I wasn't comfortable leaving it on for more than a few seconds with the 200W bulb. I'll be more comfortable testing it now that I have a 40W bulb. Also I can use my scope to check for oscillation. It didn't occur to me.