Hammond AO-39 conversion to Supro 24 cuts out at full volume

[bmccowan]

I've been busy building a cabin, but caught a bad cold which gave me a chance to get back to amps. So, I added another 9 pin socket and point-to-point rewired a Hammond AO-39 into a Supro 24 style amp similar to the Ken Watts 24 Lite. The amp sounds great with either NOS 7963s or 6cz5s. And except for some some lead dress, worked first time, whereas typically I have to hunt down a mistake.


 

[bmccowan]

Update:
Later as I tried the amp at full volume, it cut out and came back after about 10 seconds.
Clues: The earlier lead dress problem was discovered as I brought it up on a variac. It started a high pitch oscillation at about 90v AC. Chopsticking, I tracked that to the leads from the power tubes to the output transformer. They ran close to the signal path at the PI, so I moved them and corrected the initial problem. With the current issue I am thinking that an oscillation out of hearing range might be going on. or?? Any ideas before I do something silly, like shield the OT wires?
Last night I re-flowed all the connections - no luck. I will carefully trace the circuit to look for a mistake. Will post some pics when I get a break from New Years cooking.
There is no schematic for this particular version - only a layout from when Ken Watts sold a kit. I followed the layout and two schematics for similar Valco amps. Voltages check out.
Any ideas? Thanks,
B Mac

[sluckey]

Did you use new filter caps in this conversion? Have you tried substituting tubes? Is your amp built exactly like the Watts layout?

Post your measured voltages for the filter caps and all tube pins.

Connect your meter to monitor a voltage point in your amp. I'd start with the first filter cap. Try to make the amp cut out and take note if the meter reading changes during the cut out. If not, move the meter to the next filter cap and repeat. Continue moving the meter to the other filter cap, then the plates of each tube and trying to make the amp cut out.

[Colas LeGrippa]

It happened to me last week. Most of the times the filtering section is responsible. In my case, I added a 20uF in parallel with the 20uF that was already in there at a certain place, and the oscillation went away, the volume pot problem disappeared.


Colas

[bmccowan]

Thanks to both of you.
I followed the Watts layout exactly in terms of component wiring sequence. But of course since I was squeezing it point-to-point style, while reusing some of the Hammond tag boards, the lead dress is very different.
I have substituted tubes. I reused 3 of the 20uf filter caps. I think I should try replacing before I go down too many other routes. The process you outline Sluckey makes sense - will try it. Need to find my ear protection as I really cannot take it full volume for long.
Pics soon.
Happy New Year,
B Mac

[sluckey]

I reused 3 of the 20uf filter caps.

Hmmm. My original AO-39 had two cap cans. Each can had two 30µF caps. There were no 20µF caps. So which 20µF caps are you talking about?

[Greg]

Hi. I see there is no grid stopper resistors on the power tubes on the schematic. It is pretty much standard procedure nowadays to add some. If you install one with the body as close as possible to the socket lug, it might cure the oscillation which only occurs at high volume. The original vintage was probably wired PTP with the cap directly soldered on the lug, so there was not that much wire length. Values are usually around 1k to 22k. The smaller that cure the oscillation, the better.     

[Greg]

Ok, I just saw a 1k5 grid stopper in your layout that wasn't in the schem.... You could try to go larger incrementally until it stops. Or to go down until it starts. 

[bmccowan]

Happy New Year all!
That was a typo on the filter caps - I used 3 of the 30uf caps that are in the original 2x30 cans. I will be bypassing those and putting in one 20 & one 2x16. Even if the caps are not the problem - they are likely pretty tired.
Thanks for the thoughts on the grid stoppers.
B Mac

[Colas LeGrippa]

I don't think the 1k5grid resistors are causing the oscillation problem. I am pretty sure the filtering section is faulty. Grid problems due to lack or insufficient resistors value  would cause oscillation at any level of the vol pot and in many cases, wouldn't cause nothing. But I may be wrong.
Colas

[bmccowan]

Blizzard is over - power is back so I could get back to this amp. Problem solved I think!
I installed new filter caps - amp sounded tighter but still cut out at full volume, so I clipped in my meter as Sluckey suggested and saw the B+ voltage drop dramatically during the cut out. (from 320 down to 80 at the power tubes!) Although I had swapped power tubes previously I did so again with a fresh set and problem gone.
So I'm curious - All power tubes I've tried so far are 6cz5. And the tube with the issue at full volume is fine when I back off the volume. I running them at about 320v. And to my ears there is a high pitch overtone in the sound. I'm considering adding a snubber circuit.
On the other hand I may take it apart and build something else - as these amps only seem to come to life cranked, and my tinnitus, and wife, can't take that.
Or another option, I could wire in a VVR. I did that on my Z-28 type build and it sounds pretty good a lower volumes.
Any thoughts on these ideas?
Thanks for the help - I really appreciate it.
B Mac

[Apexelectric]

I’m pretty sure the 6cz5’s can’t handle the load that the 6973’s can. They are a popular substitution but might need to make sure the dissipation isn’t too high or possibly try using the 6973’s.