Hammond AO-43 Conversion

[Pdavis68]

I just completed the first stages of a Hammond AO-43 conversion. So far I've done a fairly minimal conversion. Added a 1/4" guitar jack (with 33K grid stopper and left the existing 470K grid leak resistor in there, though I was tempted to replace it with a 1M).

I added a master volume control after the PI. Based on some advice, I used a signal canceling master volume with the 2nd and 3rd pot connectors going to each of the power tube grids (well, just before the grid stoppers on the power tubes). And I added a speaker jack.

I also replaced the filter caps. I didn't connect the reverb tank (I don't think I need to, but I could be wrong).

I haven't yet put in a fuse, but I have it running through a current limiter. I turned the volume all the way down (counter clockwise) and turned it on. After the tubes heated up, it went from silence to a high pitched whistle (my wife thought it was a fireworks rocket, to give you an idea of the sound).

I did the grid stopper after the grid leak. I've seen it both ways online, so I don't know if that makes a difference.

I didn't use a shielded wire for the jack (I have some if I need it, but I was being lazy and there wasn't one there to begin with, so I figured it'd probably be okay without.

Also, I have no idea what the actual state was before I purchased it. It hadn't been turned on in a really long time, I do know that. And mice had eaten away at some of the external wires (internals were pristine) and the reverb filter cap (it had a cardboard covering that they nibbled away at the upper edges). There was a little rust on the power transformer, though not much at all and otherwise it was in really good shape and cleaned up nicely.

Any ideas on where to start looking?

[Pdavis68]

Also, one of the 6BQ5 heaters is noticeably brighter than the other. I only have the two 6BQ5s that came with it. I also have an EL84 that I could swap for one of the two 6BQ5s. (All the tubes in it are the original ones). I mention this because there was a suggestion to replace preamp tubes in a post I read elsewhere. I replaced one and was about to replace the other, but noticed the heater differences in the power tubes.

I have two 12AX7s that I know are good. I have to run out for a bit, but I'll try replacing with the two known good ones next. If I have to, I can run out and buy a couple of EL84s later today. You don't need to re-bias changing from 6BQ5s to EL84s, right? They're electrically equivalent, aren't they?

[Pdavis68]

I replaced the 12AX7s with known good tubes and it's the same. I attached an MP3 recording of the sound. An MP3 is worth a thousand words...
I'm guessing new power tubes at this point. Any opinions?

[DRH1958]

Have you installed a shorting jack for the input? It could oscillate if the input tip isn't grounded without a jack plugged in. Does the oscillation stop when you plug in your cord? Also, did you twist the input wires nice and tight since you didn't use a shielded cable? Install the 1Meg or 470k on the jack and the 33k grid stopper right on the tube pins.
For the 6BQ5, no re-bias necessary as this amp is cahtode bias. The heater looking brighter sometimes is a result of the heater extending higher up in the top assembly of the tube. The 6BQ5 and EL84 are the exact same tube.
That reverb driver circuit is the same as a single-ended power amp. The 12BH7 is the output tube and that small tranny on the end of the chassis is the OT. It, along with that 50uF cap are the way the signal couples to the reverb tank coil, so if you don't hook up the reverb, pull the tube or unhook the power to it, otherwise it will be like running an amp with no load hooked up.

[Pdavis68]

I couldn't find a shorting jack when I installed it (just found one, so I'll put that in) and no, I didn't twist the input wires, 'cause I'm a noob (I did twist the speaker wires). I'll go ahead and replace with a shielded cable.

I didn't actually have my guitar plugged in, maybe I should have tried that. But first things first. I'll put in the shielded cable. And I won't power it up again without the reverb tank hooked up. Didn't think of that as an SE amp, but makes perfect sense. Hope I haven't damaged the reverb tranny in my testing.

I'll do all this and report back when I'm done. Thanks for the advice.

[Pdavis68]

Thanks! That fixed the whine. Unfortunately, other than the mild hum of an idle amp, I've got no sound. I've gone back and checked all the input connections with my DMM and they're fine (verified with and without a plug in it to verify it was grounding without a plug) and that the resistance was 39K with the plug in (I inadvertently wrote 33K in my initial post, but the grid stopper is 39K).

There's some static when I turn the volume knob, which makes me think: 1> I need a new volume pot (used a cheapy from a Tayda Electronics in China) and 2> the output tubes are good. For now I can live with staticy volume.

When I replaced the input jack, I put the grid leak on the other side of the grid stopper, so it basically goes directly from grid to ground. It simplified the soldering a bit. The grid stopper is soldered directly to the grid pin.

So at this point, I guess the next step is probably going to be to take the oscope to it, which is a pain because for testing, I really need to do it in my shed where I have space for everything, and it's 90 degrees outside (and feels about double that insides the shed). I don't relish the idea of sweating over 340V.

Maybe I can clear out some space in my office to work on it.

[Pdavis68]

Okay, now I think I have a short. For the first time since I built it, the 200W bulb in my current limiter is coming on when I turn the amp on. It lights up, fades away, then after about 5-10 seconds, lights up again.

Any quick ways to finding a short circuit? I've already visually inspected everything I've modified and nothing sticks out.

[Pdavis68]

DRH1958, you posted a reply, but now it's disappeared. The attached image has 2 red circles showing where the master volume is wired in. If I have it turned all the way counter clockwise (volume all the way down), it should short those two points, correct?

[g1]

Above the first post at the top of page, check that the display button is set to "linear". That may be why you don't see all the posts.
Please post the schematic. Looking at the layout we can only guess what is connected to the points you mentioned.

[Pdavis68]

g1, I was referring to a second post by DRH1958, which I believe was deleted, but THANK YOU for showing me the linear layout. That's been driving me nuts. lol... (I also discovered there's a setting for it, so I set it permanently for Linear. Nice!)

The schematic of the unmodified an is attached to this message.

[DRH1958]

I posted the next items I would have done to see if things were correct and one of those things was taking voltage measurements. I posted my reply, but you got your post about there being short up first. In light of there being a short, I didn't think taking voltage measurements was appropriate, so I removed it altogether.
The fact that it changes in about ten seconds makes me think it is in response to either bias voltage building up or power tubes staring to conduct. I would replace your output tubes.
Another thing is the output tranny. If it is shorted, maybe the power tubes conducting fully going to the OT is a clue as well. Try to ohm out your OT. The primary should be maybe a few hundred and the secondary about one ohm.
It looks like you hooked up the master correctly.

[Pdavis68]

I'm pretty sure the OTP didn't short. I accidentally shorted the last output cable I made for my other amp (fortunately it didn't hurt that tranny before I figured it out), so I was extra careful with this one and I was very careful to make sure the speaker was plugged in every time I tested (once burnt, twice shy).

The OTP is measuring 400 ohm on the primary side. To measure the secondary side I'll have to desolder it, correct? Right now it's measuring 0.1 (which is actually lower than what my DMM measures connecting the two probes, 0.3), but I'm guessing it's because it's still in the circuit.

Is there an easy way to test the tubes without a tube tester? Maybe checking for shorts between plate/grid/cathode, or is it likely not that obvious?

I was going to get some EL844s anyway since I'd like a quieter version that breaks up at a lower volume anyway, but I'd sure hate to have blown these.

And thanks so much for all your help. I really appreciate it!

[DRH1958]

I suspect something to do with the output section kicking in since it starts to short after about ten seconds. This is how long the tubes take to start to conduct.
I just measured the speaker jack for the secondary resistance. The one end is grounded and the other connects to the NFB resistor. Since it is around 14k ohms and the secondary winding is in the low single digits, the NFB resistor shouldn't affect it. I suppose to make sure, unhook the secondary from everything else and measure it. It should be about one ohm or so. 400 ohms looks good for the primary.
Unless you have a direct short in a tube, you will need a tester to find partial shorts or leaks. It needs voltage on it and to detecet where the short is.
I would suspect something in the output section or filter section since it takes about 10 seconds to show a short. i suppose it could be a result of the rectifier tube,but the 5u4 has a directly heated cathode, so it develops voltage almost right away.

[Pdavis68]

Thanks. I'm going to go ahead and order some EL844s as well as a fuse holder and fuse. Seems like putting in the fuse would be the prudent thing to do given that there's obviously a problem. I'd rather blow fuses than transformers.

I'll probably desolder the OPT later tonight and measure the resistance, but I don't expect that I'll work on the amp again until the fuse & tubes arrive.

I'm also not terribly happy with the way I have the filter caps. They're inside the chassis and it's a bit crowded and messy. I think what I'm going to do is take them out, remove the can caps (which I have disconnected from the circuit, but still attached to the chassis) and then put the filter caps on a board mounted where the cans are and I'll fashion a doghouse out of scrap metal for it.

[DRH1958]

You could go back to multi-section caps.This isn't too bad a price and the clamp is $1.85 or so: https://www.tubesandmore.com/products/C-EC50-50-500 They are running a 10% off special for July. Sooner or later, you will probably use the caps you have in the amp for some project. Good time to stock up on commonly used parts, resistors, bypass caps, etc.