My A-100 now hums.

[tdintbl]

My A-100 has developed a hum only at first warmup.

- It is 60hz, not 120hz. I did check.
- It is coming from the AO-39. Tapping the AO-28 for the Leslie is without any hum.
- It tapers to inaudible over the course of about 1 minute. It does not come back until the organ is fully powered down.
- It started about a week ago.
- Reseating tubes does nothing.
- All electrolytics were replaced about 5 years ago by myself. Organ has not seen use since May, but has been in use 20 or so hours/week the last 6 weeks.
- All tubes are original, as far as I can tell. So, 1962.

My suspicion is that the rectifier is going but not gone. It goes away in about the time it would take for tubes to reach full temp. If it were one of the filter segments I'd expect 120hz, it to not fully go away, and for it to vaguely worsen when I play lots of bass or heavy clipping overdrive.

I do not have another 5U4 at the moment to swap out, otherwise I would have done so to see.

Thoughts? On the right track? If so I'll go get a 5U4.

I also have audio, but have to upload it.

[KC9UDX]

I don't know how a 5U4 could cause this. It's directly heated. So the only defect could be a difference in current at either plate, which will cause lower output voltage, rather than hum.

You can temporarily parallel another capacitor with a filter capacitor to eliminate that. But I expect that's where your problem lies.

[nonreverb]

I would try the 12AX7 first myself. Also, there may be a power tube getting old and the two not coming up at the same rate. As they're coming up, try adjusting the hum balance control to see if it makes a difference. If it does, you might be in the market for new power tubes.

[Jaim]

It is possible one of the 6BQ5/EL84's is starting to go bad. Swap the posistion of the tubes and see if that changes things.

Jim

[David Anderson]

I agree with nonreverb. These amps rely on some degree of hum cancellation in the output stage for quiet operation, and with older output tubes, they may not come up to full operating temperature at exactly the same rate. Or maybe the two 12AX7 sections are not heating up evenly. It's not an uncommon situation in tube amps with older tubes. I doubt it's the rectifier tube.

[Jyvoipabo]

From : http://www.dairiki.org/HammondWiki/CausesOfHum

Bad grounds

The impedances of the ToneGenerator output vary from a few ohms to a few tens of ohms. This is not much --- any extra resistance in the ground paths within the organ can cause hum or other funny noises.

In particular, check and tighten the ground connections at the preset panel, and the grounds at the ToneGenerator and the manuals. It is sometimes hard to tell by visual inspection whether a soldered ground connection is okay or not --- some people recommend resoldering all the grounds as a matter of course.


JP

[eblues]

...try adjusting the hum balance control....

Eh? Time to hit Google... I don't recall reading about this adjustment this before. I have the exact same hum symptom.

[KC9UDX]

You can adjust that for diagnostic purposes, but in this case, not the cure. If the hum can be reduced at warmup by adjusting the hum balance, then almost certainly it will be worse after warmup.

You can isolate a hum balance versus filter capacitor problem in this case by pulling the 5U4, waiting for the other tubes to warm up, and then plugging the 5U4 in. If the problem persists, it's a filter capacitor. If it goes away, it's an imbalance, more likely at a 6BQ5 (or less likely R20 or R21, still less R22}, or less likely the 12AX7 (or the bias resistors there).

You could see if the problem is a 6BQ5 by watching the emission curve at warmup on a tube tester. The slower tube is the problem. But really they both may (likely) be different than a new one. You can test a 12AX7 the same way by comparing the emission curve (versus temperature of course) between plates.

Because of the way the hum changes at warmup, it would be very unlikely that a bad connection is the cause (unless the change is very abrupt, which doesn't seem to be the case here; even then it's unlikely).

[Jaim]

If you look at the schematic, the "hum balance" adjustment sets the cathode bias on the 6BQ5 output tubes. The pot has the wiper to ground and the adjustment sets how much of the 100 Ohms to allocated to each tube. See attachment,



Jim

[tdintbl]

Thanks all. Learned some things already. I was under the impression when a rectifier starts going it'd introduce more ripple. Good to be corrected.

I do thankfully have a plethora of 12AX7s at a former bandmate's. Might be a few 6BQ5s in there as well, but not so sure there. Not in my lair this evening to can't do the useful tests listed, though I will be. Only test I can't do is the emissions curve, no one I know nor any businesses I know have a tube tester. Such things are truly forgotten relics around Fresno.

Now, I'm under the impression if I do have to replace the push-pull 6BQ5s, they need to be replaced as a pair yes? Then of course readjust the hum balance for best operation.

One last.. food for thought. If it's the 12AX7 (which I do not yet know) is that the price I'm paying for completely removing C1, C10, R8, and R9, and often also removing R3, R4 by way of isolating them? Basically driving that tube well beyond its intended operating range? Or is that of no concern for the life of the tube?

[KC9UDX]

Frankly, if the organ performs satisfactorily under every condition except warmup, and the test I described earlier, removing the 5U4G and reinserting it after warm-up, I'd be inclined to do one of two things: forget about it, or, install a standby circuit to turn on the 5U4 after a delay, which could be as simple as a switch inline with one of the filament legs.

You could be opening a whole can of worms otherwise.

[tdintbl]

I'm inclined to disagree. Again, I've not tried any suggestion offered as the organ (as well as all my other gear) is stashed away in my work office as they really don't mind loud organs in the back 40 after I clock out. Still, most of the suggestions have to do with ageing tubes, which are built to be replaceable. No worries there. Eventually they're going to have to go. These are at least 55 year old tubes. That is true build quality!

[eblues]

Frankly, if the organ performs satisfactorily under every condition except warmup, and the test I described earlier, removing the 5U4G and reinserting it after warm-up, I'd be inclined to do one of two things: forget about it, or, install a standby circuit to turn on the 5U4 after a delay, which could be as simple as a switch inline with one of the filament legs.

You could be opening a whole can of worms otherwise.

I want be be sure I'm clear on the test procedure... Pull 5U4G and turn on the organ to warm the other tubes, then insert 5U4G while the organ is still on?

When turning on the organ with the 5U4G pulled, am I checking to see if it still hums while 5U4G is pulled, and if so then it is a capacitor issue? Or am I checking for hum when 5U4G is re-inserted?

If it's determined to be the filter capacitor, I assume the advice will be to replace it ASAP. Will serious damage eventually occur if it's a bad capacitor and I don't replace it?

However, if it's a balance issue, then there's no potential harm to the amp and I can just leave it alone?

I don't find the hum that bothersome, since it only lasts for a minute.

[KC9UDX]

There's no damage happening in any case. If the hum doesn't bother you, then I really recommend not to do anything with it at this point. Replacing the offending component at this time could reveal deficiencies in other components. Your aged components currently work, together. But introduce a new component and you could end up with hum or distortion at other more intrusive times, and the need to do significant repairs to get all the components up to spec. Granted this is the worst case scenario, but it's possible.

However, there's not much chance for harm doing a little diagnostic work. And if you end up swapping tubes and things get worse, you can always put the old tubes back in.

So if you want to do this, this is what to do. But please be careful. There is high voltage present at the 5U4 tube. But just use common sense and you'll be fine. In this modern day it's not considered safe to do something like this, but really you have to do something extraordinary to get hurt.

1) Turn off the power and disconnect the power plug.
2) Remove the 5U4. (With one hand, and keep your fingers away from the pins, not unlike when you unplug something from a wall receptacle.)
3) Turn the power on and let the organ warm up, at least twice the amount of time that it normally takes for the hum to go away.
4) Carefully insert the 5U4. (That is, you really don't want to touch anything metal when the power is on. Unless you have a genuine metal envelope 5U4, in which case it's pretty safe to touch the tube envelope whilst plugging in it. I assume you actually have a 5U4G or 5U4BG with a glass envelope though.)

If the organ behaves as before, that is, if it hums exactly as before, then the hum is being caused by the filter capacitor, or something else related to its operation.

If the hum doesn't happen, then the problem is due to a tube cathode that's warming up slower than another cathode. This is normal. It's difficult to find two tubes that behave exactly the same. That's one of the ways that solid state components are superior. (And why you have a hum balance control.)

But really, unless the hum is detrimental to something you're doing (disturbing a live performance, for example) I wouldn't worry about it. I have several tube radios that do this, and it's just part of their character. Now if they start buzzing after warmed up, then I'll do something about it.

[eblues]

Thanks for the detailed walk-thru, KC9UDX!

I tried the test, and my organ hums about 1/2 as loud and half as long when I plug my 5U4BG back in after warm up, compared to a normal start up. Perhaps that means I have both a filter cap issue and a balance issue?

I'm a bit surprised you're not suggesting to replace the filter capacitor. But I'm not anxious to tackle that job, so unless this thread starts getting over-run with advice to the contrary, I'm going to leave things be. Currently kinda burned out on tinkering for now, after doing the hookup to a 710, and then manual felt replacements. Would kinda just like to enjoy a tidy music room for a while, and play and enjoy.