Hammond M3- 5U4GB tube overheating

[Hugh J]

Got an old Hammond from a coworker and after reading topics on this forum I was able to get the motors running and tubes glowing. Despite this progress I still have no sound. Everything is oiled and all tubes have been replaced. I saw somewhere on this forum that none of the tubes on a Hammond should have a high enough heat to burn at the touch but the 5U4GB, the biggest one farthest left from the back, is. I'm not sure if this is the cause of the sound issue or is an issue at all but any advice helps. Thanks

[gtc]

Typically, two types of tubes will run too hot to touch: rectifiers (such as the 5U4GB) and power output tubes.

However, if they are running 'red in the face', that is their plates are glowing, then that indicates they are overworking -- usually caused by a shorted or nearly-shorted component, such as a capacitor that needs replacing.

BTW: with old gear, it is usual practice to replace original electrolytic and paper capacitors as a matter of course as they are well past their use by date. The capacitors in the tonewheel generator are another subject. They are not in high current circuits; they are part of filter circuits and affect the tone generation and some like to leave them original.

Just a reminder: if you intend to do any of that work yourself, be aware that lethal voltages exist and serviceman precautions are needed.

[Hugh J]

I'll start working on the capacitors then, figured it would need redone eventually anyway. I appreciate the advice greatly

[muckelroy]

"No sound" was your description of the problem, but even if the amp is malfunctioning, you should typically at least hear a little bit of hum coming out of the speaker. There could be dirty connectors, dirty tube socket pins, loose wires.. Lots of things to check here.

Among the usual work on these amps, you can do a very basic test of your 5U4. Use a tube tester to check for shorts. If you do not have access to a tube tester, you could at very least pull your 5U4 (when it's cool of course) and use a multimeter to check for shorts. Pin 4 and 6 should not short to any other pins. Pin 2 and 8 should be connected to one another (that's the cathode / heater), but they should not short to any other pins.

Sometimes the tubes will only short when they are hot or conducting, and you can't test that without taking measurements in-circuit while the amp is on, or by using a decent quality tube tester. 5U4 tubes are inexpensive enough that you could replace them, and not stress over it if in doubt.

Fuses are a good idea, too. Rectifier shorts can easily cause power transformer meltdowns if not protected by a fuse. There's enough information on the forum that you can research this topic. No single fuse is foolproof, but a 1 amp slow-blow on AC PT primary is better than no fuse.

[gtc]

Yes, there's a logical method to follow when diagnosing faults. The usual practice is to start from the output and work backwards, however checking that the power supply is providing the correct voltages is also a good starting point.

Note that the M-3 has an electromagnetic speaker (i.e. its magnetic field is created electrically, not via permanent magnet as in more modern speakers). It also has a B+ voltage disconnect feature built into the connecting plug between pins 2 and 4. That is to prevent the organ's output tubes running into a no-load situation when the speaker is disconnected.

Again, be very careful around these voltages.

[muckelroy]

Note that the M-3 has an electromagnetic speaker (i.e. its magnetic field is created electrically, not via permanent magnet as in more modern speakers).

Right, though later M-3 organs did have permanent magnet speakers.

[gtc]

Right, though later M-3 organs did have permanent magnet speakers.

Good to know. Will be interesting to hear back from Hugh J on what he has.

[David Anderson]

Also, in earlier M-3s, the speaker field coil is essentially the cathode resistor for the output stage.

In diagnostics, it's pretty standard for techs to start with the power supply because it's common to all stages. Any amp is basically a modulated power supply, and if the power supply isn't healthy, nothing will work right.

[Hugh J]

Found burnt wires around a charred transformer, the smaller one next to the vacuum tubes, and I think this may be causing a short of some sort. I believe this M3 was made in 1960, serial number "137530".

[gtc]

Well, that's certainly toast!

From your photo that appears to be T3, the audio output transformer, sitting between C62 and C14.

[Jyvoipabo]

Hello,

I think also this is T3 : the output transformer.

It is supplied with 310V and a short to ground must explain the 5U4 failure but no more.

We see that the short circuit is in the wiring.

To be sure this transformer is damaged insulations must be checked looking for a short with the ground or input-output.

JP

[gtc]

Also check what tubes are actually in that amplifier vs the correct tube listing. It's not unknown for people to stick any tubes that happen to fit the sockets into equipment, sometimes just to make it look complete.

Edit: I note that you did mention that all tubes have been replaced, however were they correct in the first place? Did you buy a complete tube set from (for example) an eBay seller or replace one by one what was already in it?

[Jaim]

The type of OPT used in the M3 is dependent on whether it drives an EM speaker or a PM speaker. The secondary in an EM type has a grounded center tap.

Jim

[David Anderson]

The type of OPT used in the M3 is dependent on whether it drives an EM speaker or a PM speaker. The secondary in an EM type has a grounded center tap.

Not quite true. There are several running changes in the AO-29, and there is a version (schematic is in the manual) that has the field-coil speaker and an output transformer with no center tap, probably because the center tap was deemed unnecessary.

In later M3s, there is negative feedback around the OPT, so a ground reference is required, but not a center tap.

[Hugh J]

The T3 being destroyed wouldn't have an effect on the amp in the organ right? From what I can tell it's just a transformer for the separate Leslie. I'll check the tubes and make sure they're in the right order, most were missing when I got it so I put them in wherever they fit and glowed.