open circuit induced hummmm

[mhuss]

A customer fired up his (tube) amp with no speaker connected. After he reconnected the speaker, he heard a low hum that wasn't there before. Hum is still there with all preamp tubes removed. It works fine otherwise.

He replaced and re-biased the output tubes, but no improvement. What other open-circuited voltage-arcing failure might cause this?

https://hiwatt.org/Schematics/DR_50wOutput.gif
https://hiwatt.org/Schematics/DR_50wPS.gif

[Helmholtz]

Does the amp deliver full output power?

Is it 60Hz or 120Hz hum?

No-load operation is likely to cause excessive OT primary voltages, which may damage tubes, OT and tube sockets.

[mhuss]

I don't have the amp in hand, but he said it's still working fine otherwise, so no obvious reduction in power. The hum is 60hz with 120hz overtones.

[Helmholtz]

I don't have the amp in hand, but he said it's still working fine otherwise, so no obvious reduction in power. The hum is 60hz with 120hz overtones.

I was hoping for an output power measurement. Don't trust the ears for power assessment.

If the hum frequency is predominantly 60Hz, check both HV rectifier diodes and the bias filter cap.

Can you provide a scope pic of the hum?

[g1]

Maybe not heavy power tube damage, but just got thrown out of balance (mismatch)?

[mhuss]

He replaced and re-biased the output tubes, no change in hum, so it's not the tubes.

[nosaj]

I vote the problem is in the OT.
nosaj

[pdf64]

I wonder if he’s just become aware of a previously existing slight hum?
The NFB loop should provide some degree of protection from loading issues.

[Helmholtz]

I wonder if he’s just become aware of a previously existing slight hum?

Yes, that's a possibility I considered.

The NFB loop should provide some degree of protection from loading issues.

Not sure about that.

Missing load greatly increases loop gain and thus the risk of oscillation.

[The Dude]

I'd also check screen voltage (screen resistors) on all sockets. New tubes doesn't guarantee that they are all conducting and, since we don't have an accurate power measurement, we don't know that they are.

[J M Fahey]

Just being Devilīs advocate:

1) maybe *slight* Hum was always present, but since he got through a scary stituation "now he listens with 8 ears and looks with 8 eyes" like an old Argentine proverb says. As was mentioned above.

2) rebiasing may have gotten proper average tube curren at idle but not *necessarily* balanced both halves, so slight hum appears.

Compare Blackface "single control for all tubes" to Silverface bias balancing system to some mps having individual bias pots, either one per half or one per tube.

Not sure what system that particular amp has.

Simpler systems or even non ajustable ones such as favored by Mesa rely on using tight matched tubes; mesa even offered graded tubes; modern systems are pragmatic and recognize current tubes are all over the place.

Just fuzzy memories, didnīt old Hiwatt have non adjustable bias?
Fixed -38/-40V or something like that?

[mhuss]

This is a HIWATT-like amp, that has a single bias adjustment for the pair of output tubes. A stock amp is very quiet at idle, with mostly hiss when you turn up the volume.

He sent us an audio recording, and while it's impossible to be certain, the hum does seem excessive in comparison with the volume of the standby switch 'click.' He swears it was dead quiet before, now he can hear it from two meters away.

The hum is approximately the same with two different sets of tubes. Its possible but unlikely both are mismatched in the same ratio.

[g1]

Could a loss of NFB account for a difference of that kind of proportion?

[Helmholtz]

Schematic in post #1 shows no global NFB.

[mhuss]

There is NFB, see https://hiwatt.org/Schematics/DR_Pre4Input_v1a.gif. And that's not a bad guess (given the symptom), but it seems unlikely something in the NFB loop would fail as a result of output overload.

This would be so much easier if I had the amp in hand, sigh.