Music Man HD150 hum

[g1]

Yes it sounds excessive, however, some meters will read DC as AC. Try measuring a 9V battery with the meter set to AC, what does it read?
Still, I don't think it could translate 22VDC into 50VAC. Is D7 ok?

[jvm]

I dug out the old benchtop meter and measured 4.4 VAC at D7. D7 has been replaced already and tests ok.

[Jazz P Bass]

Lift one end of R52 to isolate the supply.
Remeasure the Vac ripple on D7.

If it is still 4 Vac, parallel another cap over C31.

Something is odd, as 4Vac ripple is pretty severe.

[jvm]

I now measured 2.4 VAC at D7. ? I lifted one end of R52 and measured .05 VAC. Verified hum is no longer present at D7 by using signal tracer. However, the hum is still present at the screen resistors. So I need to look at the B+ supply next. I assume that my hum is feeding back through the +22V supply.

[jvm]

I connected my signal tracer at the 470 ohm screen resistors. I have isolated the hum to the +350 volt supply by disconnecting the OT center tap and also disconnecting the +22v supply. If I unhook the feed wire from C30 to the 470's at the tube socket, no hum. Trouble is, I can't seem to find anything wrong. I have replaced D5 and D6, subbed new caps for C29 and C30. I even lifted one end of R49 to eliminate the neon lamp. Still have hum at C30. Any suggestions where to go from here?

[nickb]

I now measured 2.4 VAC at D7. ? I lifted one end of R52 and measured .05 VAC. Verified hum is no longer present at D7 by using signal tracer. However, the hum is still present at the screen resistors. So I need to look at the B+ supply next. I assume that my hum is feeding back through the +22V supply.

You should expect some hum on the B+ and screen grids. It's of little consequence as it cancels in the output transformer.

Feeding back thru to the 22V supply? I doubt it very much unless some unexpected leakage path has developed. You should have next to no hum on the 22V supply as D12 acts in conjunction with R52 to remove most hum. I don't think 4Vrms of hum at D7 is excessively high either. My guess is D12 is misbehaving in some way. Check the DC volts at D12. Check the part number - you did say you replaced it - it could be wrong.

Afterthought: If D12 failed shorted then R52 will have been baking too. At least check it's value and consider replacing just to be sure.

[nickb]

[]Music Man HD150 with a loud hum present at screen resistors. It has been recapped but I went ahead and replaced both 100uF/450v caps to eliminate them as potential problems. When I got the amp D12(1N4748) was shorted and one power tube was dead. I replaced both and also replaced D7 but have the same hum that was present before. Hum is present with tubes removed. Any suggestions?ATTACH]29337[/ATTACH

In these days of large variations between tubes, replacing one tube unless it's a ball park match with it's partner could be troublesome. The pair won't share the current equally and so one will run hotter than the other, maybe even redplate in the extreme.

[jvm]

Thanks for the advice nickb. I am fairly certain that D12 is not the cause as I have the 22V supply disconnected at D12. I have the same hum at the screen resistors as I have at C30. R51 (150k) measures 163k. Aside from that, I can't find anything amiss in the power supply. What else could cause this?

[nickb]

Thanks for the advice nickb. I am fairly certain that D12 is not the cause as I have the 22V supply disconnected at D12. I have the same hum at the screen resistors as I have at C30. R51 (150k) measures 163k. Aside from that, I can't find anything amiss in the power supply. What else could cause this?

I guess it's not clear to me what you mean by "disconnected the 22V supply". What are the power tube grids connected to in that case? Ground I hope!

Some kind of mismatch between the two sides could cause the hum - tubes, bad driver, screen grid resistor, open grid stopper and bad OPT, bad connections are the candidates. Measure the DC voltage across R40 and R39 and we'll be able to see if both sides have the same current.

[J M Fahey]

Music Man HD150 with a loud hum present at screen resistors.

So what?
Define "loud hum" .
Do you have a loud , unbearable hum at the speakers?
How much are we talking about? (in mV AC)

Hum is present with tubes removed. Any suggestions?

Yes, probably thereīs nothing wrong with your screen supply.

I measured 370 volts at all 470 ohm screen resistors, 23 volts at D12, 16v/-16v at D13 and 14.

Then everythingīs fine.

I am using a signal tracer to verify that the hum is still at the grids with tubes removed.

Sorry but a signal traces is not a measuring instrument, such as a voltmeter or a scope.
You might have incredibly low 50mV ripple there (anything below 1VAC would be fine and up to 5VAC would not hurt *that* much) and yet high gain available in typical audio tracers might make it burst your earphones.
Not forgetting that less than ideal grounding or simply a ground loop would show high hum levels where thereīs none.

I unhooked the orange PT secondary wires to shut off the 22 and 16v supplies. Still has hum at grids.

As explained above, that hum at grids is irrelevant.
With D12 passing current and "Zenering", its internal impedance is very low and will kill any stray ambient hum being picked up (what I think you hear through tracer); with it not passing current, itīs open and the opposite happens.

This amp does not have C33.

If the holes are there, place a 100uFx35V there, with proper orientation.
If not, solder it across D12.
It will definitely help.

85 VAC at D7 and 50 VAC at D12. Seems a little excessive, no?

Still, I don't think it could translate 22VDC into 50VAC.

Cheap multimeters just put a diode in series with the DC meter they already have (crude, isnīt it?), and multiply whatever they read by 2.2 .
Might multiply by 2.3 to sort of compensate for the diode drop.
22V x 2.2= 48.4V
22 x 2.3= 50.6V
There you have it

Something is odd, as 4Vac ripple is pretty severe.

Agree. But thatīs a piss poor supply anyway.
To begin with itīs halfwave and then it has only meager 20uF filtering.
I would also put at least a 100uFx50V there, even better 220 or 470uF if available space allows it.
They placed too much confidence in that 22V Zener.

I connected my signal tracer at the 470 ohm screen resistors. I have isolated the hum to the +350 volt supply by disconnecting the OT center tap and also disconnecting the +22v supply. If I unhook the feed wire from C30 to the 470's at the tube socket, no hum. Trouble is, I can't seem to find anything wrong. I have replaced D5 and D6, subbed new caps for C29 and C30. I even lifted one end of R49 to eliminate the neon lamp. Still have hum at C30. Any suggestions where to go from here?

Donīt hate me but Iīd start again from square 1 , reasessing priorities.
You have chased and found problems which are not too relevant.
Iīd start but putting tubes back in there, setting all controls to 0, listening to the hum level at the speaker and measuring it, so as to have a starting reference amd checking whether we are improving things or not.

If no scope available, but you have a notebook (preferrable because it "floats" when run from battery) or a desktop PC you can download a free software scope and Iīll suggest a safe interface so you can measure inside your amp without blowing the soundcard input.

And, as nickb said, maybe all you have is mismatched tubes not cancelling supply (normal) ripple.
Remember that supply is even worse than a half wave one, itīs a voltage doubler, much harder to filter.

[jvm]

I reinstalled the output tubes including the new replacement (current matched) for the original shorted tube. Amp works fine with no hum at the speaker. So, as suggested by several of you, the original problem was obviously the shorted D12 and the dead 6L6. I have been trying to eliminate a hum that was there to begin with but is cancelled out by the output section. I apologize for wasting the time of so many of you. I guess this is how we learn, though. As always, I appreciate EVERYONE who lends their knowledge and experience! On the upside, I am now very familiar with the inner workings of a Music Man amp. As long as I learn something, always a good experience.

[J M Fahey]

Amp works fine with no hum at the speaker.

Happy to hear that