Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on March 12, 2022, 02:50:48 pm
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Hey guys...
I have a 50Hz hum in my new build I can't seem to find the source of..
It's a tone king imperial clone. I've built it before without hum...
I added a LarMar mv. When the mv gets past noon an audible hum is getting more present. I tried to hook it up without mv and there is the same problem.
It's not in the V1 to V4 (preamp+reverb) because it still hums when I pull those tubes. When I pull the PI tube it stops. Changed that tube but still hum.
It's not the filter caps, nor coupling caps. I paralleled another cap on all of these, one at a time.
Swapped the rectifier tube for another, swapped power tubes for others. Same hum.
There's 0.01V AC on the B+.. so.. what's left? Heaters? Hum don't change when chopsticking or moving any wire. It's on both channels...
It sounds like a grounding issue. It's the sound of an instrument cable plugged in the input, but without guitar attached.
I'll have to study the grounds.. that probably is the culprit. Do ground loops sound like that?
Oh.. and about the mv... the hum increases with the dial. But at the very end of the sweep it starts to buzz like crazy..
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Oh.. and about the mv... the hum increases with the dial. But at the very end of the sweep it starts to buzz like crazy..
Which end? Mute at CCW or full volume at CW.
Did it do that buzz when master was removed?
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Oh.. and about the mv... the hum increases with the dial. But at the very end of the sweep it starts to buzz like crazy..
Which end? Mute at CCW or full volume at CW.
Did it do that buzz when master was removed?
Full CW it buzzes. It's a new dual 250k pot, with a 2M2 resistor parallelled from lugs 2 and 3
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Redid some grounding wiring. Buzz at max is gone.. but hum is still present..
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I'll check all solders at turrets and everywhere
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I wish this was your problem but I doubt it.
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I wish this was your problem but I doubt it.
😄😄😄
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Djeeeezzzz!!! I tried everything in the book.. hum remains.
things I did:
- checked filter caps / parallelled other electrolytics too. no change
- pulled tubes 1 by 1. Hum when pulled V1 to V4. Hum stops at V5 (PI)
- put rubber isolation between chassis and PTx to prevent mechanical vibration finding its way in. no change
- dismounted the relays power supply and moved it around. no change
- dismounted the relays board and moved it around. no change
- hooked the 100+100R artificial CT for heater wiring to 6V6 cathodes. no change
- moved the grounds to various places. no change
- grounded all grids 1 by 1. no change (see attached schematic: red dots keep humming, green dots mute the hum)
- reflowed all solder connections, moved wires.. not the slightest change whatsoever...
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Are you sure it's 50Hz and not 100Hz? Have you swapped power tubes?
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Are you sure it's 50Hz and not 100Hz? Have you swapped power tubes?
Yes Steve, pretty sure about that. It's the exact same pitch like touching the tip of a plugged in instrument cable.
I also tried 6V6 tubes that run perfectly in another amp.
Here's a long shot: the amp is cathode biased, but the tremolo is connected to the 50V tap with a diode for negative bias. Is it maybe better to branch that 50V tap from the rectifier pin 6 (+diode +1M resistor) instead of the dedicated 50V tap on the winding?
Also, I used a lot of shielded wiring. Of course only 1 end of the shield is grounded, mostly at the back of its pot. There is no direct connection between those shields and any groundbuss...
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I'm going to open her up again today. My gut feeling it's something with the relays and /or wiring there...
Although I've already tried that and removed the power to the relay board. But maybe not long enough. The relays PS involves an electrolytic cap.. Maybe that wasn't discharged completely...I don't know.
Given none of the pots affect the hum except for the master implies the problem is somewhere between X and the mv. That narrows it down to the relays circuit and the PI...
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Ground the point labeled OSC right at the MV pots. Any joy?
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Ground the point labeled OSC right at the MV pots. Any joy?
That's how it's connected right now I think.
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Ground the point labeled OSC right at the MV pots. Any joy?
That's how it's connected right now I think.
No it's not. It's connected to the depth pot.
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Ground the point labeled OSC right at the MV pots. Any joy?
That's how it's connected right now I think.
No it's not. It's connected to the depth pot.
You're confusing me.. if I look at the schematic the OSC is the connection between the PT grid load resistors and the wiper of the depth pot. What do you you mean else?
The ground of the MV pots goes through the depth pot you ground, or am I drawing the wrong conclusions?
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Pulling my hair out...very frustrating.
Well, I can rule out the relays an its PS. I disconnected the PS completely to no avail.
I also removed an underboard wire that connects the 3 turrets marked as X. I jumpered the X point after the preamp to the X point at the PI, omitting the reverb... same hum.
I also disconnected the MV and put the 2x220k grid load resistors directly to ground, omitting the tremolo.. no change.
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The ground of the MV pots goes through the depth pot you ground, or am I drawing the wrong conclusions?
Exactly. And I was trying to get you to ground the MV directly to chassis which would eliminate the depth pot and tremolo circuit. Doesn't matter now though because you have eliminated all of the trem circuit.
Another source of hum is unbalanced output tubes. Have you verified that the current through each tube is the same?
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The ground of the MV pots goes through the depth pot you ground, or am I drawing the wrong conclusions?
Exactly. And I was trying to get you to ground the MV directly to chassis which would eliminate the depth pot and tremolo circuit. Doesn't matter now though because you have eliminated all of the trem circuit.
Another source of hum is unbalanced output tubes. Have you verified that the current through each tube is the same?
I already tried another 6V6 matched pair from an amp that doesn't hum. The ones in this amp are supposed to be matched. I'll order a couple of pairs anyway. My next build will run on those too.
This amp here is cathode biased.. is matching tubes that critical? Can I do something about that without tubeswap?
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Here's the hum
https://www.dropbox.com/s/rrhz72jb0s85pgy/hum%20rockin%27bird.m4a?dl=0
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Any other ideas?