Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tapsnap on October 27, 2020, 09:54:26 pm
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I’ve built a Hoffman 5F2A Princeton amp and it sound great At low volume, but as I turn it up the hum becomes unusually annoying. I’ve built single ended amps before but the hum has never been this loud. The filament wires are tightly twisted and well away from the preamp area. I’ve pretty much tried everything I can think of to fix the problem, but the hum is still there. I’ve replaced the filter caps, bought new Mullard tubes, replaced all the shielded wire. Replaced the pots, coupling caps and switched to insulated jacks. A single grounding bus terminates at the jack.
Here are my voltages.
A 395v
B 330v
C 289v
R5 1.5v
R9 1.4v
R12 2.1v
Any thoughts on what I should test next.
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I had this issue on a vintage amp that did not use shunted input jack.
When I grounded the tip thru a shunt the hum went away
If you used a 12A properly wired then that issue removed.
Is the hum without a guitar plugged in?
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I understood that the amp is quiet at low volumes? So there is not any ground hum but the hum exists after volume is increased?
In this case the issue might be related to the wiring inside the amp.
Is the hum 50/60Hz or 100/120Hz?
Does the filament have center tap or have you used 100R resistors there?
/Leevi
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I understood that the amp quiet at low volumes? So there is not any ground hum but the
hum exists after volume is increased?
In this case the issue might be related to the wiring inside the amp.
Is the hum 50/60Hz or 100/120Hz?
Does the filament have center tap or have you used 100R resistors there?
/Leevi
I agree.
Pictures may help
{EDIT - untangled quote--PRR}
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I used a signal generator app and found out that this is 120 cycle hum.
It’s present regardless of whether the guitar is plugged in or not. The hum is there at low volume, but gets louder as the volume gets turned up. I’m using a power transformer with a center tap on the filament supply. 120 cycle is a filter cap problem isn’t it? I’ve changed the filter caps but it made no difference. What is a shunt on an input jack?
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gets louder as the volume gets turned up
then look left on the schematic, sloppy sockets, move wires, wiggle tubes
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Nothing I move, rattle or shake makes a difference.
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I'll share an experience I had recently.
Are you playing the amp at home, in the same room? I recently built a vintage Fender amp that has always been very quiet. But, one night I experienced the symptoms you've described: it was humming (very quiet at low volume), and it got louder as I turned up the volume.
My room is over a staircase that has a ceiling light fixture that had an LED bulb in it, and probably on the same wiring as the outlet in my room. I turned off that light and the amp became very quiet again, as it had always been before. So, my amp was picking up some type of interference/noise from the house wiring, light fixture, and/or LED bulb. :dontknow:
I just thought I'd share that.
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It looks that the amp is grounded via input jack which is ok.
The chassis is painted, check that the jack has a proper ground contact?
/Leevi
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Unfortunately, even with the photos ( and to name few ) I can't quite see if the wiring is a problem or not. And there are the solder on the potentiometers that I would like to see better.
Sorry, I can't help
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We do have a lot of LED lighting, but I turned everything off. Still the same hum. I unscrewed the input jack and used Emory cloth to clean the connection between the jack and the chassis. Still the same hum.
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gator clip a 100uf across the cathode cap for the PA tubes, any change?
re-solder your braided copper, there are 2 turrets I see that "look" to be just glued without much flow
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One grounding point works normally well in a simple SE amp. By grounding it on the input jack and using a ground bus through the amp should be enough. A good practise is to solder the bus on the pots. If you still have hum you can try to add an extra filter stage before the B+ stage.
/Leevi
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The 100uf cap has no effect. After prodding around I think the problem might be in the volume and tone controls. I’m thinking of bypassing them and connecting the two halves of the 12ax7 via the 0.022uf cap. Is this something you could recommend?
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Isolate the star grounding point (visible in the picture) from the chassis and connect it with alligator clips to the input jack ground.
/Leevi
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> even with the photos ( and to name few ) I can't quite see if the wiring is a problem or not.
+1.
The lighting is spotty; more to the point, the build is much too tight to trace-out without getting an eye inside the box.
120Hz strongly suggests the connection from the PT HV CT to the first filter cap. I don't see that; it is in the darkest crampest corner? The PT HV CT should go *direct* to first filter cap "-", no side taps. Power for the amp should come from the - of the first filter cap. If you get these out of sequence, huge (part-Volt) 120V spikes get injected all over the amplifier.
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I re-wired the volume and tone controls, but it hasn’t helped.
PRR, currently the power transformer’s high voltage center tap and the filament center taps connect to the AC power ground lug. See the photos . The red striped wire is the HV CT and the white striped wire is the filament CT. The negative side of the filter caps are connected to the bus that ends at the jack. Is all this wrong?
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The negative side of the filter caps are connected to the bus that ends at the jack.
NO! Ground the filter caps at the same place the HV CT is grounded. Do not connect them to the pot buss.
The ground symbol on the right end of the pot bus on the layout is kinda misleading. The pot bus is actually connected to the ground lug of the input jack.
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Ha, there's your hum most likely, what Sluckey says is correct. PRR is telling you the same thing.
Also, I would never ground the A/c in ground to same place as the Star ground, I always ground the A/C coming in to the chassis at the closest point, by it's self.
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PROBLEM SOLVED!!!!!!!! Thank you, Thank you, Thank you!!!!