Help with 60Hz hum in Foreplay I in custom chassis

atalcott · 12548

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Offline atalcott

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Reply #15 on: July 02, 2020, 10:23:00 AM
Awesome, thanks for the suggestions. I'll try these maneuvers today/tonight and report back.

That Hammond 155J choke is in place of the second power supply resistor.



Offline atalcott

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Reply #16 on: July 03, 2020, 06:51:17 AM
No joy. After rerouting the grounds for the power supply caps and the terminal where the tube grounds attach as you describe I tried a few different configurations with the chassis and signal grounds. Without the ps grounded to the chassis things got way noisier, varying from very loud 60hz hum with the power cord ground connected to the chassis, to hum+buzz without it attached. Reconnecting the ps ground to the chassis at terminal 3 got it back to its usual very quiet hum. I tried connecting the signal ground to the chassis at the first coupling cap  terminal strip and then back to the “tube-ground” terminal, but the hum was the same in both configurations.

Things are getting messy enough in there at this point that I’m definitely looking at a full tear down and rebuild once I figure out a layout that gets rid of the hum.



Offline Paul Birkeland

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Reply #17 on: July 03, 2020, 07:48:15 AM
Does your power amp have a 3 wire power cord?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline atalcott

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Reply #18 on: July 03, 2020, 07:52:39 AM
Does your power amp have a 3 wire power cord?

It does not, no.



Offline Paul Birkeland

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Reply #19 on: July 03, 2020, 08:51:21 AM
Well, adding a 3 wire cord won't hurt anything.

I finally found a diagram of the KT-550 and it's a very high gain amp (30+dB).  With everything powered up and running, what AC voltage do you get at the speaker terminals? 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline atalcott

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Reply #20 on: July 03, 2020, 05:27:12 PM
The KT-550 manual rates the input sensitivity at 1v RMS for 50 watt output. The pre-amp that was designed to go with it was the KT-600. Schematic here: http://www.mcmlv.org/Archive/HiFi/LafayetteKT600.pdf.

With everything up and running and no input signal, just the hum present, I'm reading 0.000VAC with my Fluke 115 across the speaker load. With an input signal the voltage varies, naturally. At low-to-moderate volume the peaks look to be about 0.6VAC. At moderately loud the peaks are up to around 1.6VAC.

I will try installing a 3-wire power cord on the power amp in the next couple days. It's been on my to-do list anyway.

The hum is pretty dang quiet, but definitely noticeable in between songs at ~8 ft. from the speakers as I sit here.



Offline Paul Birkeland

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Reply #21 on: July 03, 2020, 07:32:45 PM
You could definitely try some -12dB Harrison Labs attenuators at the input of the amplifier.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline atalcott

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Reply #22 on: July 04, 2020, 12:25:39 PM
Just installed the 3-wire cord on the power amp--no difference in the hum. :|

I ordered the attenuators too. We'll see how those do.



Offline atalcott

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Reply #23 on: July 08, 2020, 04:11:42 PM
Alright, while I'm waiting for those attenuators, a little update. I've been reading whatever I can find on grounding and hum issues, including this page http://cognitivevent.com/av_hum.html, which has some specific info about the Foreplay. I haven't had any big "ah hah" moments about my problem in particular but it's been good to delve more deeply into the principles. I tore down all the ground and signal wiring and rebuilt it based on what I've gathered. Pics attached. The first is a general overview with the VA/CF boards in their normal spots, the second two are closer views of the two sides of the thing with the boards moved out the way.

  • Twisted pairs for the input signal wires, with grounds split off from the signal wires near the source selector switch and attached to a terminal where the volume pot ground is also attached.
  • I debated about where to ground the output jacks, since the original FP has their first ground at the output jack grounds and then on to the volume pots, but I also read in several places that the output jacks should be grounded as close to the cathodes grounds of the first stage as possible. I decided on the latter, so ran a twisted pair from the output jack grounds to another terminal on the same strip. The cathode and heater center tap grounds also go to this terminal.
  • The grounds for the VA/CF boards go to the third terminal on this strip.
  • I then have a 18awg twisted trio from these terminals to the ground of the third filter cap.
  • Shortest possible 18awg wires run from the negative terminals of the second and third filter caps to back to the negative terminal of the first filter cap.
  • I've tried connecting the PS to the chassis at the third filter cap ground, first filter cap ground, and not at all. I've ended up leaving it connected to the third filter cap ground.
  • Lastly, since I've got those attenuators on the way and Mr Joppa mentioned my attenuating resistors possibly affecting the treble, I removed those and the 10ks between output and ground and reinstalled 470k resistors between output and ground.
  • For S&Gs I put rubber washers between the PT and the chassis plate.
  • Also tried adding a .1uF cap with a 10k resistor across it between the third filter cap ground and the chassis, based on the grounding schematic on the page linked above. I confess I don't understand exactly what that's supposed to do, but I had the parts and it seemed worth trying.

I think this all amounts to a star-of-stars type scheme, with all the runs kept as short as they can be, given the configuration.

The result? The hum is about the same as it was before the re-wiring. It's definitely louder than with the attenuating resistors. It's possibly a bit louder in one channel, but it's been like that all along.

But at least my wiring is a bit more respectable now!

I hope the attenuators bring the hum down to an inaudible level, but I'd love to be able to attain a silent build of the circuit without them. This being such an extreme scenario, it seems like an excellent opportunity to fully explore the mysteries of hum and noise.



Offline grufti

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Reply #24 on: July 08, 2020, 07:46:14 PM
Nothing but a shot in the dark: unbolt your choke and move it around while checking the hum. Make sure that you don't zap yourself in the process.



Offline Thermioniclife

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Reply #25 on: July 09, 2020, 01:37:43 AM
Probably not related but the 1k resistor at the HV rectifier looks like it has been toasty, is it shorted across the first filter cap?

Lee R.


Offline atalcott

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Reply #26 on: July 09, 2020, 05:04:52 AM
No luck with moving the choke around. That 1k resistor connected to the HV of the rectifier isn't shorted either. I'll see about replacing it just to be safe. I plan on rebuilding the PS entirely with new parts at some point anyway.

One of the Pauls had asked earlier what VAC I was getting at the speaker terminals. When I tried to measure that earlier I wasn't doing it properly. I have 57.5mVAC on channel A (tube A) and 12mVAC on channel B at the speaker terminals. At the preamp outputs I have 6.5mVAC on channel A and 3.9mVAC on channel B.
« Last Edit: July 09, 2020, 08:06:09 AM by atalcott »



Offline atalcott

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Reply #27 on: July 09, 2020, 09:36:43 AM
I made two small changes that seem to have helped a bit. First, I made the connection to ground at the terminal where the output jack and the cathodes attach. I'd read in this article from the Valve Wizard about the chassis connection being as close as possible to the outputs.

Second, noticing the imbalance between channels in the hum voltage, I remembered that I'd replaced the LEDs with newer HLMP-6000s and also had replaced the transistors on the quieter channel (at some point when I was moving things around with the ol' chopstick I had shorted that side out and fried the transistors). I replaced the transistors and LEDs on the noisier channel, and though the hum is still clearly audible, it's about as quiet as it has been. I'm measuring about 12mVAC on both speaker terminals now.
« Last Edit: July 09, 2020, 09:39:35 AM by atalcott »



Offline Paul Birkeland

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Reply #28 on: July 09, 2020, 12:58:36 PM
With Harrison labs -12dB attenuators plugged into the amps, you'll probably be in good territory.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline atalcott

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Reply #29 on: July 10, 2020, 06:30:07 AM
The attenuators came yesterday and they did help a ton. The hum is still there with my ear right up against the speaker, but from normal listening distance it’s not audible. I bet the configuration could stand yet another set of -12db attenuators in series. Even with these installed the system is still louder than I’m ever going to listen to it at 1/4 volume. The Foreplay is really just not a good match for KT550, but it still sounds better than my other current option.