Champ clone hum eliminated.

[Delidroid]

Hello.
This is a report on what Bob Ross would have called a happy accident.

I built a 5F1 kit. This is my third one. It was a combination of two different kits. An Ampmaker WF55 and a Modulus one. (I’m in the UK).

Because they were both slightly different I had to resort to a certain amount of working out things for myself. I
tested it and ran it and I had zero Hum or noise. I was well pleased. But when I looked through the instructions later on I realised the first tube socket was 180 degrees out. What an excuse to fiddle with it. So I unsoldered the wires, cut them shorter and soldered them back in.

Well from 1/2 volume I could hear mains hum and at about 80% volume it would break out in some horrible noise. I prodded the wires with a chopstick and managed to change the noise to what I think you call motorboating.

I reversed the socket again after I had replaced the wires which were too short now. (Note: I changed the yellow wires from yellow to black for the cathode wires. They’re different in the pics).

I delighted to say the amp is again silent and I can play at full volume when the Wife goes shopping.

[pdf64]

...I had zero Hum or noise...

Well done! But 3 of them, you may be a sucker for punishment, those chassis are too cramped for me to be bothered with. Simple circuit but awfully finnicky build.

Well from 1/2 volume I could hear mains hum...

It may have been the valve, as the unbypassed cathodes make the design susceptible to hum from leaky heater-cathode insulation. So if one triode section had that issue but the other didn't, when the leaky section was configured as the input stage, the hum would be amplified 60x more than if it was in the 2nd stage, but would be controlled by the vol setting.

...at about 80% volume it would break out in some horrible noise. I prodded the wires with a chopstick and managed to change the noise to what I think you call motorboating...

I guess that the slightly different lead dress may have altered the amp's margin of stability for the worse. Cramped chassis, finnicky build Dunno that it would be classic motorboating though, as that's usually an actual circuit issue, eg bad HT cap, resulting a positive feedback loop via the HT.

...I delighted to say the amp is again silent and I can play at full volume when the Wife goes shopping...

Yey, have fun!

[Stevem]

When I build amps for myself or when I am repairing or restoring a amp for a person that I know is picky and concerned with its noise floor ( especially like for a recording studio amp ) I replace the heater wires that feed the first preamp tube from the second preamp tube with 22 ga wire as in my photo comparison here.

Your gauge wire used in your amp being twisted as little as it is does hardly anything to reduce 50 to 60 HZ induced heater hum in the audio signal chain, and the first gain stage with its input signal level is below .400 MV is the most susceptible to that issue!

A 12AX7 and that family of tubes only needs .300 ma of current for the heaters, so 22 gauge wire is more then enough to feed / power the first preamp tube.

If you want added noise reduction in that stage , or any stage the uses a AX7 tube then JJ electronics makes a E83CC tube that has spiral filiments which will help out also!

[pdf64]

It's received good practice to ensure well twisted heater wiring, and it's endorsed by Merlin, but on the other hand, plenty of builders here report hum free amps when using non-twisted heater wires, and Soldano built high gain amps like that.
So realistically, it can't be that critical.
Whereas other aspects of regular good lead dress tend to be.

[Stevem]

It can't be denied that that especially if your building a amp in regards to wiring done like this that it does not take any more time or money to do it a potentially better way, I mean come on now!

[pdf64]

Of course, but we should recognise and acknowledge that the actual benefit from doing it may be unmeasurably negligible.
Somewhat akin to the ‘outer foil’ cap thing.
Or sprinkling holy water and saying a prayer

[ChopSauce]

Thanks for sharing your experience with troubleshooting your Champ build and congrats!

This is nevertheless frustrating not knowing what was the main responsible for that trouble...

It's received good practice to ensure well twisted heater wiring, and it's endorsed by Merlin, but on the other hand, plenty of builders here report hum free amps when using non-twisted heater wires, and Soldano built high gain amps like that.

Yes, and the pics below prove that twisted wires are neither better, nor mandatory for the purpose of limiting heater circuit hum.



I retain the suggestion to use thinner wire for preamp tubes, though...

[trobbins]

Were the 'input triode' socket pins on the side closest to the chassis side wall for the 'no hum' or hummy situation? I assume the hummy situation is the photo with the yellow cathode wires ?

[Delidroid]

Hello trobbins.
Yes. The all yellow wires were the original wires but when I turned the socket round to the “correct” way I shortened them. When i swapped it back I hade to replace a couple of wires so I used black for the Cathode leads.

Thank you all for your replies. I’m sure the feedback was caused by where the wires were run. I put this post up in the hope that someone else having trouble could try it before chucking their amp, toolbox, chair and soldering iron out the window