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I rebuilt a champ to a (2) 6V6, long tailed driver, cathode bias, bf fender tone stack. The hum is not obnoxious, but it makes it almost unuseable. From the driver forward is all grounded to the input jack. From the driver back is grounded to a PT lug. I have installed 4 additional 20mF caps on the B+ in various places where they reduced hum. I pulled tubes starting with the first (only) preamp tube and it did reduce the hum. I pulled the driver and some hum did remain, and with the preamp and driver tubes in it amplified it. So I'm pretty sure it is in the power tube area, but I don't know where. The cathode bias resister is around 470 ohm, so it isn't running real hot. Screen grids are 470. Don't know what else to do. Any ideas?
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The Silver and Black front Champs I have seen had their 6.3V heaters hooked up with one lead to the chassis. A popular mod (especially in my shop:-) is to lift that grounded lead, connect it only to the tubes and pilot lamp socket, and install a virtual center tap referenced to ground.
It reduces the hum in unmodded (SE) Champs, and if you didn't do this for your souped-up version, you should certainly try it. -
It is set up as per your mod....each leg to tubes only and 100 ohm reference to ground on each leg at the pilot light. Still hums................Originally posted by Euthymia View PostComment
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Yeah.Originally posted by MikeT View Post
You need to rework the grounding.
The CT of the PT is a very special wire. All of the huge, spiky currents from rectification run through that wire. For lowest hum, it must go to the negative terminal of the first filter cap and nowhere else.
This is because those rectifier currents cause the "ground" to move around through the voltage created by the currents through the wire resistance. This is then amplified by the whole amplifier and produces hum.Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.Comment
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I forgot to mention that rectification is by means of (4) 1n0007 diodes. OK, I'll rework the grounding ...I'll tear out the additional (5) 22@500's and start again with just the original can cap. Then I'll move the CT (red/white stripe?) to the chassis ground nearest the +cap node on the can where the rectified voltage enters and take it from there..... Hey R.G. you're famous, man....thanks!Originally posted by R.G. View PostComment
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I reworked the grounding and it is much quieter. I had all the pot backs connected and then once to the chassis. I removed it from the chassis and moved it to the input jack. A couple other things I cleaned up too. It had been so long and I had rewired and added so much stuff that I had no idea what shape the grounding scheme was in.
There is still a real bad hum when I turn the volume up. There is no cathode resister on the second triode, and it squeals with the volume up when the treble and midrange are increased. The sound of the amp itself is decent, although I would like more headroom. The voltage on the plate resisters of the first and second stages is around 120 volts...I have a resister to ground where the 2 plate resisters meet and I will try increasing its value to get the voltage up to around 160 or so. That may help the headroom but not the hum. But there is to much gain there and thats why it is squeeling. I think the cathode resisters are 1.5K. If I lowered the one with the cap accross it, that would lessen the influence of the cap, right? Maybe I should just remove the cap.....???Comment
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