5E3 Hum and transformer mounting

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Voxbox
Junior tube assistant
Posts: 62
(2/19/04 4:52 pm)
 5E3 Hum and transformer mounting
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 Hi guys,
I have to share this with you.
This may be beginners bad/good luck, depending on how you view it.
I have been pulling my hair out over a hum that has been a killer to get rid of. The 5E3 has a standard (not Hoffman) tag board and standard Fender component layout and wiring on the board, but star-earthed with input jacks isolated from the chassis. I have tried every earthing system I have read about or dreamed up, but have not been able to reduce the 50 Hz hum till now. The hum is 15mV peak to peak (Vpp) with all the controls at minimum, and increases with either volume control to a max of 0.2Vpp at full vol with either volume control. (HT is 330V.) Not a lot when the amp is cranked, but I always felt it should be less.

The amp plays well, and sings when its cranked, but having a bee in my bonnet about grounding, and being also a bloody-minded perfectionist I have tried everything. Re-orienting TFs, changing the wiring to a Hoffman layout, Feng Shui, chanting Runes.....

I noticed with the scope that the hum was greater on one of the six steel mounting bolts holding the mains TF in place. The bolts pass through the laminations and into the chassis. Removing the bolts and isolating the TF from the chassis reduced the hum at full vol to 50mVpp. The TF is of the same style as Doug's, but does not have the bottom cover and is made by Majestic Transformers in the UK.

So, my theory is that the steel bolts form additional turns round the core and can induce hum into the chassis. This would be worse if one or more of the bolts had rubbed through the varnish on the laminations and made contact with the metal of the lamination. This would seem to be the case with one of the bolts, hence the higher level of hum at that bolt.

I recall a Marshall amp that I worked on once, a 2203(?) 100W head, that had brass bolts holding the TF down. I remember this because I thought it was odd at the time-why not steel? However, this could be to avoid the problem I found. I dont have a lot of experience with valve amps, so does anyone have any experience of this?

I remember someone stating in another post that they would not use the TF bolts for making a ground point - maybe this is why.
I'll be down to the hardware store tomorrow to get some brass bolts! Wonder what it would cost to get a toroid made for the next project...

One other thing - sometimes, when both vols are turned up full, a LF oscillation starts up. Any ideas?

I'm off for a beer

Cheers
Vox
 
 
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