Major Hum Problem

[steve080]

Hi All,

I have just finished building myself an SE-5W tube amp and have a Major hum issue. I have wired it so the chassis earth is isolated from the ground. Yet when I have my fingers on the strings of my guitar and put my toe on the chassis of the amp the hum is greatly reduced. Where am i going wrong? I always thought it was best to isolate the ground from eath on a tube amp.


Any other ideas where the problem might be?

[HollowState]

Originally posted by steve080
Where am i going wrong? I always thought it was best to isolate the ground from eath on a tube amp.

That's totally backwards. The chassis of a tube amplifier should be directly connected to earth ground by a third wire. This is for safety reasons. If something inside the amp, like a transformer, shorted to the core or case, ths chassis would become hot. And possibily deadly. Shockingly uncomfortable at best.

Edit: OK, if the chassis is earth grounded but the B- is kept isolated, try grounding it to chassis. That will probably stop the hum issue.

Victor

[Hi-Q]

Is it not the case that the B- and chassis come together at one place, normally at the audio input connector? The chassis should also be earthed at this same point. I am not sure if this is the standard convention but I have seen it on many amplifier circuits.
The older amplifiers that used non split bobbin mains transformers used to earth the chassis at the point where the inter winding copper screen of the transformer was also earthed.
I suppose the best guys to answer this question are the those that have had most experience at DIY design/construction and highlights the priceless value of Forums like this one.
Les

[steve080]

Yes, the chassis is earthed to the mains input, just the B- that is isolated. I understand that connecting a 100 ohm resistor between the chassis earth and B- can help reduced ground loop effects introduced by connecting B- to the chassis.

I will try this and see how it helps.

Thanks for yuor replies

[VictoriaGuy]

Originally posted by steve080
Hi All,
Yet when I have my fingers on the strings of my guitar and put my toe on the chassis of the amp the hum is greatly reduced.

When the guitar cable is unplugged from the amp (ie input shorted- assuming you are using a shorting input jack) do you still have the hum problem?

Guitar pickups (especially single coil types) are great for picking up all kinds of 'hum' from things like dimmer switches and halogen light SMPS units.

Incidentally I do agree that you need to look at the safety issues other folks have raised.

Cheers
John

[VictoriaGuy]

If you haven't already found your way to Aiken Amps tech pages, they are worth a look, IMO.

Grounding

Cheers
John