Typical 2A3 hum?

[bobhayes]

Hi all,

I've been listening to my 2A3 amp for about a year, and love it. But I recently plugged my TV into it to watch a film, and the hum is unbearable for movies because of all the silence in films.

I just wondered what the typical hum figures are for these types of amps.
I have 10.8mV left and 10.6mV right at the speaker outputs, when loaded. Is this more than should be expected from sovtek 2A3's?

I know its originating in either the 2A3's or the output transformers and reckon this is typical filament hum, so I thought I'd just check.

It's 100Hz and is there whenever I switch the power on. It remains when I pull the rectifier, so it must be the AC filaments right?
It also remains when I pull the driver tube, and only disappears when I pull the 2A3's. It is an octave above the 50Hz hum I get when I wind the hum pots to one end.

Any information would be greatly appreciated, if only to put my mind at rest.

Thanks, Bob

[Bas Horneman]

Hi Bob,

I have read that 2mV hum on your output is attainable even with AC heated filaments.

Does your filament supply have a hum pot?

Regards,
Bas

[bobhayes]

Both channels heve hum pots.
I can minimise the 50Hz hum, but am left with 100Hz hum remaining both channels.
I read through some other posts but did not find the exact problem I described above.

[SvErD]

Do you have cable-TV? Does the hum go away if you pull out the antenna-cable?
Then you need a galvanic isolator.(Balun?)

[Bas Horneman]

100Hz hum remaining both channels

Probably a ground loop somewhere...since PSU ripple hardly ever is a problem. Is it a homemade amp?

[bobhayes]

Thanks SvErD,

The problem is the same whatever the source, or even without one, so I don't think the TV is the problem.

Bas Horneman,

Yes, the amp is home built. Where should I look first for ground loops?
It's such a simple circuit (JE Labs/Angela 6SL7/2A3) and as far as I can tell there should be no ground loops.
It is however, built on an aluminium chassis. Could the ground plane resistance (one ohm end to end) play a role in this?

Thanks

[Bas Horneman]

Hi Bob,

I'm still learning about grounding..it's not easy. What does your current grounding scheme look like? Star ground has worked well for me...except in the last amp which I built for someone else...someone else took over and fixed my mess... and made a seperate star earth for signal and psu and joined the two star earths..worked better...because you don't want large currents near small currents...

[bobhayes]

Bas,

Every component which requires grounding is grounded by it's lead (or short wire) directly to the chassis, at the closest socket or transformer mounting nut.

The chassis layout is (from left to right):

Mains In
Mains Tx
Mains switch and fuse
Rectifier
Choke and HT caps
Speaker terminals
Output Tx's
2A3's
6SL7's
Volume and inputs

Its fairly conventional in order to eliminate such problems.

[Bas Horneman]

Hi Bob,

A picture would say more than a thousand words..

Because from your description...it does indeed sound as if that could be a problem...In Morgan Jones's book he describes how moving one wire 2 centimeter solved the hum..

Regards,
Bas

[bobhayes]

Which wire?.. He,he,he!

Looks a mess, doesn't it?