My new Mark IIIs

[rockdrigo]

Hi,

Last night I fired my new Mark IIIs.

They play loud!

However, both channels have a noticeable 100Hz hum floor which changes intensity when I approach my hand to the 6SL7 envelope.

I suspect on the 1k resistor going from the RCA connector to the grid of the input tube (see top-right corner of figure below). Is it possible that it is acting as an "antenna"?

The aluminum plate is not grounded and the power cable has only 2 wires ( Live and Neutral without ground/earth).

Any hints or suggested measurements?

Thanks!

--Rodrigo.

[DeathRex]

What's the resistor going from the 6SL7's grid to ground? Lowering it might help. Of course making it 0 will get rid of everything going to the 6SL7.

[rockdrigo]

475k. It's a DIY-modified Poseidon topology.

I am currently 1500km away from my amplifiers so I can only dream about them. I thing there must be something wrong with the grounding. Perhaps connecting the signal ground to the chassis or elevating the heaters may get rid of the hum.

The interesting thing is that both monoblocks have exactly the same symptom, even after swapping/replacing tubes, switching PSU polarity, etc...

[20to20]

Your choke seems to be very close to the unshielded power tranny. Could be a source of EM radiation that isn't getting filtered enough.

Also, try creating a chassis ground system temporarily with the top plate at the mult-cap ground using some clip leads and tying the RCA input shield to the chassis ground.

[rockdrigo]

Hi,

Got some news:

1) I completely removed the hum from one of the amplifiers. It was a cold solder joint in the cap from the heaters CT to ground. Now is dead silent with my B&W 684 but some very low level hum can be heard placing the ear a few cm from my Fostex BLH.

2) The other amp still hums. Both amps share *exactly* the same wiring and components.

3) I noticed that when I touch the 0.22uF coupling cap (C1 in Poseidon's nomenclature) the hum increases. This does not happen in the hum-free amp.

4) I replaced the cap with a new one but the hum is still there.

5) I swapped the tubes from one amp to the other and the hum persists.

6) I am using octal tubes with a Poseidon driver. Can this be part of the reason?

Any hints?

Thanks!

--Rodrigo.

[20to20]

Good to hear of the progress.

Sounds like two different hum problems.

RCA input jacks and cables can be a source, if not tight.

Are you using the same cables and signal source to test each amp, alternately?

[rockdrigo]

More news:

I disconnected the .022u cap from the heaters CT to ground and the hum level dropped drastically, but the amp still hums a lot more than the silent one.

I noticed an unbalance in the AC voltage of the heaters. Rather than 3.15-0-3.15 I have something like 2.7-0-3.6 and changing.

I measure about 20 VDC between the heaters CT and ground....I suspect on the power transformer.

I'll try adding extra Farads close to the B+ near the tubes.

--R.