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All,
I just wrapped up one of my MK3's and I am having an issue. It HUMS like mad as soon as the B+ comes on. So Im looking through everything I've done, and I realize that the SDS board isn't grounded to anything....Is this correct ? Seems odd..
The connections I have are (from top to bottom):

Pin 8 from the 5AR4
Choke
Choke
Line to K
Line to J

That's all...Nothing to ground.


Thanks !

PS - maybe something else that may help - Im able to get -76vdc at the wiper of the bias pot...Sound OK ?

No, that's not right. The SDS board goes to ground. Lemme see if I can remember where...

It might be through the metal standoff that's close to the speaker terminal strip. In retrospect, that seems kinda cheesy - I would have hoped for a soldered connection.

The ground should go from the same point where the transformer's
high voltage center tap is connected, to the chassis.

Steve -

Are you sure it's power supply hum and not oscillation due
to the output transformer primary phase being backward ?

Look over the Poseidon manual carefully. This happens to
lots of people, different make output transformers do
different things.

IIRC,the ground wire connects to the rear of the cap board. When everything is correct, it will be dead quiet!

AMEN !

Yepper - that did it. Thanks a bunch for the super fast responses....I was pretty sure that the ground was the issue - but I just wanted to make sure.

FYI - My SDS board does not have any ground to the mounting post...I will post pics in a minute..

Been listening to it for about 30mins...Sounds pretty good through my test speakers.
Unfortunately the switch I had acquired for the triode mode section isn't what I need. Its a weird switch that has non-standard throws....But that is what the extra socket is for; along with the extra turrets at the front of the cap board.



Thanks again to everyone for their super-fast help !

Regards,

TomMcNally wrote:Steve -

Are you sure it's power supply hum and not oscillation due
to the output transformer primary phase being backward ?

Look over the Poseidon manual carefully. This happens to
lots of people, different make output transformers do
different things.

Tom,
Ya - that was my first thought....But as it happened to me last time w/the ST70 - I was sure it was 60hz hum and not the oscillations that greeted me last time :D

Neat. It looks like the holes on your SDS cap board are through plated. I guess you got the "new & improved" version. Mine aren't, and the copper traces are real thin. They lift easily if you aren't careful and overheat them while soldering.

Ty,
Yes, they are through-plated. However, naturally the mounting holes are not connected to the ground-plane.

Actually, it is better to not have the SDS board's mounting holes connected to the circuit ground, as th is eliminates the dreaded (hum generating) ground loop situation. The common ground should connect to the chassis at only one single point.

HTH

/ed B

SteveH wrote:Been listening to it for about 30mins...Sounds pretty good through my test speakers.

Steve, have you built anything like Shannon's GetSetGo? I've got a GSG and a set of Poseidon Mk3. I find it difficult to find the words to describe the way I feel about the two different amps. I'm curious to know what others think. Your suggestions are particularly relevant, since I know you also have a set of Klipsch Chorus.

Ty,
I do not have anything like that yet....My 'test' speakers are a pair of fostex full rangers in a bookshelf enclosure. I haven't yet hooked up the MK3's to the Choruses though.
Eventually I do want to build a <3w amp; just haven't had the time yet...