Probable ground issue

[oldvstar]

Last summer I built  Quickie to use in my new tube stereo system. The music sounds fantastic but I have been hearing a low level hum in the background. Today I decided to track down the source of that hum. Here's my system: VTA ST-70 amp, Quickie pre-amp, Lesbox modded Yaqin MS23b phono pre-amp, and  Music Hall MMF 2.2 turntable.
I started out by disconnected the Quickie from the ST70. With it disconnected the only noise I hear is from the transformers. I re-connected the Quickie and the hum returns at the level I normally hear when playing an album. I then dis-connected the MS23B from the quickie so that there is nothing connected to to the inputs, with the selector switch in the off position there is hum, and with it turned to select either of the input positions the hum became very loud. Any suggestions as to what I can do to solve this hum problem?
Thanks.
Terry

[aragorn723]

Do you have a wire running from the Quickie ground to a metal chassis?  This helps a lot with noise issues.  Another thing is to keep it away from cell phones and wireless routers, after doing that it almost completely eliminated him in my system.  Hope this helps.

Dave

[Paul Birkeland]

Does the VTA ST-70 have a 3 prong cord?

What resistance do you see between the RCA jack shell and the chassis on the ST-70?

You could run a drain wire from the chassis to the ground on the Quickie, but the connection should be made inside the power amp if possible (IMO).

-PB

[oldvstar]

I have a 2 prong cord on the ST-70, and the resistance between the RCA jack shell and the chassis is 0.1 ohms.

[Paul Birkeland]

It's probably time to upgrade to a 3 prong cord, for safety, reliability, and quietest operation.

-PB

[oldvstar]

After checking that I have a good ground all the way from the ST-70, through the Quickie to the MS23B, plug polarity, and even running a ground wire between each component I think the problem resides with the Quickie. I'm using a pair of 1940's vintage RCA 3S4 tubes, which sound fantastic, but I find that if I come within 3 feet of them the hum increases, I can also change the intensity by moving my hand around the tubes. Could it possibly be the tubes, or is there some circuit within the Quickie that is causing it to exhibit this behavior?

[Paul Birkeland]

if I come within 3 feet of them the hum increases.

That still sounds like a grounding issue.  Replacing the ST-70 power cord ought to solve all of this, and should be considered mandatory from a safety perspective.

[oldvstar]

I do have a power cord I put together for the ST70 I hadn't gotten around to putting on so I guess it's time to do it.
Thanks for your help.

[Paul Joppa]

There remains the possibility of an electromagnetic hum source - motors, nearby power transformers, light dimmers, etc. etc.

[oldvstar]

Maybe my equipment is a bit close together, I can try to get some separation between the Quickie and the ST70. They're only about 18 inches apart. Otherwise there's only the turntable motor, no dimmers, no wifi AP's or routers.

[oldvstar]

I want to follow up and thank all of you who responded to my plea, everything is now nice and quiet. I ended up doing multiple things; I found a loose nut on an RCA connector, tightening it quieted things down quite a bit. I also ran a ground wire between the Quickie and the phono-preamp, and replaced my RCA 3S4's with a pair of Telefunkens. The only sound I hear now is the low hum of the ST70's transformers. 

[Grainger49]

Don't forget to put on a grounded power cord.  That is the first thing I do when working on any old tube amp.  It really could save your life as the old transformer insulation breaks down.

[oldvstar]

The ST70 is actually brand new, I built it from one of Bob Latino's kits. I do have a nice piece of power cable that I'm going to install, it'll address the safety issue, and I'll be able to see for myself if a power cable really can have an effect on the sound produced by the amp.
On another note, I'm really liking these Telefunken tubes. I do like the sound produced by the 1940's vintage RCA 3S4's that I have, but I've found the Telefunken's to be much less microphonic, the bass is much tighter, and the treble isn't as edgy and forward.

[Paul Birkeland]

The ST70 is actually brand new

The brand new ST-70 doesn't come with a 3 wire power cord?

[Doc B.]

They are apparently two wire power cords, and the transformers are all connected directly to the metal chassis. I would suggest being careful handling the chassis. If there happened to be a fault in the insulation of a transformer there will be no path to safely ground the high potential that could end up on the chassis. Modern standards require the installation of a three wire power cord and connection of the third wire of the cord to the chassis with a number 8 screw, solder tab, nut and two lockwashers. This will serve to safely shunt any high potential on the chassis to your AC mains ground.