Quickie hum [resolved]

[halvoric]

Just built a Quickie 1.1 with PJCCS and hooked it up to my homebuilt (from the Bob Latino kit) ST-70.  I had been running sources straight into the ST-70 and it was always dead quiet.  With the Quickie there was an obvious 60 cycle hum which I solved by running a ground wire from the Quickie RCA ground bus to a magnet stuck to the large radiator that the whole system sits on.  (Grounding the shell of the RCAs to the chassis of the ST-70 had no effect on the hum.)   Using a pair of 1950s NOS Made in Canada Westinghouse 3S4 tubes from Tube Depot and the preamp sounds great.  Somewhat microphonic but nothing freak out about.

[Paul Birkeland]

Does the new ST-70 have a 2 wire or 3 wire power cord?

[halvoric]

It has the 2-wire.  I have noticed that there is stray AC voltage in the chassis, which I can feel if I put on a hand on both chassis and the giant iron ground plane (radiator).  I will need to find a safe way to ground that chassis.

[Paul Birkeland]

The first and most important modification to do to an old ST-70 is to remove the 2 wire power cord and install a 3 wire power cord.  This is for safety and noise considerations, and is ultimately what I would most emphatically recommend doing. 

[Zimmer64]

+1

I have built a Latino VTA ST-70 and a Quickie 1.1 with PJCCS myself as well. I have used a 3 wire power cord and grounded the chassis of the ST-70. No hum problems and no stray AC either.

Michael

[Doc B.]

Aside from the ground loop issues you guys have experienced a two wire power cord poses a safety risk. A three wire power cord with a safety ground connection to the chassis and lamination stacks is strongly encouraged.

[halvoric]

I'm going to order a 3-wire cord post haste.  I assume the ground wire should go a copper ground tab anchored with a star washer under one of the power xformer attachment bolts (and not to any particular green xformer lead(s), correct?

[Paul Birkeland]

I would use a solder tab on the closest power transformer screw.  Since the power transformer isn't electrically isolated from the chassis (which we also recommend), the transformer screw ought to work. 

[halvoric]

3-wire cord installed.
The 60hz hum that was fixed by earth grounding the Quickie, is now gone even with the Quickie "ground" lifted.  Now to install the cap upgrades in the ST70 (Russian K40 PIOs and bigger filter e-caps on the driver board) and the Quickie (K75 PIOs).