Quick Fix of 60-Hz Hum in My ST-70

[dcriner]

I was getting a little too much hum coming out of both channels. The hum was coming from within the amp, not the preamp. Pulling either 7199 driver tube eliminated the hum on that channel, so the hum was getting into the driver circuit, and then amplified by the EL84 output stages.

I could tell by ear that the hum was 60Hz, not 120Hz -- so the source wouldn't be from ripple on the high-voltage B+ plate supply. (A full-wave rectifier produces 120Hz, not 60Hz.) The hum had to be coming from the 60-Hz line and/or the heater circuits.

First, I biased the two heater ckt center taps with about 70-V DC. This keeps the filaments, particularly the 7199 drivers, from acting as tube elements with emission of electrons. That helped.

Then I connected a 0.01-uF cap from the neutral side of the line cord to the chassis star ground. That virtually eliminated any hum whatsever.

The original ST-70 design did not have a grounded chassis and adding a solid earth ground via a 3-prong grounded plug is not recommened -- could cause ground loops with the pre-amp, etc. I replaced the line cord with a polaraized plug, so the neutral leg is always connected to the fuse and the hot leg to the power switch. I connected the 0.01-uF cap to the fuse, and thence to ground.

Modern codes dictate an "X-Y" AC safety cap for this application. They are available from www.justradios.com

Because the chassis isn't connected to an earth ground, there is a risk of shock if somehow the 120-V line shorted to the chassis (with or without the added 0.01 cap). Some measure of shock protection can be achieved by powering the amp (and pre-amp) from a recepticle protected by a ground-fault interrupter.

Biasing the heater winding CTs is easy. I hooked up a simple voltage divider between B+ and ground, using two resistors in series: a 220K and a 47K, with the smaller one connected to the chassis ground. The connection between the two resistors gives about 70-V DC, which is then connected to the two heater winding CTs. The 220-K resistor goes to Terminal 3 of the quad filter can (this is the terminal marked with a square).

P.S. This is a clone ST-70 kit from Amplitube with all new components, including transformers from Triode Electronics. It's possible that the original Dynaco power transformer had enough stray capacitance that the chassis was effectively grounded to the power line via that capacitance. Just a theory.

If a metal chassis has no reference to earth ground, it can become an antenna.

Doug Criner

[Shannon Parks]

Hi Doug,

So she's quiet now?

[dcriner]

Shannon: yes, totally quiet.