Triode St70 with hum

[Bernyk]

Hi,
I built a triode electonics St70 and have a hum in both channels. I believe it is a ground loop problem. Any suggestions on how to troubleshoot?

Thanks
Berny

 

[Chris Hornbeck]

OK, to begin: why do you believe it's a ground loop problem?

All good fortune,
Chris

 

[rmyauck]

Maybe post good close up pics of the whole under chassis. Somebody may spot something. What tubes are you using?

Randy

 

[azazello]

Is there a chock in PSU. If there isn't, You can add it with 10-15 Hn between first and second filtering electr. caps after rectifier in PSU.

 

[DF96]

If there is a ground loop problem then all you have to do is eliminate ground loops!

Far more likely in a DIY amp is a grounding problem. People seem to naturally inject PSU hum into grounds, then wonder why they get hum.

A circuit and pictures will help us.

 

[Bernyk]

Thanks for the questions, here's more info and I've attached a pic.
Tubes:
4 JJ EL34; 1 Sovtek 5AR4; 1 JJ ECC82; 2 SED-EF86 Winged C
As a note, I have replaced the JJ with Ruby EL34's and the hum is louder.
When I first built it, I had to replace the 10 ohm resistor that went from the EL34 socket to ground on the right channel, since it was dead - no burning or physical damage.
I've examined all ground connections and can't see any cold solder joints or other issues.

 

[DF96]

Heater wiring seems to run very close to unscreened input wiring.

 

[Bernyk]

Thanks, can you be more specific. I don't have an electronics background so I'm not sure which is the heater wire.

 

[DF96]

The heater wiring is the thick twisted pairs going to each valveholder. Red/black or yellow/black in this case. Unfortunately the builder appears to have used the same stuff for the input wiring.