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Old 2nd March 2013, 02:15 AM   #1
Bernyk is offline Bernyk  United States
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Default Triode St70 with hum

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
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Old 2nd March 2013, 02:50 AM   #2
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OK, to begin: why do you believe it's a ground loop problem?

All good fortune,
Chris
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Old 2nd March 2013, 02:51 AM   #3
rmyauck is offline rmyauck  Canada
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Maybe post good close up pics of the whole under chassis. Somebody may spot something. What tubes are you using?

Randy
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Old 2nd March 2013, 06:16 AM   #4
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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.
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Old 2nd March 2013, 09:23 AM   #5
DF96 is offline DF96  England
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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.
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Old 2nd March 2013, 02:59 PM   #6
Bernyk is offline Bernyk  United States
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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.
Attached Images
File Type: jpg St70 1.jpg (731.5 KB, 59 views)
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Old 2nd March 2013, 03:19 PM   #7
DF96 is offline DF96  England
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Heater wiring seems to run very close to unscreened input wiring.
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Old 2nd March 2013, 04:22 PM   #8
Bernyk is offline Bernyk  United States
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Thanks, can you be more specific. I don't have an electronics background so I'm not sure which is the heater wire.
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Old 2nd March 2013, 04:59 PM   #9
DF96 is offline DF96  England
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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.
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