Triode St70 with hum

Bernyk · 2nd March 2013, 02:15 AM

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 · 2nd March 2013, 02:50 AM

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

All good fortune,
Chris

rmyauck · 2nd March 2013, 02:51 AM

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

Randy

azazello · 2nd March 2013, 06:16 AM

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 · 2nd March 2013, 09:23 AM

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 · 2nd March 2013, 02:59 PM

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 · 2nd March 2013, 03:19 PM

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

Bernyk · 2nd March 2013, 04:22 PM

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

DF96 · 2nd March 2013, 04:59 PM

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.

2nd March 2013, 02:15 AM	#1
Bernyk diyAudio Member Join Date: Mar 2013	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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2nd March 2013, 02:50 AM	#2
Chris Hornbeck diyAudio Member Join Date: May 2011 Location: Little Rock	OK, to begin: why do you believe it's a ground loop problem? All good fortune, Chris

2nd March 2013, 02:51 AM	#3
rmyauck diyAudio Member Join Date: Apr 2009	Maybe post good close up pics of the whole under chassis. Somebody may spot something. What tubes are you using? Randy

2nd March 2013, 06:16 AM	#4
azazello diyAudio Member Join Date: Aug 2008 Location: Bulgaria	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.

2nd March 2013, 09:23 AM	#5
DF96 diyAudio Member Join Date: May 2007	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.

2nd March 2013, 03:19 PM	#7
DF96 diyAudio Member Join Date: May 2007	Heater wiring seems to run very close to unscreened input wiring.

2nd March 2013, 04:22 PM	#8
Bernyk diyAudio Member Join Date: Mar 2013	Thanks, can you be more specific. I don't have an electronics background so I'm not sure which is the heater wire.

2nd March 2013, 04:59 PM	#9
DF96 diyAudio Member Join Date: May 2007	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.