ground hum issues in an old peak st-100 - diyAudio
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Old 28th April 2015, 04:59 AM   #1
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Default ground hum issues in an old peak st-100

I have been loving my little Peak St-100 I revived in the thread below, but I am having hum issues and wanted to ask the more experienced among you, where I should start trying to resolve the issue....

help with an old peak st-100single-ended 6GW8

This is the current state of the Peak ST-100. My question is whether I should rebuild the chassis now, or, which I would prefer, try to remove the hum in some way through circuit problems or earth point. I have searched online and find myself pretty confused. This is not really something I know about yet. Is there some simple tips anyone can give me for beginning to eliminate the hum?

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The earth point is here - too much? too much chaos?!?

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....any help would be greatly appreciated....


I am pretty new to all this, though I have also also built a modified kit 2A3 SE making a ridiculous copper chassis for it (the copper was lying around and I am a metalworker and TIG welding is fun....) But somehow I still love the uncluttered sound of the old Peak, even though it has no power.... I just want to make it better. and better.... : )

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Cheers,
Sean.
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Old 28th April 2015, 01:09 PM   #2
MelB is offline MelB  Canada
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Would appear your 6.3v supply is floating. Does the 6.3v supply have a center tap? If not the first thing to do is get two 100 to 220 ohm resistors and add one from 6GW8 pin 4 to ground and one from 6GW8 pin 5 to ground. (The resistors should be of the same value) You just need to do that on one of the 6GW8 tubes. I would also remove those 6.3v wires from pin 4 and pin 5 and twist them together before re-attaching them while you are at it.
There is a nice example of a twisted wire on your 2A3 amp near the top.
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Old 28th April 2015, 02:09 PM   #3
DF96 is offline DF96  England
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The first thing to do (if necessary - the photos are not entirely clear) is to remove the transformer CT connection from the 'star' point and instead wire it to the reservoir cap negative terminal.

Is that really an 820R 'bleeder' resistor? More like a room heater!

A circuit digram would help. You have given us a (sort of) wiring diagram, but azlmost certainly it does not show exactly where the grounds go.
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Old 28th April 2015, 04:56 PM   #4
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If You connect el. cap 350-470 uF in point A to ground, humm will decrease.
I don't like Your filter group....I prefer CLC / 20uF, 10 Hn, 350-470 uF/ group after rectifier and connect p.A to 470 uF.
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Old 29th April 2015, 03:02 AM   #5
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Thankyou all SO MUCH for responding.
I am going through these points now and writing as I go....

azazello - you are dead right there - I will get a cap for this - I have no HV caps here except for a 30uF, but i tried that and it made a huge difference.... And can you tell me what you mean by the filter group? I am pretty new to all this, so any pointers, or links you can send me would be great....

DF96 - I connected CT to negative of reservoir cap - you were right - it was straight to ground point. Yes - that resistor was here in my box and I used it. : ) Will make up another circuit diagram soon too!

MelB - The centre tap of the transformer is hard wired to one side of the heater supplies, so that does go to ground. I disconnected the heater wires and twisted them as you described....

When I put in the cap that azazello suggests I will report back.

Cheers.
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Old 29th April 2015, 09:01 AM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by DF96
Is that really an 820R 'bleeder' resistor? More like a room heater!
I now realise that it is the supply rail feed. I was thrown off course by the peculiar drawing convention.
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Old 29th April 2015, 09:30 AM   #7
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sorry - the drawing is awful i guess - this is not my field at all! i am pretty dumb with all this - not knowing what a bleeder resistor is or the like - so i have been looking up lots of stuff, learning a little, like CLC circuit for power, etc....
lots to learn!
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Old 29th April 2015, 12:38 PM   #8
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Filter circuits - Inductor Filter, LC filter, CLC or PI filter, Capacitor filter
Cs are caps 20-40 uF and 470 uF, L is iron chock about 10 Hn /from Hammond or EdcorUSA/
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Old 30th April 2015, 04:21 AM   #9
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thanks azazello - i am in australia, getting a choke mailed as there isn't much around i can find cheap.... then will make and report back....

thanks for your time and knowledge - much appreciated!
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Old 16th May 2015, 01:45 AM   #10
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thanks so much to azazello and the other posters - i have replaced the power filter setup and the hum is so so so much better. makes me so happy....
what really buggs me off is that this old peak sounds clearer and more alive than my 2A3 stereomour kit which i lovingly put together. so now i have plans to rebuild the components of this little peak into a proper enclosure. give it what it deserves. next project all lined up. : )

again - thanks - this has helped me learn a little about electronics circuits, hum, and power filters specifically. much appreciated.
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