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Old 14th January 2004, 02:35 PM   #1
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Default disorted hum

What could be the cause of distorted hum ? Everything measures out ok in my new amp although the B++ is a bit low (460 instead of 494 as per schematic) After switching on in stand-by mode I have 4 mV hum on the speaker terminals even though the end tube is DC fed. After flipping the stand-by switch I get about 13 mV very ugly distorted hum from my speakers.
I would very much appreciate some feedback.
Thanks in advance

Martin
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Old 14th January 2004, 02:36 PM   #2
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And the powersupply.
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Old 15th January 2004, 11:04 AM   #3
dhaen is offline dhaen  Europe
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I'm not sure if I understand the term "distorted hum".
If it's just hum:
How much hum?
Please measure the AC voltage across the speaker terminals with the amp input shorted.
A value of 5mV or less would be expected on this design.
Try removing the 0.33uF coupling cap. Does the hum disappear?

Alternately, do you mean that the signal is modulated by hum?


Have you asked Mr Van de Merwe
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Old 15th January 2004, 11:19 AM   #4
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John,

By distorted I mean that there is an alarming, scratchy sort of modulation on the hum when listening to it on the speakers. This is in stand by mode. No signal whatsoever. It measures 3 mV at the speaker terminals. Also the 300b heaters are fed DC, so wouldn't you suspect the hum to be much less ? And in stand by mode, totally absent ? I also noticed that in stand by mode I still had 34 V DC on the B+ and 126 V on the B++.
Does this mean anything to you ? After flipping the stand by switch, the hum level rises to 12-13 mV.

thanks very much for your input.

Martin
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Old 15th January 2004, 11:27 AM   #5
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Oops, correction. The hum level rises to 5,5 V after flipping the stand by switch. This is the lowest I can get it with the hum pot.
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Old 15th January 2004, 11:38 AM   #6
dhaen is offline dhaen  Europe
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Hi Martin,

Yes, the 5mV figure may be a bit high.
What is the figure with the 0.33uF cap disconnected?

Can you un-bolt the output transformer and rotate it? Do you se the hum change?

The standby voltage is expected. The valves will draw very little current in this condition.
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Old 15th January 2004, 12:12 PM   #7
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Hi John,

Removing the 0,33 uF cap in stand by doesn't make a difference. After flipping the stand by switch to on, hum drops to 1,9 mV without this cap.
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Old 16th January 2004, 04:18 PM   #8
mcs is offline mcs  Denmark
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Just a small unrelated comment.

The cathode bypass cap in the schematic is underrated. It is shown as a 100V cap with 92V DC accross it. Remember that the AC signal is added to the DC, so the cap should be a 200V type minimum...

Best regards,

Mikkel C. Simonsen
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Old 16th January 2004, 04:23 PM   #9
dhaen is offline dhaen  Europe
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Quote:
The cathode bypass cap in the schematic is underrated
I respectfuly disagree
There will be a small momentary rise in cathode voltage if the stage goes into overload. Otherwise, the voltage will not exceed the ratings.
I can forsee no other AC voltage component on the capacitor.
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Old 16th January 2004, 04:24 PM   #10
dhaen is offline dhaen  Europe
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Martin,

Have you tried turning the output transformer yet?
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