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Old 15th April 2012, 02:04 PM   #21
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An update to the original problem posted - I've found the source of the buzzing/humming. Very, very odd, never come across anything like it:

I disconnected the AC 35v transformer, so that only the filament 6.3v AC was connected. Lo and behold - the exact same buzzing was there! Then I disconnected the 6.3v AC transformer and used a 6v DC wall wart (plug pack) - identical buzzing.

After this, I changed the 6v dc wall wart (plugpack) for a sealed lead acid battery (6v dc) - no buzzing whatsoever!

What on earth could cause this?

Last edited by lordearl; 15th April 2012 at 02:32 PM.
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Old 15th April 2012, 03:55 PM   #22
dgta is offline dgta  United States
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What could cause this is your filament circuit somehow being connected (or shorted) to the stage's output. But this doesn't explain your claim of 440Hz. Your filament is at line frequency.

Is this occurring on both right and left channels?

I recommend that you do a lot of reading and see about getting some test equipment. Without more understanding and tools, you will find building and sorting electronics quite frustrating.
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Old 15th April 2012, 03:59 PM   #23
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Quote:
Originally Posted by dgta View Post
What could cause this is your filament circuit somehow being connected (or shorted) to the stage's output. But this doesn't explain your claim of 440Hz. Your filament is at line frequency.

Is this occurring on both right and left channels?
Yes - happening on both channels, there's between 10mV and 20mV of dc at the output of the rca jacks. The buzzing also ceased when I test the dc offset with my multimeter.

The 440Hz was just a guess at the frequency - middle C was the best reference point for me.

What is the best way of testing where the shorting is occurring? Why would the buzzing disappear with a battery filament supply?
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Old 15th April 2012, 04:18 PM   #24
DF96 is offline DF96  England
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The heater supply is obviously the source of the buzz. You just have to find where it is getting coupled in to the audio. Grounding or capacitive coupling are the most likely.
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Old 15th April 2012, 05:26 PM   #25
dgta is offline dgta  United States
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Quote:
Originally Posted by lordearl View Post
Why would the buzzing disappear with a battery filament supply?
Because the buzzing is AC and the battery is DC.
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Old 15th April 2012, 06:03 PM   #26
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Did you leave the heater floating? In that case first try grounding one side.
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Old 16th April 2012, 12:04 AM   #27
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Originally Posted by revintage View Post
Did you leave the heater floating? In that case first try grounding one side.
Ok so just connect one of the filament terminals to the chassis earth? It seems like a systemic problem with the pcb as I have built two with the same issue. I have a spare pcb and 6n16b tube, if anyone would like it (free).
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Old 18th July 2013, 06:17 PM   #28
maton00 is online now maton00  Mexico
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Try to follow the wisdom of merlin blencowe, and also read the "the power suply section/heater suplies/artificial centre tap "
How to design valve guitar amplifiers
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Old 18th July 2013, 09:32 PM   #29
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Quote:
Originally Posted by lordearl View Post
An update to the original problem posted - I've found the source of the buzzing/humming. Very, very odd, never come across anything like it:

I disconnected the AC 35v transformer, so that only the filament 6.3v AC was connected. Lo and behold - the exact same buzzing was there! Then I disconnected the 6.3v AC transformer and used a 6v DC wall wart (plug pack) - identical buzzing.

After this, I changed the 6v dc wall wart (plugpack) for a sealed lead acid battery (6v dc) - no buzzing whatsoever!

What on earth could cause this?

research hum balancing circuit.
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