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Hum in homemade amp
Hum in homemade amp
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Old 30th December 2017, 08:39 PM   #1
Antanas990112 is offline Antanas990112
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Default Hum in homemade amp

When I turn on my amplifier after a few seconds appears an increasing hum and a vibrating sound from the power transformer (there are two main toroidal transformers inside one cover). After 20-30 seconds everything disappears and the amplifier works normally without any extraneous sounds. Those first 30 seconds make me feel bad. Where to look for a problem?
Kindly Antanas
mk3ampschematic (V9).pdf

Inside view.jpg

mk3psu.jpg

300B XLS.jpg
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Old 30th December 2017, 09:08 PM   #2
Alllensoncanon is offline Alllensoncanon
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Hum in homemade amp
I wonder what the current through the diode in the center tab of 450V~ are in the first 30 seconds. You could measure that with a small value resistor put in series.
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Old 30th December 2017, 09:30 PM   #3
russc is offline russc  England
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I'm no expert so I'll ask -
Why is the B+ connection to the 5v winding & not direct to the cathode?
What is the purpose of the diode?
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Old 30th December 2017, 09:54 PM   #4
lexx21 is offline lexx21  United States
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Hum in homemade amp
I second the question that Russc asked...
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Old 30th December 2017, 11:00 PM   #5
MelB is offline MelB  Canada
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Sounds like it is about the time it takes for the 6SN7 to warm up. The 300B and GZ37 will warm up right away. Is the 6SN7 somehow involved in biasing the 300B??
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Old 30th December 2017, 11:54 PM   #6
PRR is offline PRR  United States
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Quote:
Originally Posted by russc View Post
Why is the B+ connection to the 5v winding & not direct to the cathode?....
If it was a naked filament rectifier, the "best" connection is to the center of the filament or filament winding. Should be about 2V less 100/120Hz buzz. But I once goaded someone into measuring this and we could not squint a for-sure difference.

If this is a heater-cathode rectifier, the B+ should come off the cathode pin.

This, either way, is not his warm-up hum.

If a POWER tube's bias supply comes up slower than the B+, there can be a big overload on the power supply until things all come together.

Yes, small resistors to monitor current, or just monitoring for unexpected voltage sag, will give good clues.

EDIT- monitor 300b grid at start-up. I suspect there is a +20V jump as the 6SN7's several caps come to final voltage.

Last edited by PRR; 31st December 2017 at 12:00 AM.
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Old 31st December 2017, 12:30 PM   #7
kodabmx is offline kodabmx  Canada
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The second half of the 6SN7s' grid will be at B+ before the tube starts to conduct. Installing a 1N4007 diode between pin 2 and pin 6 will shunt the current through the cathode resistor untill the tube warms up and the diode falls out of the circuit. From Broskie: "it protects the second triode at startup, when the cathodes are cold and the cathode follower’s cathode sits at 0V and its grid sees the full B+ voltage—never a good idea, as the cathode can see portions of its surface ripped away by the huge voltage differential."

Last edited by kodabmx; 31st December 2017 at 12:32 PM.
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Old 31st December 2017, 06:17 PM   #8
Antanas990112 is offline Antanas990112
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Default Hum in homemade amp

Quote:
Originally Posted by russc View Post
I'm no expert so I'll ask -
Why is the B+ connection to the 5v winding & not direct to the cathode?
What is the purpose of the diode?
Hi,
I found in "Audiophile vacuum tube amplifiers" book by I.S.Popovich "that adding suitable polarized solid state diode between the HV secondary's center tap and ground significantly reduces IM distortion".
Another recommendation I received from Jac van de Walle: "5V heater for the GZ37 alsready seems to have a center tap, you should use it to connect the output to. That gives less ripple at the first capacitor, 4uF in the schematic". I tried to implement these recommendations.
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Old 31st December 2017, 06:31 PM   #9
Antanas990112 is offline Antanas990112
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Quote:
Originally Posted by Alllensoncanon View Post
I wonder what the current through the diode in the center tab of 450V~ are in the first 30 seconds. You could measure that with a small value resistor put in series.
Hi,

~105 mA.
I was expecting to be at least about 200mA
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Old 31st December 2017, 06:38 PM   #10
Antanas990112 is offline Antanas990112
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Default Hum in homemade amp

Quote:
Originally Posted by lexx21 View Post
I second the question that Russc asked...
I found recommendation in "Audiophile vacuum tube amplifiers" book vol 2 page 128 that adding suitable polarized solid state diode between the HV secondary's center tap and ground significantly reduces IM distortion". I hope that it is true.
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