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Hum in homemade amp

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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
View attachment mk3ampschematic (V9).pdf

Inside view.jpg

mk3psu.jpg

300B XLS.jpg
 

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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.
 
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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."
 
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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?

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.
 
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??

Hi,
I removed 6SN7 and left only GZ37 and 300B XLS (V4), but nothing has changed. The same 30-second hum and the mild vibrational sound of two main torroidal transformers which are placed underneath the cover. Both transformers are closely aligned. Maybe they interact there somehow? :confused:
 
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.

Hi,
I realised recommendation of Jac van de Walle: the 5V heater for the GZ37 alsready seems to have a center tap, you should use it to connect the output to. So where the 200mA fuse is. That gives less ripple at the first capacitor, 4uF in the schematic.
The current through the diode in the center tab of 450V~ are in the first 30 seconds is about 105 mA ( I was expecting to be about 200mA :confused: )
I tried to turn on an amplifier without 6SN7, but the same problem persists. After about 30 seconds, the hum grows and then it disappears.
 
hum in homemade amp

buzz at beginning smells like high spike consumption, until everything stabilises..
you can unplug 300b, and listen for magnetic feedback between irons. -it should be dead quiet

i use toroid 12H+1mF, with good results.
you should add some uF foil caps, it clears up the amp sound.
old
Hi,
I would like to have dead silence, but I hear a weak, constant vibration. Such sounds were spread by old, poorly tightened transformers. These new toroidal transformers did not triggered any sound until I inserted them into one cover and put epoxy on them. I will try to turn them on separately. Maybe they somehow interact with each other?
 
old
Hi,
It's interesting that when only the 300B is removed, the vibrational sound from the toroidal transformer remains, and it's natural that there is no any hum.
But after removing only the GZ37, there is any noise from the toroidal transformer, but it remains constant hum, even larger than with all the tubes. What causes it if there is no HV ??? :confused:
 
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
View attachment 653787

View attachment 653791

View attachment 653792

View attachment 653794

I do not understand what's going on: under one cover there are two Amplimo 7N1476 transformers. They are somehow self-guiding with each other. If I connect ~ 220V to only one of them I get standard voltages in secondary windings (450V-0-450V), but also I get voltages in non-connected transformers' secondary winding (about 300V-0-500V. ??? Why???). I also found a sufficiently high voltage between the midpoint of connected transformer and the secondary terminals of disconnected transformer.
If I connecting their primary windings in parallel, in secondary windings of both transformers I get more or less the correct voltages, but also unwanted sounds mentioned above for the first 30 seconds.
 
Hi Antanas

Maybe those two Amplimio transformers are back-to-back. Its hard to tell without knowing all the details. Is it safe to assume that you did not actually build it yourself?

If you did not build it, then I would advise contacting the original builder. If this is not possible, I think the only thing to do is trace the schematic yourself by scratch...

BTW, looking at the schematic, as well as EML 300B plate curves the quoted voltages and millamps don't seem to make sense to me...

Ian
 
Hi,
Unfortunately, it was my mistake. It's me put them into the cover with epoxy resin. Now it's a desperate thing to distinguish them. I did not think that they could interact with each other in this way. Maybe I needed to separate them with a metal plate ??? it's too late now. I see only one way - it's replace them with new ones (400 Eur ��) and how to avoid a similar destiny....in the future??
Kindly, Antanas
 
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