Humming power transformer

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rp

Well-known member
Joined
Feb 23, 2014
Messages
45
Location
Los Angeles, CA
I've just built this standalone tube PSU, which I'm using to power a two-channel NYD 6sn7 one-bottle preamp. The transformer is humming a bit and running warm. Is it under spec? I figure the total heater requirements are 1.2A (2 6sn7) + 0.3A (2 lamps; one in psu, one in pre) + 0.063 (humdinger) = 1.56A. Is that running too close to the rated 2A?

Am I right to think the problem is the heater circuit? I feel like the B+ circuit is well with in the norm, and is not running close to maximum current. (I haven't measured, but the one-bottle schematic lists B+ current as 12ma, so I should be using only 24ma).

Other than the power trafo hum, everything seems to work fine: correct voltages, sounds good, low noise in the preamp.  It's my first tube build completely from scratch, so I'm happy that it mostly works and I haven't killed myself.

The preamp circuit can be viewed here: https://groupdiy.com/index.php?topic=45369
Original 6sn7 one-bottle thread here: https://groupdiy.com/index.php?topic=12637.0
 

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Transformers tend to hum. The rated power is limited by melt-down burn-up.

How warm is "warm"? At full power, you may not want to keep your hand on it forever. Anything less is probably OK.

Measure primary and secondary actual voltages and compare to rated voltage ratio. Is output lower than computed?
 
After about an hour of being on, it's warm, but not so much that you can't keep your hand on it. I think it's probably fine. I might try to play around with damping some of the chassis. The hum is better when the top is off.

The voltages measure higher than spec coming off the transformer. I just measured the wall voltage at 124vac, and the secondaries of the transformer at 290vac and 7.1vac.

While I was trying to figure out design errors, I was wondering if the transformer might be working too hard to fill the first cap for the B+ supply. I've noticed that some power supplies have a low-value resistor before the first cap. Is this to reduce the current demands of the cap?
 
you could do a simple test whereby you hook everything up, turn the power on so like normal, listen to the hum as you pull the tubes out of the sockets. 

 
I tried the pulling the tubes as suggested by CJ and found that it made no difference. It turns out that the transformer hums even when there is nothing attached to the psu! Nothing seems loose on the transformer, so I feel like hum is a design flaw. I've mocked up the circuit in the Duncan PSU app (sans raised-heater voltage divider), and it seems to indicate that the transformer exceeds its rated current limit.

The hum is loud enough to warrant trying to fix it, but I'd like to avoid getting a larger transformer $$.

Could I

- make the first cap after the rectifier small, so that the transformer doesn't have to dump as much current into it?
- Put a high-watt, low-ohm resistor between the rectifier and the first cap?

Are these even the right questions to be asking?
 
I assume that the HUM is from the big tx and not the small one. The basic problem is that the lams are LOOSE. Most tx's are impregnated with a varnish to stop mechanical hum. You could remove the transformer, place it in a small plastic cup, poor in varnish to cover the transformer, let it soak, remove it, let it dry, reinstall it. This process will take several days as a DIY.
 
Ive had luck curing transformer buzz from the laminations by applying laquer or varnish between the bobbin and the core , ocassionally you can have loose lams that rattle . Dipping the entire transformer in varnish is another possibillity .CJ might know what the best kind of varnish is to use ,you dont want to end up causing deterioration of the insulation on the wires which certain solvents might cause .
 
What is this, a tube mike preamp supply?

Those transformers are way bigger than needed.

However transformers can hum even unloaded. Very high currents, the 60Hz field can tear them apart. Normal currents, everything vibrates 60Hz, though very-very small. Large transformers (thousand times bigger than these) often have specs on how loud they can hum.

You say taking the lid off changes it. This means that chassis is acting as a soundboard. Do what you do if you DON'T want to hear that acoustic guitar. Rubber pads under the bridge (the transformer). Pads on the soundboard (chassis panels). If there's a buzz, tighten/loosen or pad it.

You can probably throw a few hundred Ohms between PT and rectifier. You have plenty of headroom, and it may soften the pulses.
 
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