Running Hammond Power Transformer over-spec..

[chptunes]

Hey folks.. I'm brewing up a 6G6 based single-ended amp.  What do you guys think about running the Hammond 260A Power Transformer just over-spec regarding the heater amperage?  It's a 22VA transformer with a 6.3v Center Tapped Heater supply rated for 1A of current.

My heater current would be 300mA + 150mA + 600mA = 1.05A

Safe or not?

Thanks,
-Corey

[chptunes]

..and here's a link to the transformer page:

http://www.hammondmfg.com/260.htm

[davent]

Unless you've already got one that's a pretty expensive small Hammond, need the universal primary? I was running another Hammond (262F6 with voltage doubler) at close but not over it's ratings, after maybe half an hour would get a mechanical buzz so changed to a bigger transformer and never had a buzz since.
dave

[PRR]

Unless you need 24/7 non-stop for 20 years, or in an un-cooled hut in the desert, I would not worry about any small PT working 5% over spec. In this case, one winding 5% over is only 1.4% on the total VA. Bah, forget it.

PTs under 50VA, the rating is usually about regulation not heat. Your 6.3V may come out 6.2V, but tube heaters are not that fussy. (Also many Hammonds are specced for 115V input, you probably have 120V.)

[Rob Strand]

I agree 100% with PRR.

What's usually the problem with transformer ratings is if you are pulling x watts out of the DC rail you need a transformer rated at, 1.5*x to 2.0*x  (+ power from heaters); depending on specifics.   If I had to pull a number out of the air I'd say 1.6*x.

See,

http://www.hammondmfg.com/pdf/5c007.pdf

[tca]

Hey folks.. I'm brewing up a 6G6 based single-ended amp.

What is it?

Cheers.

[PRR]

> problem with transformer ratings is if you are pulling x watts out of the DC rail you need a transformer rated at......

Yes; but this series of Hammonds are rated for _DC_ output current (with 2-diode rectifier). They are very old blueprints, and aren't used for much more than making-DC.

Fender 6G6 is a big push-pull-5881 amp. 22VA won't begin to light it up. If "6G6 based" means one 6V6 plus a small bottle, he'll likely run 37mA in the big jug, a few mA in little jugs, and be just-about spot-on the rated 40mA.

However his heater-list does not show a 6V6. So he may have other plans and potential pitfalls. (His "600mA" is not ringing any bell for me.)

But ass-uming he isn't over-loading the HV DC winding, I would not fret about 5% over on the heater winding (1.4% on total VA).

Indeed a "general purpose" PT (rated for _AC_ current) needs to be significantly re-rated for DC current.

[Rob Strand]

Yes; but this series of Hammonds are rated for _DC_ output current (with 2-diode rectifier).

Ah, that helps the cause.

[chptunes]

It'll be a small 5f1-type circuit with a 6G6 for power and a 6X4 for rectification.

I'm gonna use it for bedroom guitar and ipod playback.

Thanks for the feedback.

-Corey

[PRR]

> 6G6 for power

Ah! I misunderstood.

Then the HV DC side is -so- underloaded (perhaps half of rating) that the PT will run well under design temperature.