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why do hammond PT get so hot?

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3/29/2004 4:49 AM

why do hammond PT get so hot?

Is it just me or do Hammond PTs seem to get really hot? For one of my past amps, I had two 6L6s drawing 100mA, and the PT could supply 150mA....after burning it in for about 4 hours, the PT was hot ot the touch, so hot, you couldn't leave your hand there for longer than a few seconds. For my latest amp, I had 4 6V6s drawing about 115mA, and the PT I was using supplied 250mA. I figured that the PT wouldn't even get warm, but it still got pretty hot (though not quite as much as the one previous...)

Do Hammonds just run hot?

BTW, the 6.3 and 5v windings were well under there ratings as well.

Ricky

3/29/2004 12:56 PM
Wild Bill	Ricky, I've never heard of your problem. In fact, what I've heard and experienced is just the opposite. Could you shoot us some more info on what kind of rectifier you used, or a schematic or whatever. My suspicion is that something besides the transformer must be going on... ---Wild Bill

3/29/2004 2:10 PM

I've heard of this with Hammonds before. It's probably because the primary windings are designed for 115VAC, but most folks' incoming line voltage is more like 125VAC nowadays. This is right on the verge of saturating the core, so the transformer will get hot even with no load. If you have an extra 6.3V or 5V winding, you can wire it in series-adding with the primary to get a true 121VAC primary winding and it will run cooler.

Chris

3/29/2004 5:06 PM
Ricky	This had occured to me. I didn't think it would make that much difference though. Do you think that it poses any long term problems, or just that the PT will run hot? Ricky

3/30/2004 1:10 PM

My line voltage is 122 VAC and in 40 years I've never had such a problem with a Hammond PT.

What's more, for the life of me I can't imagine any technical reason why the primary would saturate if the line voltage was 5-10 volts higher than design. In fact, I wouldn't want to be the guy charged with DESIGNING a PT to do such a thing! If not impossible I'm sure it would be very complicated and too expensive to market.

I'll bet 5 beer Canadian that there's something else going on...

---Wild Bill

3/30/2004 7:06 PM

IME PT's can get quite hot - even if they are pulling less than their rated currents - if they are mounted on the underside of the chassis in close proximity to output tubes and the rectifier (ala Fenders). A lot of heat can get trapped and build up compared with the "tranny and tubes facing up" designs. I was surprised to see how cool PT's would run when you pull the chassis, put them on the bench upside down and let everything face up in the open air.

3/30/2004 9:40 PM

Good point,Man o'Blues.
Though,the other side of the coin could be the tranny itself,and here are two possible aspects:
1.For an unknown reason (or maybe just for commercial reasons),the Hammond designer just didn't took too seriously the "thermal factor check" when he made a certain design.There is a maximum acceptable temperature of 55 degrees Celsius (or 131 degrees Fahrenheit,if you like) wich has to be taken into account by all means if someone really cares about a given PT's stability.I remember my first self-made PT's on wich I measured 62 deg.Celsius with a surface thermometer probe and I eventually dumped them because I didn't want any surprise.At that time I didn't know about the thermal factor equation (I was using only some primitive calculations) and I was very happy when I found out about it.Since then,I never had a problem with any of my PT's.
2.The lamination quality (permeability) and thickness is playing a certain role about a PT's temperature,though it's not very clear to me what's happening in this respect.Several times,using the same bobbins,I tried different laminations.The warmest PT's were the ones having a thicker lamination with a lot of Sy.I'm not sure what kind of PT is the one Ricky claims it goes hotter than normal (he didn't mention the model),but it might be interesting if someone could establish if Hammond is using all over the line the same lamination quality and thickness for their PT's.
(my 1,27 Euro-cents...

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