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My Main Hammond 372BX Plate and filament Transformer melted and let out the magic smoke out. I want to take this opportunity to "upgrade" my transformer. Any suggestions for options that might be higher quality than the Hammond 372BX. This is EL84 based set amp.
I'm Jeff also. I custom design SE special custom power transformers for ultra-discriminating audio people and I will be the first to tell you there is nothing at all wrong with using Hammond Power Transformers..... IF you know how to spec them, and under-use their ratings by a fair margin. Your 372BX was only rated to do 92 VA / 100 mA. on the secondary.
Question one, are you really needing an INTERNATIONAL primary ??
By that, I mean do you need choices from 110 VAC home line voltage to 240 VAC home line voltage,... as in Europe? If so, simply use the 3XX series and just UP the current rating. Order the 372JX which does 250 mA. and costs $149.95 Your melted trannie, rated at 92 VA / 100 mA. cost you $100.78. and its a total loss now.
If you are USA only, or nominally use 120 VAC only, NEVER order the 3XX numbered series of Hammonds - ever, (with all the fancy un-used primary taps hanging off them). Order the 2XX Hammond series, with the 115 and 125 VAC primaries only....MUCH more trannie for the money, and you can easily and cheaply upgrade the VA rating, so the trannie LOAFS and runs cool, rather than strain and spill its guts out.
Would you rather drive a 49 CC China scooter up a hill, or a 1,300 cc BMW cycle ??
Your upgrade route, if you are in the USA, is to order a Hammond 272JX, it is rated at 236 VA and it can do 600 VCT at 287 mA. of current!!!!! Guess what, it retails for $65.54!!!!!!! A fraction in cost of the 100 mA. melted one, and 2.36 times HIGHER in VA rating.
Three important questions need to be answered still:
(1) Can you fit it on your chassis?? It will be physically larger that the "P.U.ny" 100 mA./92 VA power trannie. If it does not fit, rebuild the amp on a larger chassis, or modify mounting holes and locations on your existing chassis.... so it will fit.
The amp (with the new trannie) will run cool to the touch, and it will sound at least 10 to 30 percent better. Why? Well, the HEART of the amp, the power transformer, is STRONG and will self-regulate the B+ under the demands of music reproduction. Wait till you hear this !!
Two other questions need to be answered :
2) Has another part failed, that caused this transformer to have a ultra high load, and thus fail. If so, that problem area needs to be fixed!!! FIRST.
3) Was the size of the safety fuse in your amp so high in Ampere ratings, such that the power trannie blew BEFORE the safety fuse? If so, by all means, reduce the value of the fuse for MORE protection.
If you have questions, feel free to contact me. 272JX wow, $65.54 a TOTAL steal.
It is my observation, (especially after seeing people's posts as to what they use for chassis wire - URL below), that most people on this Forum and in audio in general (99%) build their amps narrow-band and dynamically challenged, FAR WORSE than the limits of Hammond Transformers, including Hammond output transformers. Smoke on that, for awhile my friends.
Jeff makes a number of good points, especially about finding the cause of the problem. Most of the responses so far in this thread make the assumption that the transformer failed from excessive heat, and that the heat came from the current through the wire resistance.
There are other causes, and other potential problems, with power transformers.
First off (I have said this before) transformers have a limited magnetic headroom. If you use grain-oriented silicon steel laminations for example, you can according to the manufacturers run them as high as 16,500 gauss of magnetic flux. Run this hard, the core dissipates a fair amount of energy and gets hot; traditional designs try to match the heat generated in the core with the heat generated in the wires. If you run them even a little bit harder, the core temperature will rise quite rapidly because it is in saturation.
Running the core hard also leads to more vibration, hence mechanical buzzing - this comes from both magnetic forces on the current carrying wires, and also from magnetostriction - the metal itself distorts under the presence of its magnetic field. This vibration can accelerate abrasion of the wire insulation, and thus shorted internal turns followed by what appears to be heat death and loose smoke.
Running the core hard also leads to excessive radiated magnetic field, which often induces hum into other parts of the circuit. As you get into saturation, the core rejects more and more of the field, forcing it into the surrounding space.
If you use a transformer specified for a much higher current you will reduce the copper losses, as Jeff suggests. But you will not change the magnetic losses at all. To sensibly reduce the temperature, you should put a few more turns on each winding and reduce the flux.
Flux increases with voltage, so if your house power is at a higher voltage than the transformer specifies, you can easily exceed the design flux density. Flux density also increases linearly as the inverse of the frequency, so transformers rated to operate at 50Hz as well as at 60Hz will have a 20% margin of magnetic headroom. This is the important difference between the Hammond 3xx and 2xx series, and the reason the 3xx are physically larger for the same ratings. If the transformer runs 16500 gauss at 50Hz, then it will run a bit below 14000 gauss at 60Hz.
Transformers are complicated beasts, with a large number of important variables interacting with each other. I am convinced that you must respect all of them in order to get a good result. Not that it does me any good, but I feel the same way about single-issue politics!
There is another factor here. A number of the Hammond 200 series PTs tend to have a mechanical buzzing, whereas every 300 series PT I've tried was dead quiet. I totally agree about choosing a much larger PT rating, but I would pay the extra dough and get a 300 series instead of a 200.
Just what I was going to add. In the heyday of the Hagerman Cornet/Cornet2, Jim changed the power tran from 200 to 300 series because of MANY complaints of buzzing. I think it was 272 BX but it's been awhile.
Could be that Hammond addressed the issue. In any case, I always put an .040 layer of EAR SD-40 under all my transformers and chokes.
I will believe corporations are a legal person when Texas executes one!
You and I don't know, from a few small samples, what the real facts are on that, so its hard for me to comment, but I can't imagine there being any difference, coming from the same manufacturer. Doesn't make sense to me.
You know Dave, its possible that the people who order the cheapest iron are the most novice of builders, and so, they don't know to leave enough "overhead" in their specifications. They would order the less expensive 200 series initially.
I agree that the samples are small. However, just do a google search for "Hammond transformer buzzing" and you'll find other people who have had the same experience.
OK, but realize, about 95% of the novice designers and builders will order Hammond, and most newbies are on a budget, so there will be a larger number of such reports, when they don't fully understand how to de-rate.
Its sorta like my Crown Vic Police Interceptor daily driver. There are LOTS of reports of rear-end crashes where the gas tank explodes and kills occupants. Tell me, does that have something to do with the CVPI's popularity as a Police Car, and being parked on the sides of the roads to give tickets? Should I fear a gas tank explosion? Well, I'm not parked on the side of a busy road, so I don't Dave.
Would you know some of the specific model numbers which are noisy? Seems like a lot of extra $$$ to pay to go to 300 series, if you don't need the extra taps on the primary side. And what about the 50/60Hz 200 series trannies? Are they quiet?
I don't remember which ones I tried. It's been too long ago. I do recall that one of them was the PT spec'd in the Vacuum Tube Valley Octal Linestage. That had a nasty buzz. Sometime after I built that, I did some research on the forums and discovered that a lot of people had the same complaint about the Hammond 200 series. I then ordered a pair of 373BX transformers for some 6L6 monoblocks, and they were very quiet. Since then all of my Hammond PTs have been 300's.
My all-time most-quiet PT is the Angela Universal. I've used a total of 4 of them and they are 100% silent. I can put my hand on it while the amp is cranking away and there isn't the slightest vibration.
This is fantastic information! You gave me a answer that was exactly what I was looking for. I'm ordering the new one today. Thanks for giving the "what to order" and "Why this one"
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