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Old 10th January 2008, 11:59 PM   #1
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Default Power transformer: How Big?

I need a little help sorting through information in a data sheet to pick a power transformer. Actually I already built the amp, I just want to be sure that I made a good choice with this power transformer.

I have 4 KT88s at 50mA quiescent and various class A voltage and current gain stages that I estimate at 70mA max. My total zero signal current will be less than 270mA.

According to the KT88 data sheet, they can dissipate up to around 150mA at max signal in the configuration that I am using them. (ultralinear with 560V B+) This must mean that that they dissipate this at the peak of the wave. It can't mean that this is an average value, since we would be way over the plate dissipation.

Working backwards from the max signal Pa+g2 of 33W, that gives us about 59mA per power tube +70mA for 306mA at max signal.

The power transformer that I bought is a Hammond 278CX which puts out 465mA. I figure that is plenty. Did I make any bad assumptions here?

What rules of thumb if any do you guys use when selecting a power transformer as far as current rating is concerned?

Thanks in advance.
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Old 11th January 2008, 01:41 AM   #2
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The power should be 2X the idle current of the outputs, plus whatever current the driver/preamp tubes will eat up. The 400-0-400, 465 mA Hammond trannie will work, but it's a tad low on the current requirement. It may stress out (and distort) at high volume levels. I would get something at least 500 mA. Alden at Heyboer built me a 400-0-400, 500 mA transformer for only $125. Believe me it's worth it....these guys have been in business since 1957!
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Old 11th January 2008, 03:51 AM   #3
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I am going to build a second unit, so I may just give that a try. I would really prefer something with a 120V primary so that when I plug it into a 120V outlet I don't get overly high output on the secondary. The 115V primaries of all the Hammond line kind of bug me.

I just wanted to get an idea of how much I should shoot for. I got an 800mA transformer quoted from Electra-Print to see how much it would cost and it was a bit expensive, so I'll probably give them a chance to quote a smaller unit as well.
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Old 11th January 2008, 06:14 AM   #4
agent.5 is offline agent.5  United States
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Default .

why not just get another Hammond transformer and make dual mono blocs?
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Old 11th January 2008, 02:42 PM   #5
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I could do dual mono blocks. The Hammonds are certainly cheap enough. It kind of bugs me that they buzz real loud when I pull about half the rated current. I was really thinking about getting one custom made for the second unit to see if it buzzes less. Since I am using SS rectification I could switch from CT full wave rectification to bridge rectification, get rid of taps. I don't need filament windings as I am using a separate 10V transformer for regulated DC filaments.

I was hoping a couple of more people would weigh in on good targets for current output like TubeHead Johnny did. (Thanks TubeHead Johnny)
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Old 11th January 2008, 03:01 PM   #6
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Hammond transformers are a tad inconsistent in manufacturing, and the trannie you purchased may have been a defective unit. I've received 11 of these units in the past 4 years, and never had a problem with buzzing. I would give them another chance. Just don't cut any wires until you are happy. AES is excellent with returns.
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Old 11th January 2008, 09:18 PM   #7
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Were any of these a 278CX? I have bought two of them, about a year apart and they both buzz. It's not really loud. I was exaggerating a little bit, but it is definitely audible and increases with load. And considering the fact that the one I was replacing was completely silent, I was a little disappointed.
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Old 11th January 2008, 09:23 PM   #8
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You don't say what filter topology you are using; if it's a critical inductance choke input you can use a higher fraction of the PT rated current, but with C input filter the peak ripple current can be quite a lot higher than the DC load current.. This may be a source of your buzzing. For C input the PT might need to be sized something like 2X or more, depending on the DCR of the PT and ESR of the 1st capacitor.

Michael
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Old 11th January 2008, 11:53 PM   #9
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I am using C input filters for both amps.

The first amp (a Fisher X-101-B) that I used this PT on runs about 230mA quiescent and has about 180 Ohms in series with the first cap(70uF) to drop some voltage(since the Hammonds output voltage is high due to the 115V primary) and solid state rectification. The previous transformer(Fisher original) was totally silent. I had to add more series resistance(voltage too high) and eliminate the tube rectifier(no 5V winding) for the new transformer.

The second amp is my own creation that dumps current directly from the diodes into the 400uF cap. Huge peak currents here.

The funny thing is that the second amp doesn't buzz that much more than the first. I'll admit that I'm abusing the second transformer a bit, but the first one probably shouldn't buzz like it does.
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Old 12th January 2008, 06:35 AM   #10
dejanm is offline dejanm  Austria
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I think TubehadJohnny is right. It seems to me that your 400 mA might be on the edge. I am building KT88 PP in triode mode, class-A, with SS bridge + CLC filtering and I am using 250 VA transformer with 1 A for B+. When I was making this decision I was googling and it seems to me that this is a typical transformer sizing for this kind of amp. It may be a bit oversized, but I think that supply should be always oversized ... Very good transformer that you can buy for these purposes is Lundahl LL1649.
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