[AMRadio] Mod transformer turns ratio |
jeremy-ca
km1h at jeremy.mv.com
Sat Nov 17 17:26:51 EST 2007
Don, it is rated at 7.34KW and weighs in around 250-300#. Came from a 10 KW Martin BC TX; PP 4CX5000A's modulating another pair. No other markings on it other than the mfg (Avdin Energy Corp) part # and date code of 7752. Its in oil and about 17" square and maybe 22" high. Serious overkill but the price is right and Ive been looking over a year for something. The size doesnt bother me, just as long as I can use it. Maybe I should dust off the 304TL's and forget about the 810's? OK on the choke requirement. I'll start looking once I determine exactly what the heck Im doing. Carl KM1H ----- Original Message ----- From: "D. Chester" <k4kyv at charter.net> To: <amradio at mailman.qth.net> Sent: Saturday, November 17, 2007 4:18 PM Subject: [AMRadio] Mod transformer turns ratio > Carl KM1H wrote: > > "Looking at a modulation xfmr that is marked 4596VCT RMS Primary and 3747V > RMS secondary. This appears to be a 0.82:1 voltage step down or a .64 > impedance step down if I did the math right. Question is what sort of > wiggle > room is there in both impedance ratios and actual voltages used? > > The power rating is way more than sufficient for my needs and it is rated > down to 50 Hz. I can use modulator tubes to supply sufficient power to > overcome a reasonable mismatch but what is reasonable and how does the > mismatch affect distortion? I havent been able to find any formulas or > graphs." > > That would be more normally expressed as 1.23:1 turns ratio, or almost > exactly 1.5:1 impedance stepdown. > > Does it give a power rating or current rating? Or load impedance rating? > Without that information it is difficult to determine what the optimum > impedances are, but that usually isn't critical. That's the way the > popular multi-match modulation transformers like the UTC VM series works. > With a given turns ratio, a wide variety of impedances can be used. The > only thing that cannot be changed is the impedance ratio. So you might > use a pair of modulator tubes working into a 15000 ohm plate-to-plate load > to modulate a final amplifier at 10000 ohm modulating impedance. Or your > modulator tubes may operate into a p-p load of 9000 ohms, so the > modulating impedance would be 6000 ohms. A 4000 ohm modulating impedance > would yield a 6000 ohm p-p load. > That is a good ratio for using a common power supply for modulator and > final, for achieving good positive peak capability with plenty of > headroom beyond 100% in the positive direction. > > How big is the transformer and how much does it weigh? That might give a > clue to its power rating if that data isn't given. Given the voltage > ratings you listed, if the power rating or current rating is known, the > optimum modulating impedance can be calculated using Ohm's law, and thus > the power rating if that isn't given. It sounds like broadcast iron if > it's rated down to 50 Hz frequency response. > > That brings up another issue. Is it desined to carry the DC to the final > through the secondary? If not, you will need a modulation reactor and > blocking capacitor to go with it. A good way to find out, if it open > frame where you can see the core laminations, is to see if there is a gap > in the core. If the laminations are stacked in such a way that there is a > gap, filled with paper or some other kind of insulation, then it is > designed to carry the DC. If it is cross-laminated like a power > transformer, with adjacent sets of laminations reversed so that there is > no gap in the iron core, then most likely it is not designed to carry the > DC. Most broadcast modulation transformers are NOT designed to carry the > DC to the final. > > Don k4kyv > > > ______________________________________________________________ > Our Main Website: http://www.amfone.net > AMRadio mailing list > List Rules (must read!): http://w5ami.net/amradiofaq.html > List Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio at mailman.qth.net > To unsubscribe, send an email to amradio-request at mailman.qth.net with > the word unsubscribe in the message body.
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