Please excuse my ignorance among the experts but shouldn't one be careful not to push the voltage rating on the secondary of the modulation transformer when increasing the plate voltage on the RF amplifier? or on the primary winding when pushing higher voltage on the mod. tubes.
On all of my 32V's at the Collins Radio Center, I put the toggle switch on the rear to 600 volts, not 700 volts. Know where I can get a replacement mod. transformer for the 32V transmitter? I don't! Inquiring minds would like to know! Why sacrifice a modulation transformer, which are expensive and rare these days. Thank you Dave, W3ST Secretary to the Collins Radio Association Publisher of the Collins Journal www.collinsra.com ----- Original Message ----- From: "Donald Chester" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Saturday, January 24, 2004 3:47 PM Subject: RE: [AMRadio] 4D32 rig testing > > > >Collins runs them at over 700 volts in class C plate > >modulated service (32V series). > >If they don't arc over, and you don't exceed the plate dissipation, > >what would be the problem? > > > > That's a good point. I recall an article pre-WW2 in QST which described > getting high peak audio power from modulator tubes by running unusually high > voltage on them, but staying within the rated dissipation rating. A point > was made that for example, the 807 is rated for a maximum of 600 volts plate > modulated. That means the peak voltage would be at least 1200 volts, and > that doesn't hurt them, and is still within factory specified ratings. So > for intermittent service, you should be able to run as much as 1200 DC volts > on an 807. I think the limit on plate modulated voltage would be the point > where arcovers would begin. > > However, I do recall using a quad of p-p parallel 2A3's in my audio driver. > I didn't have a proper driver transformer at the time, so I ran them @ 400 > volts/30 m.a. each tube. They are rated at 300 volts/40 m.a. maximum in the > RCA manual. That way I could get plenty of peak driving voltage to the > class B grids, despite having a driver transformer with too much stepdown, > but with exactly the same plate dissipation as per RCA recommended ratings. > They worked great, and drove the modulator tubes with very little > distortion. But I noticed that within a few months, the 2A3's would get > weak. I went through a couple of sets that way. Finally, I obtained a > proper driver transformer with the proper turns ratio, and slowered the > plate voltage back to 300v, and changed the bias so that each tube would > draw 40 m.a., and now I have had the same set of 2A3's for almost 10 years, > and they still test good. Maybe just a coincidence, but at the current > price of a new 2A3, I don't feel like trying to find out. > > My conclusion is that some tubes may not take overvoltage very well, while > others will not be harmed, and the only way to find out which ones is to try > them and see. If your tubes seem to be short lived, go back to the > recommended parameters. > > -K4KYV > > _________________________________________________________________ > High-speed users-be more efficient online with the new MSN Premium Internet > Software. http://join.msn.com/?pgmarket=en-us&page=byoa/prem&ST=1 > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio
