========== > From: "Geoff/W5OMR" <[EMAIL PROTECTED]> > > > Yow! > > > > 3000VDC @ 300mA = 10,000 ohms. (This is where the transmitter lives > > according to the notes I have on it) > > perhaps thats what it's capable of, but you're not going to run it there are > ya?
I had not thought about it.. actually the maximum recorded operating conditions at carrier are [EMAIL PROTECTED] (on an old Texas State Guard freq.). The plate voltage is adjustable from 0 to 4000V. > > Come up with a more realistic power level, and go from there. For that > matter, 60 Hy will probably do you nicely, as long as the iron can handle > series current through them. > > > 10,000 ohms = 80H @ 200 Hz > > I ran 2000V at 250mA for the longest time, with only 40Hy of choke, and > never had a problem. the highest rating of current thrugh one of the chokes > was 275mA (whiich is why I stayed down around 250mA - besides, 500w > DC input is ~around~ 350~375w of carrier out... but, man the positive peaks > I had then... (http://www.qsl.net/wa5bxo/asyam/aam3.html) I could go as low as maybe 2000V (considering getting enough power from the modulator). These conditions aren't recordrd, but with the variable L pi-network it could probably be tuned. I'll have to experiment, don't want to go on the air with too much power. It'll be on a dummy load for a while during shakedown anyway. > > > And I suppose I had better avoid swinging chokes here too. > > > > Anyone have a suitable reactor for sale? I have a couple of 10's, but > > obviously this would not be near enough. > > > > I guess the main point in putting reactors in series would be to place > > the highest voltage-rated one at the modulated voltage point. > > Voltage? No... Current? THAT's what carries the mail! All the chokes are rated for 400-500mA. I just thought I should put the best-insulated ones in the 'chain' nearest the point with the highest peak voltages. > > 73 = Best Regards, > -Geoff/W5OMR > ========== > From: Brett Gazdzinski <[EMAIL PROTECTED]> > > I went out an bought a 50 henery choke from Peter Dahl. > It cost $200.00 I think, but is very nice. > It works fine at 2000 volts and 400 ma on the 813 rig. > I noticed that playing with the cap value changed the sound of the > transmitter > quite a bit, more is not always better. > I cant say removing the current from the cvm5 seemed to help the sound > of the rig much. > > Maybe the cvm5 is not so bad to start with... > > Brett > N2DTS I agree, looks like I should plan on a 250-500W output from the modulator. Looking at the size of that mod xfmr of mine, I think it is larger than the CVM-5. I see the CVM-5 is 7"x12"x9" and weighs 60 lbs., but it also has large end bells. I think the 12" dimension is the length including the end bells. I don't know what the actual core size is. I may not really have to worry much about DC in the secondary. Its core is pretty large, 5"x12.5"x9" on the outside, and the cross section volume is 4.25"x4.25"x5". The weight on this open-fram unit is 124 lbs. But I will start anyway with the chokes and sweep the audio once the rig is up, to see what the response looks like. Maybe I have an oversize transformer? Patrick ========== >From: "John Coleman, ARS WA5BXO" <[EMAIL PROTECTED]> > > I guess the "more is better statement" is strictly a personal > preference. What I should have said is "If you want the audio waveform > to not be modified by the output circuitry then more is better". Having > to little capacitance will modify the audio wave form and can shift the > phase of lower frequencies for instance if a 200 cps note and a 400 cps > mote or together and appear as natural harmonic causing certain peaks of > a non sinusoidal wave to go positive then the shifting of the phase > relationship between these two frequencies may cause a repositioning of > the peaks and there by cause the loss of the peaks. This can sound > better. I guess for some, better sound is not always the natural sound. > It can even shift it so that the peaks go negative. This is also done > with certain settings of tonal control circuitry. > > 73,John, > WA5BXO This is another thing I had not yet considered. Not DSP, but ASP? I can see I will have some experimenting to do. It seems this might also depend on my voice as well as I read in the other posts. Patrick ========== > From: Brett Gazdzinski <[EMAIL PROTECTED]> > I was talking about the pass capacitor, using a big one seemed > to interact with the choke and mod iron, a big cap had the sound > muffled, or something, I was disappointed with the results. > I tried changing the size of the cap, and found 4 uf I think > sounded natural. > I had tried all the way up to 40 UF. > > The power supply has 70 uf or more in it... Mine's going to have between 40 and 80. It depends how I decide to put it back together. One feature the rig has is to switch the modulator rectifier filaments separately from the PA rectifier filaments, so actually there are two power supplies. The modulator is going to take what looks like the same current at zero signal and 100% modulation as the final, any suggestions where to put more of the capacitance? Patrick > > Now that I actually have a good mike and preamp, maybe I should try > other values again. > > More farads in the power supply is always better, but you need step > start or some way to bring it up gently. > Some run the voltage on all the time and switch the cathode, I never > tried that, easy with triodes likely, not sure about tetrodes.... > > WA3JVJ has a push pull parallel 805 mod deck with a huge string of > computer type caps, gives a full farad or more, and he runs the voltage > on all the time. > > I think its quite crazy, about 60 computer grade caps in series parallel, > at 2000 volts or whatever, if one cap fails, the explosion and mess > will likely be very nasty! I have 20 of the 450V 2000uF caps, but was not going to use them, was going to stick with some 8uF 4KV caps I have, and maybe put them in series parallel, I just don't have anything higher voltage except the original four 2uF 6KV caps. Duncan's power supply simulator shows that there would be quite a bit of ripple with the 10H-4uF combination at 250mA. The issue is that the HV XFMRs are 5KV each, and presently in the choke input configuration. The HV is controlled by two variacs. The circuit is unusual, as each transformer has a single secondary winding and one side is grounded. Each HV output feeds one side of a full wave center tapped rectifier circuit. Therefore, you must bring up both variacs together, and balance the HV AC output or you will get hum I suppose. The DC resistance of the transformer secondaries is about 25 ohms, they are old pole pig cores. So you see the power supply is yet another challenge to work with. I am having a great deal of fun though, more than I have had in a long time. I thought to throw more of my spare chokes at the power supply rather than as a modulation choke, but maybe the caps will make it up. Patrick > > Brett > N2DTS > ========== > From: "John Coleman" <[EMAIL PROTECTED]> > > I agree on the explosion Brett!=20 If it's going to explode, it should be a proper explosion with PCB's.. > If the choke and coupling cap are both large enough there should be > no change in the output waveform than if it were connected to a > resistive > load. Perhaps it is the capacitance of the choke windings that is > throwing > it off. It would be interesting to know what the free resonance of the > choke is. Or perhaps too much RF bypass capacitance in the RF deck. At > any > rate I can't help but think that a smaller coupling cap would create > resonance which would put a reactive load on the modulator as would too > little reactor. This would make the modulator to see a different load Z > depending on modulating frequencies. In turn there would be > intermodulation > distortion. And a shift in phase relationships of frequencies that make > a > certain wave shape. > > Interesting subject! > > John, WA5BXO ========== > From: "Donald Chester" <[EMAIL PROTECTED]> > > > > 3000VDC @ 300mA = 10,000 ohms. (This is where the transmitter lives > > > according to the notes I have on it) > > > > > >perhaps thats what it's capable of, but you're not going to run it there > >are ya? > > That's what the first high power homebrew rig I remember ever actually > seeing ran - an old fellow in Kentucky. He used a pair of 100TH's in the > final, modulated with a pair of triode connected 813's and a homebrew > modulation transformer. His audio sounded like a tin can, but did he strap! > > His rig inspired me to build a single 304TL final moulated first by a pair > of 211's (@ 600w input) then later with triode/813's then 810's and later, > 833-A's. My 866A's kept flashing over @ 3000 volts, using a pole pig, so I > settled for 2500v @ 400 ma. Besides that gave a better match to the > modulation transformer. > > Don K4KYV So maybe it's not unusual to run a rig there. It all comes down to the rules I guess. I need to look them up, see what the limits are for AM. I understand the interpretation or measurement method has changed. Patrick ========== > From: "Donald Chester" <[EMAIL PROTECTED]> > > >I cant say removing the current from the cvm5 seemed to help the sound > >of the rig much. > > > > I tried it with an older version, the VM-5. It didn't radically affect the > sound of the rig over the air, but with the DC thru the winding, it talked > back like a speaker in the transmitter cabinet. I couldn't crack up the > audio without accoustical feedback. With the reactor, the VM-5 was quiet > as a mouse, and the waveform at low audio frequencies (< 100~) looked more > symmetrical on the scope. I haven't run DC through a modulation transformer > secondary since about 1963. This is a good point. I am interested in very good low frequency fidelity if possible. I've been building audio amps for years, I figured that experience might help me with this transmitter. Patrick
