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4cx15000 input swr problem

S

scottdarling

Guest
I have a aerocom transmitter with a 4cx15000a tube.Have almost a 100x input multiplication factor. Amplifier works great but if i key it for more then a minute or two the input swr goes from flat to about a 1.5 between the radio and the aerocom.The aerocoms output drops a few thousand watts and I lose all foward swing out of the aerocom.Antenna system checks out ok.Can anyone help me.Thanks
 
Im pretty sure its fine I had a amplifier with a 6000a7 yu148 tube and never experienced any problems swrs are around 10 watts reflucted per 10kw out.A freind said possibly a capacitor in the input circuit or the dc blocking caps im stumped.Would hate to start ripping the wrong things out.
 
I meant to say reflected.Reflected power decreases as power output of the aerocom decreases and swr between radio and aerocom goes up.If a plate dc blocking capacitor were to drift from heat would i see an increase in input swr or a decrease.Are there any other components that could be causing this problem.
 
Could be a bad capacitor on the input of the amplifier causing the resonant frequency to change.
Check the neutralization of the power amplifier. It could be breaking into oscillation as the tube warms up.
 
Remember, as you increase the ijnput power to the 4CX15000A, its stem will expand a bit, and this will change the interelectrode capacitances somewhat. Can you tune out the VSWR if you keep it jkeyed? If so, the tube is changing inside, otr one of the perepherial components in the grid circuit is changing. If it's the tube itself, you may have to hit a happy medium on the input tune... or leave the radio keyed.
I have half a dozen transmitters which use this tube and a similar one, we try to keep them tuned where they will cold start if necessary, and then warm into tune.
 
so would anything in the tank circuit cause a symtom like this.I can tune the swr back down but i dont leave drive applied all the the time so what should i check out first the input circuit or neutralization.If it were atarting to ocilate wouldnt the output power increae not decrease.Also wouldnt i not be able to see the dip in the power output while tuning it with audio.I never had this problem before but since i changed the class of operation to closer to ab1 now the amplifier swings foward and sounds great but i have this problem.I only draw a quarter of an amp of no signal plate current and 1500 volts of screen voltage.Also put in 3 250pf 25kv 33 amp each doorknob caps in parellel so i have a total of 750 pf at over 100 amps 25kv for the plate dc blocking caps.I have a 1000pf 15kv doorknob on the back of the roller inductor that hooks up to the grid of the tube.The drive and the nutralization caps hook up the the other end of the 100pf cap.
 
Ham and 10 kw? Congratulations

Is this is a continuous duty transmitter running intermittant? It is common to see during a first start high vswr that bottoms out as the filament voltage warms the tube.

The filament stays "hot" all the time? All you are doing is turning the drive on and off?

The doorknob caps are relatively cheap. Shotgun the whole mess. I don't see coils "go bad" but caps are lightning related usually. Not much to go wrong on the input.

Driving it with 250 watts?
 
It does run intermittant.Not 100% duty cycle.SWR Is flat when I first key but after talking for a a little bit over 10 seconds or so the output power drops and the swr between the radio and the amplifier rises.I dont run the amplifier up to 10kw it will do 20 i run it low.Take about 100 watts to get a 10kw carrier or around 100 watts of total rms swing to get it to swing up to 10kw.The input doorknob is cheap but the ones I changed in the output are over 120 bucks each.Would hate to waste a bunch of money.Im refering the plate blocking capacitor.
 
scottdarling said:
It does run intermittant.Not 100% duty cycle.SWR Is flat when I first key but after talking for a a little bit over 10 seconds or so the output power drops and the swr between the radio and the amplifier rises.I dont run the amplifier up to 10kw it will do 20 i run it low.Take about 100 watts to get a 10kw carrier or around 100 watts of total rms swing to get it to swing up to 10kw.The input doorknob is cheap but the ones I changed in the output are over 120 bucks each.Would hate to waste a bunch of money.Im refering the plate blocking capacitor.
Click to expand...
Never seen a plate blocking capacitor do anything other than short...generally with a huge explosive sound.
 
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