[AMPS] Weird matching problem...

[Richard]

>
>I have an Ameritron AL-80A that I've modified for a higher B+ voltage 
>(about 3600 VDC at 400 mA) and it generally works quite well.  I've also 
>added new, much quieter, brushless DC fan which now works works fine after 
>some shielding between it and the RF deck, etc.  However, I'd noticed that 
>the amp never loaded up well on 80 and 160 m after the mod: it was as if 
>there weren't enough tune or load capacitance.  My perception was right on, 
>because the original B+ was about 2700 VDC at 400 mA, so (according to 
>Eimac's data sheet) the output impedance was around 3500 ohms.  At 3600 
>VDC, it's closer to 5000 ohms.  This changes things quite a bit for the low 
>bands.
>	To maintain the same loaded Q under these new conditions, the pi part of 
>the output network (it's a pi-L) needs more inductance and less capacitance 
>for the same output impedance (the ideal inductance needed for the L isn't 
>as critical, though it changes slightly).  Without increasing the 
>inductance, the capacitance needed for resonance rises significantly, as 
>does the loaded Q.
>	I found it easy enough to guesstimate how much to move the taps on 40 and 
>80 m, but 160 m isn't as straightforward, because there are no taps to 
>move: it's all there is. So, I added a small inductance -- about 10 uH, 
>or  13 turns of an AirDux 1609A I had on hand.  There isn't enough room to 
>add this coil to the end of the existing inductor, extending it's length, 
>so I placed it perpendicular to the 160 m coil.  This also places it 
>perpendicular to the 10-15-20 m coil a bit more than an inch from the end. 
>Afterwards, 40, 80 and 160 m all tune much better and efficiency seems to 
>be up.  20 and 15 m are as good as ever, but 10 m is now wacky.

€  Do you have forced air cooling on the tank inductors?

>	Initially, everything on 10 m tunes up fine but, after about 15 s, the 
>output begins to drop. The output continues to drop and the plate current 
>rises. I know it sounds nuts, but overall, the match goes to pot, and I am 
>completely unable to tune the output! No combination results in a dip in 
>plate current or significant output.  

€  A Pi-network tank behaves more like two L-networks in series than like 
a L/C parallel resonant tank.  A Pi-network is tuned for max out with max 
rated drive.  There is little dip at this point. 

>Obviously, plate dissipation rapidly 
>goes through the roof.
>	I'd guess that something is getting hot. It's not the parasitic 
>suppressor, since the resistor looks fine, and is without any 
>discoloration. I haven't screwed up the nerve to quickly rip the cover off, 
>throw a short across the B+ supply, and go touching everything to find the 
>suspected hot item, 

€  There is no DC on the tank.  Stand on a matt, remove drive, and you 
should survive.  

>...

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end


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