[Amps] 4-400 output problem

TexasRF@aol.com TexasRF at aol.com
Fri Jan 6 12:46:10 EST 2006


 
Hi Steve, I have not been there/done that so this is a speculative answer  at 
best:
 
When you increase drive and then load heavier to reduce grid current, the  
out put C is reduced, not increased. It sounds as if you run into a limitation  
caused by the minimum C in the circuit. If the plate inductor is off a bit, 
the  circuit may be acting more like an L network instead of Pi network in which 
case  the output C would be quite small or not at all.
 
I would have to think about which direction the inductance would need to  
change in this case, but you can figure that out. Like I tell friends: hey, I am  
the concept man you do the details!
 
73,
Gerald K5GW
 
 
In a message dated 1/6/2006 11:16:13 A.M. Central Standard Time,  
steve at dx4win.com writes:

Hello  all,

Was sitting here thinking about a problem with HB amp I built  about 4 years 
ago, which still perplexes me. Put is aside at the time,  figuring I would 
take a fresh look at it when I got the time.  Well,  never found the time....

This is a pair of old, well worn 4-400's in a  grounded grid configuration, 
mono band for 10 meters.  Plate voltage  is about 4400 no load, with about 
4000 or so loaded, as I recall.
Plate  circuit is very compact with few, if any long leads.  Most things are  
directly connected to each other.   Used #2 solid copper for the  L coil and 
vacuum and air caps for C1, C2.
Tuned input using 2,  itty-bitty, end mounted, nickel plated, padder type 
air variables and a  toroid core.  Forget the core #, but it was 'black' and 
recommended  to me by, Frank, W3LPL They are located right at the tube 
sockets, so no  long leads
Everything looks as I would expect it with a grid dipper , or my  Autek box.
Amp is stable, and makes power.

Here is the 'hitch in the  get-a-long':

(With plate current dipped) As I increase drive, the grid  current goes 
up.  I tweak the C2 cap, grid current goes down; RF  output goes up.  Maybe 
touch up the 'tune' cap a bit. All as  expected.
At some point, when drive is increased and grid current goes up,  no 
adjusting of the C2 cap will cause this to be transferred and seen in  the 
RF output.
In the past, as probably most of have seen, this  condition happens when you 
run out of C2, at which point you would  parallel a bit of extra fixed 
capacitance, but I have plenty of C2
I  can not make the extra RF on the input, be amplified and appear on the  
output.  Drive goes up; grid current goes up; tubes go RED; output  stays 
the same.

Driving it with about 100 w.

These same two  tubes put out 1200 watts on 10 meters for years in another, 
band switched,  amp at 55-60% efficiency.   All I can get in this setup is 
about  850-900 watts out.  Same power supply also (same voltages.)

Being  the tubes are old (paid $20 for the pair in 1981 as 'pulls' from  
commercial station in area) I've always needed to drive the grid harder,  
running 250-300 mils, but current can be driven to 400 ma when the above  
happens.  In any case, I've gotten my money's worth from these two  tubes.

Everything else seems to work so well, could I be doing  something like 
saturating the core on my input?  It is wound with  something like #16, or 
maybe #18.

(Disclaimer:  Understand, I  am not an 'electronics professional', so don't 
bust my chops if I have  taken liberties with technical terms and concepts.)

Any help, or  comments, from the 'gifted gray matter' out there would  
appreciated.


73 de Steve,  NR4M


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