[Moon] W6PO 2m Amp I/P circuit help.

[TexasRF at aol.com]

Hugh, your description of the problem gives some really good hints about  the 
cause.
 
First, the original input circuit design worked very well. Your added turns  
on the input coil may well be the culprit.
 
At lower drive power, the cathode impedance will be higher than with normal  
drive. The input coil is one leg of an L network that steps the 50 ohm input 
up  to some higher impedance. If the input inductance is higher than ideal, 
then the  stepped impedance is also higher, hence the good match at lower drive  
levels.
 
I would suggest you remove a turn or two from the input  inductor and try 
again.
 
If you would like to make the input circuit adjustable, you  can add a series 
variable capacitor of about 30 to 50pF maximum. Every ohm of  capacitive 
reactance will reduce the inductive reactance a like amount. In this  case, you 
would purposely add some inductance to make the total reactance reach  the 
desired amount.
 
73,
Gerald K5GW
 
 
 
In a message dated 7/2/2007 11:24:44 A.M. Central Daylight Time,  
hduff at cogeco.ca writes:

I'm  having an interesting problem with my newly built W6PO 2m 
amplifier;   The amp was built to spec per the Eimac newsletter reprint  
to the  best of my knowledge and ability with the exception of a G3SEK 
triode  board. 

The input T circuit is comprised of the N connector - a series  1000pF 
doorknob cap - the first coil, shunt tuning cap to ground,  the   second 
coil - tube cathode.  The first coil is  airwound, not slug tuned but 
extra turns were added and the coil is tuned  by expanding/ compressing 
turns (not critical as I understand by reading  some archive posts).

I cannot get the input circuit to yield a low SWR  to the FT-847 exciter. 
The transmitter is folding back  and I'm only  able to see about 500 
watts out of the amp.  With the transmitter set  for maximum power out 
(50w) the best I can get is  between 3 and 4:1  SWR and the drive power 
drops to about 25 or 30 watts as seen on the power  meter. 500v out seems 
about right  with 25 or 30w drive and 2900V on  the plate so the gain of 
the amp is normal. (HVPS uses a pole-pig xfrmer  good for well over 1  amp.)

My observations:

- I can get a  good SWR reading when driving the amp with very low power 
but once I  increase the drive beyond a few watts the SWR  rises to about 
3:1 or  4:1. Any attempts to retune the shunt capacitor for lower SWR are 
futile.  I've even tried compressing/expanding the  coils with the input  
compartment cover removed (cover off-adjust-cover on) but can't do it  
actively since the cover needs to be  installed in order for the  blower 
air to keep the tube cool. I've wound and rewound both inductors  +/- the 
prescribed number of turns  and different I.D. in a variety  of 
combinations. I also tried mounting the variable cap in a couple of  
different places for better grounding  but still no joy.

-  Tuning of the shunt capacitor for a dip in SWR is extremely sharp, far  
more than I would expect it to be. (A clue !?)  I can almost   blow on 
the tuning control and the SWR will take off.  The fully  meshed 
capacitance is 25pF, unmeshed is about 5pF.

- I have two  3cpx1500a7 tubes (assumed to be brand new) and the amp is 
behaving the  same regardless of which one is installed.

I prefer not to use an  intermediate amplifier between the radio and 
amp.  I want to make the  amp work properly.

I know it's hard to troubleshoot from afar but I'm  all ears if anyone 
cares to take a stab at helping to locate the problem.  
I'm anxious to see the amp produce QRO power.

Tnx es 73 de Hugh  VA3TO


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