[Amps] Drake L4B to 6 Meters Help

[Jim Thomson]

Date: Mon, 12 Dec 2016 10:01:33 -0500
From: "Michelle" <michelle_melt at windstream.net>
To: <amps at contesting.com>
Subject: [Amps] Drake L4B to 6 Meters Help

Hi Group,
    I have converted a Drake L4B to 6 meters and I seem to be stuck at 750 watts output, I have tried a PI input (no change) , different tank coils (less output), The current setup listed below I have checked both input and output with a antenna analyzer at 50.125 and they both seem to be right on spec. Thanks in advance for your comments.

Plate volt 2300 under load
plate current .650
Grid 220
Drive 100 watts
Output 750 watts

Input circuit T match (works as it should, flat match)
Plate cap 3-30 pf vacuum variable
Load cap 25-350 pf (original Drake plate cap) 1/3 meshed at max output.
Tank circuit PI-L, L1 3 1/2 turns 1/4" copper tubing 1 1/2" id, L2 4 turns 16 ga 1" id.
RF choke 42 turns of 22 ga on 1" porcelain form
Bifilar choke Stock Drake 20 turns each leg of 10 ga on 5/8 ferrite rod
All grid pins directly grounded to chassis
Parasitic suppressors are 47 ohm 2 watt with 1 turn 16 ga
T/R relay changed to vacuum relays

What am I missing here? Has to be something really simple I'm overlooking,

Thanks again for your comments,

73s
Michelle
WB8QGR



##  I was going to suggest using a PI for the input instead of a T...but you tried that already. 
A buddy tried the T input on his  2 x GS35B 6m GG triode amp..and couldnt get the input swr
flat.  Swapped to a Pi, and all is well.   Less loss with a PI input, since only one coil.  Dont go cheap
on the coil gauge,  since skin effect on 6m is pronounced.  Use something like 10-12 gauge..or better
yet, cu strap, wound flat.   Like  .25 to  .375 inch wide cu strap on a real small ID.    Then u end up with
no stray C between adjacent turns.  Strap coils are like 2 x knife edges facing each other.  

##  A PI net for the tuned input is the same as a low pass filter.  You want harmonics to be suppressed, right
at the input to the cathode.   And I dont mean harmonics from the xcvr. 

##  Your loaded tank Q  with a PI-L  on the output will be sky high.  I would dump the PI-L  in favour of  a 
modified PI net.    The pair of 3-500Zs  already has loads of stray C  between  anode to grounded grids. 
That stray C is in parallel with the  3-30 pf  tune cap.   Insert a small value of uh, like .2 uh, in the form of
a strap  coil, .5 to .75 inch wide cu strap,  and small ID,   just BEFORE the main PI net. 

##  The tube C  plus the small coil forms a step down L network.  This  will transform the plate load Z 
from 2081 ohms  ( 2300 vdc and 650 ma)  down to far less, like 500- 1000 ohms.  Then the PI net
can easily be built to work with the lower plate load Z.    Ok, now you can achieve a lot lower loaded
tank Q.    Circuit is then called a L-PI.   Harmonic suppression is excellent.   EFF  will improve a bunch.
It can also be used with a PI-L.....in which case it becomes a  L-PI-L. 

##   .25 inch OD  tubing has the same aprx circumference as  .375 inch wide strap.  Strap conducts RF on
BOTH sides.  Tubing only conducts RF on the outside of the tubing.   .5 inch wide strap =  .318 inch OD tubing.
Again, strap, wound flat, has no stray C between adjacent turns. Easy to wind flat strap coils vs tubing coils,
+ overall diam is reduced greatly. 

##  what are you using  for plate block caps ?   The drake OEM should more than suffice. 
Watch out with the wattmeter setup.  A buddy with a hb  8877 on 6m had lousy eff.  He tried rewinding the
tank coil several times, and every other trick in the book,  like rebuilding the tuned input......all  with very very 
slight incremental  increases in eff.  Turns out the damned  bird  Slug he was using....was out to lunch. 
After a calibrated slug was installed, measured eff was way up.   Problem solved.   The basic amp was not
the problem, the slug was. 

Jim  VE7RF