[Amps] Tank eff... with more B+

[Jim Thomson]

Date: Tue, 9 Feb 2010 23:39:19 EST
From: TexasRF at aol.com
Subject: Re: [Amps] water cooled 160m amp.
 
Jim, if you look art the curves for any of these tubes you will notice that 
 the plate voltage swing can't be allowed below four or five hundred volts  
without excessive grid current (or screen current in the case of a  
tetrode).
 
That means the total voltage swing is plate voltage minus 500 volts  
typically. If you are using 2700 vdc on the plate, the total swing is 2200 volts  
which is 81.5% of the plate voltage.
 
If you run 4000 volts, the plate swing can be 3500 volts or 87.5% of the  
plate voltage. The difference between the two voltage levels makes the higher 
 voltage have about 7% more efficiency.

##  are u saying the tank eff will rise 7% ?? 

##  on a L4B, that works out to 73% on low voltage, and  81% on
high voltage.   That is a 8%  diff.    I don't see any 8% increase in eff.
Then again, low V  is 625 watts out,  high V is  1290 w.   Two
x diff power levels.. BUT at least the plate load Z is the same. 

##  I understand abt the V swing  vs  curves  for tubes... and I
understand what ur saying, and the concept,  I just don't
measure it in practice.   Are we supposed to be comparing
identical  DC input levels... say  2500v  @ 800 ma   vs
4000 v  @ 500 ma  ???   [ same plate load Z]   or  high plate load Z..
VS low plate load Z ??    




 
Of coarse this added efficiency may not actually be available if it makes  
the power output exceed our 1500 watt limit.

## measure the power at the ant, not the amp. Assume .5db to 1db
 of feed line loss.. [10-20%] 




 
Also, the higher plate voltage will dictate a higher plate load impedance.  
Usually higher load impedances come with a higher loss due to the Q loaded 
being  higher when compared to Q unloaded. A really good plate inductor will 
mitigate  this effect to a large extent, especially at higher frequencies.

## The  1/4"  tubing coil in the L4B  runs hot on 20-15-10m.  I can
see a higher loaded Q on maybe 10m.. but not 20+15m.  The fix  for
that is to add a tiny uh coil, b4 the main PI net.. and transform the
plate load Z way down... THEN the main PI net see's a lower loaded Q.
That's  easy to do on the GM3SEK  PI  spreadsheet. 

##  running high RF current through coils on HF is usually bad news.
tank eff drops, coils cook, and poor band switch takes a beating. 
Even if you used bigger tubing coils, you still cook the bandswitch.
High loaded Q = narrow BW as well. 

## best combo I can come up with is vac caps, big tubing coils,
small tubing coil  b4  vac tune cap... then transform the plate load Z
down to a lower value.   The tube C  plus extra  small coil b4 Pi net,
form a step down LC network.   Then eff is up on the high bands. I call
it an L-PI    [or  a L-PI-L ]   On HF anyway,  lower loaded Q = better eff.

later... Jim   VE7RF



 
73,
Gerald K5GW
 
In a message dated 2/9/2010 8:16:37 P.M. Central Standard Time,  
Jim.thom at telus.net writes:

From:  "DF3KV" <df3kv at t-online.de>
Subject: Re: [Amps] Water cooled 160  meter amplifier..
To: <amps at contesting.com>
Message-ID:  <1Nep7B-0Om3CC0 at fwd00.t-online.de>
Content-Type: text/plain;  charset="us-ascii"

That is the same efficiency as with a 8877 at that  voltage.
It will be much better at higher anode  voltage.

73
Peter

##  why should the tank eff   increase.. with higher  B+   
voltages applied  ??   I can see gain going up a bit. 
I can also see more power  out.  What has B+ level 
have to do with tank eff ??? 

later...  Jim   VE7RF