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

To: TexasRF@aol.com, Jim.thom@telus.net, amps@contesting.com
Subject: Re: [Amps] Tank eff... with more B+
From: TexasRF@aol.com
Date: Mon, 15 Feb 2010 08:37:19 EST
List-post: <amps@contesting.com">mailto:amps@contesting.com>
 
Oh, I left off one summary point: The tank efficiency does not increase  
with more B+. Due to the associated higher Q, the tank efficiency actually  
decreases.
 
The tube efficiency does increase with more B+ and enough to more than  
cover the tank efficiency decrease as shown.
 
73,
Gerald K5GW
 
In a message dated 2/15/2010 7:29:05 A.M. Central Standard Time,  
TexasRF@aol.com writes:


Hi  Jim, I will rephrase a bit.

I used peak power in the efficiency numbers  and should have used rms 
power. 
My bad.

Assuming the plate current  is 1A for both examples, low power would  
generate 2200w peak, 1555w  rms with a d.c. plate power of 2700w and  
efficiency is 
57.6%, high  power would generate 3000w. peak, 2121w rms with a  plate 
power 
of  3500w and efficiency of 60.6%. The difference then is 3% more   
efficiency. Again, this is for the tube only, no tank losses.

I  know that many tubes can do better than this and that simply means that  
 
their plate voltage swing is higher than our example. These tubes have  
lower 
minimum plate voltage than the 500v example I used.

Well, the  plate tank circuit does have loss. Assuming you use the same   
bandswitch position, the plate inductor is the same one for high power  and 
 low 
power. When you tune the tank circuit for maximum power out,  the current 
is 
higher with the higher plate voltage because the amount of  power is 
higher.  Higher power comes with higher voltages and  inductor current 
rises as a 
result.  Another way to say this is that  the loaded Q is higher.

If the tank is designed for a Q=10 at low  power, then in our example Q  
would be 2121/1555 times 10 or 13.63 at  higher power.

If the inductor loss is 200w at low power, it will be  273w at high power 
in 
our example. So, we would end up with 1555 - 200w at  low power and  
efficiency of 50.2% and 2121 - 273 at high power and  efficiency of 52.8%.
The difference being 2.6%.

Obviously this plate  inductor needs improvement but is probably in the  
ballpark for many  10 meter amplifiers.

The above example would imply an unloaded Q of 78.  If this Q was improved  
to say 200, then the loss at low power would  be 78w , power out would be 
1477w  and efficiency of 54.7%. At high  power Q=13.63, loss is 145w, power 
out 
is 1976w  and efficiency of  56.5%. The difference in efficiency being 1.8%.

The difference is so  small that it would be difficult to measure with  
ordinary power  measuring equipment. We are talking about the width of a 
meter   
needle at 1500w.

All of the commercial amplifiers with high/low  voltage switching were  
intended to tune on cw to establish a plate  power match to 1kw power input 
and  
that would produce maybe 550 to  600w output. Then upon switching to higher 
 
voltage, the tuning was  left untouched for a corresponding increase of 
plate 
current that produced  the same load impedance. If the voltage is increased 
by 44% and the  current by 44% then the plate load impedance remains the 
same and   power input and output is doubled. Today, most operators tune at 
the  
higher  voltage and leave it there. This reduces the plate load  impedance 
somewhat due  to the plate voltage sag but as others have  said, this is 
not 
all bad as  linearity is improved at the expense of  slightly less power 
output.

Anyway, all this was in answer to "why  would efficiency improve with more  
B+". This is why. Think of the  higher% plate voltage swing.

73,
Gerald K5GW



In a  message dated 2/15/2010 4:08:04 A.M. Central Standard Time,   
Jim.thom@telus.net writes:

Date:  Tue, 9 Feb 2010 23:39:19  EST
From: TexasRF@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@telus.net writes:

From:  "DF3KV"   <df3kv@t-online.de>
Subject: Re: [Amps] Water cooled 160   meter  amplifier..
To:  <amps@contesting.com>
Message-ID:    <1Nep7B-0Om3CC0@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


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