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

To: Jim.thom@telus.net, amps@contesting.com
Subject: Re: [Amps] Tank eff... with more B+
From: TexasRF@aol.com
Date: Mon, 15 Feb 2010 08:28:31 EST
List-post: <amps@contesting.com">mailto:amps@contesting.com>
 
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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