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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