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Author Topic: Drake L7 10 meters inefficient at SSB voltage?  (Read 1519 times)
WA7KGX
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Posts: 104




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« on: November 19, 2012, 07:59:40 PM »

I have been testing my L7 with Eimac bottles and noticed an anomaly on 10 meters.
On the CW power setting with 1800 volts measured Ep and .54 amp plate I get 550 watts out
for an efficiency of 56%.  Driver is an ICOM 756pro at max output.

Switching to SSB the plate voltage is 2600 under load.  Plate current is .62 for 1612 watts input.
Output is 729 watts, efficiency is 45%.  Bottles warm up to a dull red color.

On 20 meters I get 57% and 56% efficiency.  No observable color on the bottles.

Is this normal for the L7?
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W1QJ
Member

Posts: 1443




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« Reply #1 on: November 19, 2012, 08:51:17 PM »

Did you re-tweak the load and tune after you switch from cw to ssb?  What is your input swr to the radio on 10 meters?
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WA7KGX
Member

Posts: 104




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« Reply #2 on: November 19, 2012, 10:41:54 PM »

I retweeked after switching to SSB voltage.
The settings were slightly different.

I repeated the high power test using the 756pro RTTY mode.
This produces more output than WSPR does.  This time I got
900 watts for .68 amp at 2600 volts.  1768 watts input,
50% efficiency.

Actually, shouldn't that be 1868 watts in, counting the drive power?
I think I read that the drive power ina GG amp is passed through.
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W1QJ
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Posts: 1443




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« Reply #3 on: November 20, 2012, 03:59:46 AM »

Please answer the second question I asked you.  You are doing better, but there is still the other question I asked that is important.
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K6AER
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Posts: 3483




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« Reply #4 on: November 21, 2012, 01:42:15 PM »

Typicaly output on many HF amplifier have less efficency on 10/12 meers. Stray capacitance and inductance cause the amplifier to have more tank losses. The tubes in many cases have a bit less gain at higher frequencies. The input matching may also be as accurate as on other bands.

A lot of the older amplifier used the same size inductors at high frequencies and as a result the inductors were not able to handle the current.
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W1QJ
Member

Posts: 1443




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« Reply #5 on: November 21, 2012, 07:58:35 PM »

Using a solid state radio on 10 and 15 meters with an L7 is tough.  There is no input matching on the L7 for 10 meters.  I had to use my tuner in the radio to match the input.  I finally fashioned an input circuit that worked perfectly and altered 15 meters to work perfectly as well.  It was a hassle to do this but I managed to come up with the recipe over the course of several hours,  I had to remove the common cap to ground on the input of the entire network to make that work.  It slightly threw off the other bands but was still ok.  I have a pet peeve to have to use a tuner on the input of an amp.  I like to see a flat swr with no additional aids.
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