[Amps] DRAKE L-7

[Michael Tope]

Bob,

Are you using a stock L-7PS or L-4PS power supply with your L-7?  The 
numbers you are getting don't seem to make sense. The L-7 runs the 
3-500Zs in zero-bias mode similar to the SB-220. The resting plate 
voltage in the SSB position is something along the lines of 2500 to 2700 
Vdc. Using your numbers, 3200 volts x 640mA = 2050 Watts DC input x 65% 
efficiency = 1330 Watts.. If the 1800 watt number were real, your 
efficiency would have to be on the order of 90%. Also, at 1800 watts 
output with only 100 watts drive, the L-7 would need close to 13dB gain. 
That isn't a realistic number unless the anode voltage really is up 
around 3600 Vdc in which case I think you do get a little more gain, but 
regardless of gain, efficiency is never going to be any greater than 
70%. The gain of the L-7 is not much more than 10dB at stock plate 
voltage. Using a realistic numbers for anode voltage, I get 2300 Vdc x 
640mA = 1472 Watts DC input x 0.65 = 957 Watts @ 9.8 dB gain.

Using my L-7 just now on 160 meters with 90 watts CW drive (Diawa reads 
90W, FT1K meter reads 100W), I got 2100 Vdc x 700mA x 0.65 = 950 Watts = 
10.3dB gain. The actual reading on my Diawa CN-720B was 900 watts which 
would correspond to 10.1dB gain. The L-7 power meter read 1000 watts. I 
am not claiming that any of my numbers are super accurate, but they are 
consistent with what I would expect from a pair of zero-biased 3-500Zs.

FWIW, the anode voltage reading on my other L-7 reads higher even when I 
use the same power supply. I think this is from drift in the carbon 
composition resistors used in the voltage divider in the anode voltage 
metering circuit. Sounds like your watt meter calibration may have 
drifted as well.

73, Mike W4EF..........


On 4/5/2010 7:38 PM, Bob AD5VJ wrote:
> I would however like to run this by you guys:
>
> I am looking at the meter right now in ssb mode and it shows at idle 3600
> Plate Volts and zero plate amps.
>
> I know to get to power out to the antenna I multiply the ep by ip and take
> away about 40 percent since ep and ip is input power not the output power to
> the antenna.
>
> Leaving the amp in ssb mode and then placing my TS-850SAT in tune mode
> (50watts output) at key down the plate voltage shows 3350 plate volts, the
> Drake Plate current shows 580ma when the plate is dipped to minimum and the
> load is increased to full output until no further change can be detected and
> output is maximum and voltage and current are stabilized.
>
> Then I transmit in rtty (pulsing using the mark - no data) at a drive level
> of 100w (alc at midpoint - processor off) - leaving the Drake in ssb mode on
> transmit, I retune the amp using the mark pulse and the plate voltage drops
> to 3200 volts and the plate current shows 640ma of current with a power
> indication of almost 1800 watts output on the drake power meter.
>
> I think the power meter is supposed to be reading power out to the antenna
> but not sure it could be power input to the plate. Doesn't the output power
> in this case seem a little high on the drake avg power meter?
>
> Since this is an avg power meter, it would be 1.3 or so times this on voice
> transmissions for PEP seems awfully high.
>
> Bob AD5VJ
>
>
> 		_____________________________________________
> 		From: Bob AD5VJ [mailto:ad5vj at ad5vj.com]
> 		Sent: Saturday, February 06, 2010 4:47 PM
> 		To: Amps at contesting.com
> 		Subject: DRAKE L-7
> 		
>
> 		Can someone pls tell me why when I am loading up my antenna
> with my Drake L-7, the Keyer on my IC-761 gets stuck. I have to depress the
> VOX then turn the rig on and off to get it to unstuck.
>
> 		Any ideas?
>
> 		73,
> 		Bob AD5VJ
> 		
> 		
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