[Central-Electronics] 20-B power output

[Jim Candela]

On 10/7/2011 2:16 PM, Tom Elmore wrote:
> How much power output should I see on a 20-B exciter say in CW mode in the
> 160m position? The most I get is between 11 to 12 watts. I have read it
> should be closer to 20. Is this input power to the final?
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> Thank You
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> Tom Elmore
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> Anchorage, Alaska
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Tom,

     My experience with the 20A is that you get the most power output on 
80m, and a little less on 160. For 160 we have a lot of losses in the 
output L-C tank since the wire used is very fine to get enough 
inductance in such a small area. Also, the tank coil has a tap that 
approximates a 50 ohm take off point. There is no loading control to 
match other impedances. If you have a shorted turn in that tank, expect 
higher losses, and reduced efficiency. Those air wound coils on vintage 
gear often need to be replaced.

    If you have a 160m crystal, try that as it bypasses the balanced 
modulator. On one of my QRO 20A's, just this week I was playing, and I 
had 40w on 80M using the VFO, and then using a crystal I had 58 watts. I 
swapped the 6U8 tube with a 6MU8, and the VFO power jumped to 58 watts. 
I'm not saying a 6MU8 is better, but more like the 6U8 pentode section 
may have been low on emission. The 6MU8 IMHO is a great upgraded 
replacement for the 6U8 in the 20A since the pentode has beefier ratings 
with most of the other parameters similar to the 6U8.

    The B+ voltage is influenced by line voltage, and rectifier tube 
type. In general a 20A on 125 vac line has the B+ too high, and the 
filaments at about 6.9v. In fact the electrolytics based at 450v ratings 
are sometimes overloaded after the power is turned on. It might be 
better to put a variac on the 20A set to 115 vac.

    I recall playing with a stock 20A on 80M, and the 6AG7's transition 
from class AB1 to AB2 at about 15 watts RF output. If you were to hold 
the grid bias steady, the power would go past 20w in class AB2 mode. But 
since the bias just goes more negative as you draw grid current, the 
power (stock 20A) just saturates around 15 watts give or take depending 
on line voltage.

   So I would not be too worried about 12 watts on 160m. It sounds 
pretty normal to me. Might do 15w if you were to hold the 6AG7 grid bias 
steady as grid current is drawn. But going down modification row is 
tricky, and full of unintended consequences.

Regards,
Jim
WD5JKO