[AMRadio] Central Electronics 20A Power Modification

[Jim Candela]

Hi all,

	I have been working on a 20a for some time now, and I'd like to share
something here with the group.

The 20a has low RF output from the 2 X 6ag7 RF linear amplifier output. Mine
was about 3 watts AM carrier, or maybe 12 watts PEP on 80 meter SSB. Getting
a 600L linear amplifier is next to impossible, so I took another approach.

I gave a 4 times 572 G-G linear that needs about 80 watts in for 800 watts
out (10 db gain). If only my 20A could put out more...
Using my linear on AM with the 20a gives me 30 watts, and heats the ham
shack. I did this for a while, and people laughed at me for such a silly
lash up.

Here is what I have done:

1. replace 6ag7's with 7591's, and rewire tube sockets  (could use 7868 with
socket type change)

2. Add extra B+ supply (200v from small 150 vac transformer, bridge, and
cap) in series with existing 300v for ~ 500v 7591 Plate voltage

3. Remove 10k resistor across 6ba7 tank circuit. This provides more drive to
7591 grids

4. Adjust 7591 grid bias for -22 volts, and screen grid volts for 380 volts

The results have been stunning:

50 watts CW output (B+ at 475V), or  ~ 60 watts PEP SSB (B+ idles at 550v)

10 watts AM with plenty of headroom for > 100% + modulation. At 10 watts
out, each 7591 dissipates 15 watts (19 = max), and the % efficiency is 25%
(40 watts DC plate input for 10 watts rms output). The efficiency doubles at
100% modulation (sine wave).

15 watts AM at 100% +/- modulation, and 150 watts from linear amplifier.
Efficiency a little higher here.

The 7591 is about the size of a 6V6, yet has higher Gm (gain), and plate
dissipation. It fits nicely in the confined space of the 20a. Unfortunately
the 7591 can be pricey, as the audio nuts have bid the price up. Look on
EBAY, they are there. The 7868 is the same, and cheaper, but requires a
socket change.

	If any of you are interested in this, and want more details, just contact
me, and I'll share it with the group.

Regards,
Jim Candela
WD5JKO