[AMRadio] Central Electronics 20A Power Modification

[Gary Schafer]

Hi Jim,

I have a 20A also. External to the 20A I use a pair of 1625s in AB2, grid driven,
with a 4:1 toroide unun coil for the grid. Also some swamping resistors in the grid
ckt.  The 1625s drive a pair of 813s in GG.

The mod sounds interesting. There is an article titled "shoes for the 10B" in one of
the old CQ magazines around 1958 I think. The guy added a vfo in the 10B and a pair
of 6146s! Forgot what he did for the power supply.

I always found that when removing the 10k resistor across the 6BA7 that it would want
to oscillate. Also found that the top cover (perforated shield) over the final and
driver compartment had to be in place with all screws tight and the finger stock
making good connection to the front panel or it wanted to oscillate.  I have had
several over the years and noted the same thing on all.
Also the double shielded vfo cable mod that they came out with did wonders when using
an amp behind the 20A. Kept it from getting RF back into the 20A.

I wonder if you experienced any instability problems when putting in the new tubes?

73
Gary  K4FMX


Jim Candela wrote:

> 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
>
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