[AMRadio] Central Electronics 20A Power Modification

Sun Nov 10 11:50:02 EST 2002

	Gary, Jim & group,

	Thanks for the responses. I was eye-balling a 80 meter arc-5 as a "gain
block" between my 20A, and GG linear. I decided that making it multiband,
with ext. P/S, relay switching, etc. was more than I wanted to do. Having a
Johnson T-Bolt is the way to go I guess, but I don't have one.

	So far my 20a is on the side with no shielding in place, and it is stable.
Since pulling the 10K across the driver tank, I have not tried any of the
bands except 80 meters.  The 7591, and 7868 have very low Grid-plate
capacitance, and if I recall correctly, the 7868 in the Novar socket is
about the same as a 6AG7 (Cgp). The 7591 is a little higher with the octal
socket. Both types have two G2 connections at the tube base, which allows
you to have dual .001 uf (or so) bypassing of the screen. This is good for
stability.

	Jim, I agree with your 20a & T-bolt combination. Yes 400 watts carrier
should be available with two 4-400's ab1 grid driven. Are you sure about the
813 with 230 watts carrier (ab1 am linear)? As a rule of thumb, for AM
linear operation (33% plate efficiency at resting carrier output), the max
carrier available is 1/2 the tube plate dissipation. So for a 813 with a
ICAS rating of 125 watts... Maybe 230 watts PEP.
I'd run about 50 watts carrier from an 813 in AM linear output, and leave a
little headroom for modulation peaks above 100%.

	So you have had your 20a on 10 meters? tell me more. I have a VFO with the
10 meter option. It works well. Problem is that the 6BA7 output drops with
rising frequency (stock early 20a), and 1 watt output (29 mc)is about all I
could muster. Looking at the CE mods to this circuit, it looks like CE made
major mods to the 12BH7 RF cathode follower between the 6BA7, and 6AG7
grids. I'd like to hear that the later 20A factory mods solved this problem.

	One nice thing about the 7591 is that it is a ultra linear beam power tube
that is on par with the larger 6550 used in the 20A's big brother (what is
that thing called?). The 6AG7's compress on + modulation peaks, whereas the
7591's seem to be linear until clipping. This arrangement has possibilities.

Regards,
Jim
WD5JKO

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of Gary Schafer
Sent: Saturday, November 09, 2002 10:09 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Central Electronics 20A Power Modification

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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