[AMRadio] Central Electronics 20A Power Modification

[James M. Walker]

Hi Jim,
yes for the 813 in AB1, around 65 watts maximum out, I use the CE10B as
driver and frequency control. the 813 is in class "C" now and modulated
by a pair of 811A using the Kenyon 125 watt modulator transformer for
the 813
class B modulator to single 813. Works better.

Also yes on 10 meters, a 10B or 20A barefoot "sometimes" usually though
I run the 20A into the thunderbolt there as well. I couldn't figure what
the mushy sound was on 10 meters turned out my microphone was low
impedence changed it to a high impedance job and all was fine too many
rigs nor enough mikes!

Ah, did I mention I also use the VFO458 for all CE rigs I have a pair of
those as well. I think I have pictures of them on my web page listed at
the bottom of the Central Electronics group. Running them is fun because
I usually say I'm running a CE20A and everyone goes off and says "WHAT
AMP"!

I like them and they run great, I will get my folder with the vasrious
changes and see what I have can probably scan them in and send them in
e-mail.
73
Jim
WB2FCN
http://eshop1.chem.buffalo.edu go to central electronics bottom entry.

Jim Candela wrote:
> 
>         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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> > AMRadio at mailman.qth.net
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> 
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