[AMRadio] Central Electronics 20A Power Modification

[James M. Walker]

Hi Jim,
I run several 10A, 10B, and 20A combinations here at WB2FCN. For past
two
weeks been running the 20A/10B at different times, and driving a Johnson
Thunderbolt.

My 20A is "back in original" form according to the manual. Took a little
coaxing to get things back to normal in the unit. My observations for
this 20A (I have three) are 14 watts of carrier when the carrier level
pot is at 3/4 of full scale. I run this unit at 5 watts out AM to the
thunderbolt moderately loaded and get 400 Watts carrier output.

My rackmount 20A still has a problem I haven't tracked down yet, someone
solid stated the power supply and threw the voltages off to the stages,
I converted it back but the drive from the 6BA7 is intermittent. I am
thinking a bad resistor, probably, or something I missed in the rewire
process.

As for the power out and lineup for the various rigs goes mine is as
follows:

10A and 10B output is 4.5 watts on AM, driving a single 813 230 watts
carrier in AB1.

20A desk top and 20A rackmount output is 14 watts barefoot, cutback to
4.5 watts for driving the Thunderbolt to 400 watts carrier, the
Thunderbolt is stock, except for the 1n2367 HV diodes in HV rectifier.

I have worked stations on 40 meters barefoot with the 20A rigs and on 10
meters get very good reports from those I have talked to.

You changes sound intriguing and I look forward to hearing more about
your mods to the unit. Good show hope to work you on 10 maybe 29040 AM

73
Jim
WB2FCN
http://eshop1.chem.buffalo.edu


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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