[Central-Electronics] Central Electronics 20A Power Modification

[JM/CO]

This is a very interesting, and valid fix for this situation, although
running them at 500V is pushing it a bit.  My only problem here is in your
choice of tubes. 7591 and 7868 types ( which are identical, electrically )
are not being currently manufactured by anyone in their original envelope
style. Groove Tubes has a 7591 available, but it's envelope is the same size
as a 6L6-GC, and I question the actual electrical characteristics. Original
7591-A tubes are rare and their price is very high. They were designed for
audio use, and tend to have a rather short life, because they run very hot.
I would think that the 5881 would be a much better choice. A very robust
tube, it is the MIL spec 6L6. These are only slightly larger than the 7591,
and are a lot more available. There are plenty of the MIL spec ones out
there, mostly made by Tung-Sol ( who developed this type ),and the Russians
still make them.
Chuck N1LNH
----- Original Message -----
From: "Jim Candela" <JCandela@prodigy.net>
To: <central-electronics@mailman.qth.net>
Sent: Tuesday, January 07, 2003 9:42 PM
Subject: [Central-Electronics] Central Electronics 20A Power Modification


>
> I originially shared this with the AMradio list...
>
>
> 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
>
> _______________________________________________
> AMRadio mailing list
> AMRadio@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>
> _______________________________________________
> Central-Electronics mailing list
> Central-Electronics@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/central-electronics