[Central-Electronics] CE 20A and BC-458 vfo
Laurence M. Szendrei
nevecprez4 at gmail.com
Mon Apr 7 12:20:53 EDT 2025
All,
Apologies if this massage is a dupe, but I received an error message
from the CE list saying that a good part of my message was ignored
because it was interpreted as an illegal command or something. Hopefully
this one makes it through in its entirety:
Regarding the "80W CE20A," it was quite a bit more involved than just
changing the final tubes. I experimented with plenty of types, but
charging to larger tubes alone will not change the power output
significantly. One must also increase the plate voltage to the finals,
and also provide more grid drive than is available from the 12BH7
cathode follower. I went to an "economy power supply" configuration with
solid-state rectifiers and a resonant filter on the HVB+ for acceptable
voltage (~700V) and regulation, and a choke input for the LVB+, which
required an additional choke mounted under the chassis. For the driver I
swapped out the 12BH7 cathode follower for a 12BY7 conventional
grounded-cathode stage of voltage amplification. The half of the 12BH7
which served as a rectifier for the 6E5 eye tube was replaced with a
fast-recovery silicon diode. I also put in a 6W6 as a regulator for the
bias supply to sink grid current in case I drove the finals into class
AB2. I resides in the socket originally occupied by the 5U4 rectifier. I
added a 12.6V CT filament transformer, with 1/2 its secondary driven by
the 6.3VAC heater winding of the original transformer, to boost voltage
to 12.6 V for the new finals (a pair of 7984s), and the primary of that
transformer now serves as a secondary for the new negative bias supply.
That transformer also resides under the chassis. I also added parallel
resonant tuned circuits for 160M and 80M in the the 12BY7 plate/7984
grids for improved linearity on those bands, as the original design only
used tuned circuits on 40, 20, 15, and 10M. I had to neutralize the
12BY7, but got away with not having to add neutralization to the finals.
I converted the final plate tank a pi network with somewhat beefier
inductors.
As you can see it turned into quite a project! I would like to get back
to it someday and optimize the driver plate/final grid tuned circuits in
an attempt to get to the 80W output level on the higher bands.
The 6AG7 tube was a very good choice in the original design for a 20W
peak DC input rig (~12W PEP RF out on the lower bands). The tubes have
high transconductance (= high gain) and low Cgp due to a dense screen
grid structure.
BTW, I have played with several stock CE20As and have never seen more
than 5W output (peak) on 10M, on "a good day."
Another guy who has experimented with making his 20A QRO is Jim Candela
WD5JKO, and his work inspired me to take on the above project. IIRC he
ended up using 7591 tubes for the finals, but I can't recall how far he
went beyond changing out the tubes, or what output he was able to achieve.
73,
-LarryNE1S
On 4/6/25 8:20 PM, Frank Barnes wrote:
> Thanks to you both. It will be a while before I begin restoring the
> 20A and VFO but I will eventually get to it and post progress.
> Regarding the "junker" unit being upgraded to an 80 watt one: Did you
> just remove the two 6AG7's and replace them with an 807 or similar? I
> thought that the 2 x 6ag7 final amp was a bit odd but I'm sure there
> was a reason for it.
>
> On Sun, Apr 6, 2025 at 4:23 PM Cal Fuhrman <calfuhrman7 at gmail.com> wrote:
>
> Nice find Frank. I'd like to ride along with you on this journey
> when you get to it. Several years ago I was chased home from
> Orlando Hamcation by a CE-20A and a very interesting little Amp in
> an original CE case. This Amp has a pair of 832 tube's and looks
> to be in excellent condition. So, a few weeks ago I bid on and
> purchased the CE VFO from Schulman Auctions. I'm also backed up
> with projects but would really like to get these units back in
> use. I've got a CE-10A and BC -458 that I converted at our
> Northern location. I finished getting that one working some years
> ago.
>
> With several of us working on the same units it might be helpful
> to share experiences.
>
> 73
> Cal - WA3CTZ
>
>
> On Sun, Apr 6, 2025, 1:18 PM Frank Barnes <fbw4npn at gmail.com> wrote:
>
> At the Raleigh Hamfest, I acquired these two units, which
> apparently have
> not been used for decades so they will require a thorough
> restoration.
> I'm not even certain why I got them ($30) but I have always
> been curious
> about phasing-type exciters. I'm an old-time HF guy (age 84)
> and still
> like tube-type units.
> I have put together a very rough web page where I'm collecting
> whatever
> documents I can find regarding these units. The link is here.
> <https://w4npn.org/central-electronics/> It will be a while
> before I can
> restore these since I have a dump-find Heath SB-1000, a 2 x
> 807 xmtr and a
> workshop power supply in the que first.
> I also printed an old CQ magazine article (*The Command Set
> Roundup*) that
> speaks to command sets in general and which looks helpful.
> And I see some chatter on the Central Electronics blogging
> site in the AM
> Phone system.
> I haven't yet opened up the units so I don't know about their
> internal
> condition but first impression is that they can be refurbished.
> *If anyone knows of other articles, etc., about these Central
> Electronics
> 20A units, or has advice regarding rebuilding these, please
> share it! * I
> had no idea how complex the 20A is...looks like excellent
> engineering for
> the time.
> Advice is welcome.
> Frank
> W4NPN
>
>
> --
> Frank Barnes
> www.W4NPN.net <http://www.W4NPN.net>
> Chapel Hill, NC
> Grid Square FM05
> Cell 919.260.7955
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>
> --
> Frank Barnes
> www.W4NPN.net <http://www.W4NPN.net>
> Chapel Hill, NC
> Grid Square FM05
> Cell 919.260.7955
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