[Central-Electronics] CE 20-A

[Jim Candela]

Joe,

     I think your best bet is to look for a CE 600L amplifier. This has a grid driven 813, and will take the 20A output to about 350 watts PEP output. Then you could take steps within the 20a to get a clean 10 watts or so. Those 6AG7's are very linear, but other factors within the 20a are limiting the ultimate performance.

    An intermediate amplifier is also a possibility. I'd worry about instability when tuning, so a solid state "no-tune" amp might be best. All you need is about 6-10 db gain depending on 20a drive level and the gain of the big G-G amp to follow. An amp with a 8877 will have about 13-14 db of gain. This alone will take a 20A to an output comparable to a 600L without the need of an intermediate amplifier.

      The Lakeshore Band Hopper VFO is a very good unit, and I'd wager more stable than the BC-458 VFO. Still these drift, and have age related issues just like the BC-458. The 9 Mhz crystal within the 20a also seems to drift about 100 hz from a cold start. I have addressed the Lakeshore drift here with the Cumbria designs X Lock 2.0 VFO stabilizer. See this link:

http://pages.prodigy.net/jcandela/Band%20Hopper/


      I have also taken on two 20a's with QRO power increasing projects. One unit can reach ~ 60 w PEP SSB out and the other is about 45 w PEP output. These mods vary in success by the band, and in my cases have eliminated the upper bands (15-10m). They work best on 160-80m. Here are two links:


using a pair of 7591 output tubes:
http://pages.prodigy.net/jcandela/Central%20Electronics%2020a%20QRO%20%232/

Notice the AM Trapezoid patterns on this rig. A stock 20a will not make flat lines meaning the power output stage is NOT very linear. My suspicion is that the 6AG7 bias and screen voltage are caving in under load causing this as well as issues with the 1/2 12BH7 cathode follower driver. I address these issues with the QRO design.

Here is another 20a using a single e34L output tube:
http://pages.prodigy.net/jcandela/CE20AQRO/


mp3 file of 2 x 7591 unit followed by a 8877 amplifier to near legal limit power as received by local ham with a Flex 5000 set to 7 Khz Bw:
http://pages.prodigy.net/jcandela/20A_MP3/

   If you go the 20a QRO route, beware that this is not a weekend project. Consider it a winter project. Your mileage may vary.

Regards and good luck,
Jim
WD5JKO
  ----- Original Message ----- 
  From: jsloss 
  To: central-electronics at mailman.qth.net 
  Sent: Wednesday, October 29, 2008 10:53 PM
  Subject: [Central-Electronics] CE 20-A


  Have been offered a factory wired 20A, S/N 746X, that has been stored since 1973.  Am comfortable with slow power application, filter replacements etc but am wondering if there have been any S/B's or major mods for that serial number range.  Any info on increasing power out a bit might be interesting although a basic GG one or two tube linear might be a better way to go.  Will  be using it with a Lakeshore SSB VFO

  Thanks,

  Joe k7mks


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