[Central-Electronics] New to List With 20A

[Jim Candela]

Darrel,

  Welcome to the list! There is a great bunch of folks here.

I have a lot of posts in the archives starting in about 2003 and continuing for the next 5 years or so. I hope you find them interesting.

Some ideas that you might want to consider:

1.) Frequency Stability. These days, running SSB, being 50hz off is going to get comments. I once drifted 200hz, and when I switched to receive, there was a hornets nest of operators accusing me of "drifting out of the band". Your DDS idea is a good one, but also realize you need about 8-10 volts peak drive, and the DDS has to jump around band to band to work against 9Mhz. On a 20A 40M is the worst since you need 9+7 = 16 Mhz. The easy bands are 80m and 20m where the VFO at 5-5.6 Mhz is all you need.

The 9 Mhz crystal also drifts as they warm up. I had one that drifted 600hz over the first hour. Most are around 100-200hz though. A newer crystal might help. I buy CB crystals (3rd overtune) where channel 4 (27.005 /3) crystals can sometimes be found on Ebay (same size package). In my 20A I use one of these inside a Motorola RM-10 oven.

2.) Sideband suppression. The phasing network is only good to about 2500hz. The audio is not rolled off sufficiently, and as a result there will be some opposite sideband energy radiated. The SDR folks will see this on their pan adapters. I had a post in the archives to enhance the phasing network to work up to 4 Khz, a very simple, and worthwhile change.

3.) Heat. The 20A would prefer 115 vac line voltage instead of 125 vac.Running on 125 will literally cook things as the filaments are near 7 vac, and the B+ will idle at 400V +. The 6AG7's run really hard, and beyond ratings. Consider a way to lower the voltage input..could be a 10V 3Amp transformer configured to 'buck' the incoming voltage.

Now about the power issue. Look around for a Central Electronics 600L amplifier. This is the best option. Other options are an IPA such as what you suggested to drive a big GG linear, or to rebuild your 20A for increased RF output.

To QRO a 20A, this would be a significant effort. Basically put a bridge on the power supply, and with different RF tubes, you can get over 50 watts PEP output. My 20A can do almost 80W PEP on 80M. More recently, Larry NE1S did a QRO 20A, and he gets 100W on the lower bands, and still usable through 15M at significant power (60W IIRC). The power transformer can handle it. Larry worked on his project for several years until he got it right. 

Good Luck with your 20A!
Jim
WD5JKO

________________________________
 From: D Nichols <jetercreek at gmail.com>
To: central-electronics at mailman.qth.net 
Sent: Friday, July 19, 2013 12:43 PM
Subject: [Central-Electronics] New to List With 20A
 

Having seen no traffic since joining, I figured a ping was in order. HI

Always liked the looks of the 20A.  A buddy had one in high school which
drove a Globe Champion back then.  He ran the MO SSB net after school
and received with a Lafayette HE-80!  I'd hate to try that today!

At any rate, I snagged one on ebay.  The guy didn't mention and probably
did not realize it was factory wired serial 75XX-C so I got it worth
the money.  The slugs still have the gray paint intact.  All the parts
were original (paper caps had to go, but the electrolytics are good). It
drives the SB-220 to 100 watts on 75M with an 8640B and an untuned 2 transistor
amp.

http://www.pongrance.com/dds-buffer.html

Plans are to do a DDS VFO for this and other vintage gear eventually and
it's not necessary to tell you guys that an IPA is needed to really use it.
The drawing board is now full!

I'm leaning toward driving a simple 6146 circuit to the SB220 so I am tinkering
now with raising the cathodes above ground to cut down on the dissipation of
the 6AG7s.  Has anybody else on the list done this?

Been reading the archives and having fun.

Darrel, N0DBX


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