[Central-Electronics] New to List With 20A

Sat Jul 20 11:30:38 EDT 2013

TNX, Jim.  Have seen a number of your posts and am aware of the issues. 
I have a variac and would
not be without one.

One thing that is really puzzling to me is others experience with the 
9MC I.F.  I looked at mine
on the modern receiver/spectrum scope first time it was powered on.  It 
appeared to move a drastic
amount for a crystal.  After reseating the crystal and the tube, this no 
longer occurs.  8.99960
for an entire afternoon of playing.  Crystal oscillators really should 
not move around much unless
something is amiss.  I dug the crystal out of a parts set also acquired 
and it is at 8.9999, but
that just changes the calculator numbers!  Using a sig gen as a VFO just 
requires some math and
I was memorizing some of the freqs over the couple of days I had it on 
the air.

Reading about N3ZI's VFO, it seems to have many memories of programmable 
offsets.  It might add
an additional challenge to the operation, but beats the drift!  As you 
mention, the channelized
crowd has a low tolerance for that!  A friend and I on 40 one night with 
my Drake 4-Line and his
Swan with the Huff/Puff circuit set our freq to 7173.73 or some such.  
Sure enough, people dropped
by to tell us we were off frequency!

I've had quite a few direct responses to my post.  Thanks to all. Great 
information coming my way.

N0DBX

   On 07/20/2013 08:36 AM, Jim Candela wrote:
>
> 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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