[Central-Electronics] 10B harmonic problem

Sun Oct 15 22:48:37 EDT 2006

Art,

    George W1LSB is the 10B Guru on this group, so take his advice like
"listening to EF Hutton".

As for me, I'm a crazy guy who likes to "hot-rod" the 20a. After writing my
post on this, I fired up my QRO 20a #3 on 20 meters with the CE VFO. I don't
have a spectrum analyzer, but I have a good 50 Mhz analog scope with 10X
compensated probes. If the waveform is not a perfect sinusoid, you know that
there is trash in the spectrum. On 20m my 20a at 55 watts output puts out a
pure sine wave. If I go to the upper end of 75m, I see some 5 Mhz leaking
through (scope shows the modulation mix of the two frequencies). The 5 mhz
trap in series with the output tube grids is not entirely successful, as is
the 6Ba7 mixer LC tank in rejecting that spur. But on 20m without the two
traps (13 & 15 mc) I appear clean. The reason it is clean is the higher 'Q'
of the mixer tank where the mixer B+ bypass capacitor is a major contributor
to the 'Q' since the circulating current of that tank goes through that
.006uf capacitor. Bridge that capacitor with another and the stage gain
increases. Switch from Mica to Z5U ceramic, and the gain drops about 6DB.

I am currently using that 20a as an exciter for a Dentron MLA-2500 linear
amplifier project. I have converted from a pair of 8875's to a single 8877.
This is not a simple conversion. Unfortunately I need more B+ for the 8877
so I need to re-vamp the power supply. I am getting outputs of 800 watts CW,
and 1000w PEP SSB however with the 20a as an exciter on 80M, and 700 watts
on 20M. How much work, and cost will that extra 3db  require? :-)

Regards,
Jim

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of George
Maier
Sent: Sunday, October 15, 2006 9:36 AM
To: 'Arthur Delibert'; central-electronics at mailman.qth.net
Subject: RE: [Central-Electronics] 10B harmonic problem

Hello Art:

Glad to hear more people running 10B's.  There were a few factory changes in
the mixer circuit that you may or may not have in your unit.

1) There should be a 180 Ohm resistor from the cathode of the mixer to
ground, and it should have a .005 uF cap (5000 pf) across it. If the
resistor is there, but not the cap, add a .005 ceramic disc.
2) The 13 MHz trap, consisting of C27A (3 pf) coupling cap, and C27B (25 pf)
which is across L7 (yellow coil), was moved from screen grid circuit of the
mixer to the plate circuit.

The factory notice for these mods talks about improved linearity, but I
suspect that it has some effect on the way the 13 & 15 MHz traps tune.
I've done this to my 10B, but I can't remember what the ultimate rejection
is at either frequency, only that the traps seemed to tune as they should.
In addition to the items that Jim (WD5JKO) mentioned, you might consider
replacing C25, C25A, C27A, and C27B with brand new silver mica's; I also did
this as just as a precaution.  If the old caps are a bit leaky, the Q of the
trap could be down.

Thanks to Kenn Grimes, K4XL, the details of this and all other factory
changes are available on BAMA (http://bama.sbc.edu/); just download the 10B
djvu file.

73 & Good luck!

George - W1LSB

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net] On Behalf Of Arthur
Delibert
Sent: Thursday, October 12, 2006 10:30 PM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] 10B harmonic problem

Hoping somebody can help me with this problem, which I didn't see anywhere
in the archives.

I am a new owner of a 10B -- actually 2 of them, and they both have the same

symptom.  When I use the 458 VFO on 20 meters, there is a very strong
harmonic at 3 times the VFO freqency, or about 15,500 MHz.  The manual says
to expect this, and to tune L6 for a minimum output on the harmonic.  It
says the harmonic should be only a few percent of the 20M signal.

I have a harmonic that's as strong as the 20M signal, and tuning L6 through
its whole range does almost nothing to eliminate it.

Using a grid dip meter on L6, I can see some dip at about the right
frequency.

I'm using the same VFO and the same 20M coil on both of my 10Bs.

Is my 458 VFO hyperactive, and generating a much bigger harmonic than it
should?  Have the components in the 15 MHz trap drifted in a way that the
nulling action is in the right frequency range but isn't very prominent?
Something else?

Any help or thoughts appreciated.

Art  KB3 FJO

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