I'm currently comparing various linear amplifier input networks with intent
of changing the one int my HB 4-1000A Amplifier. This amplifier was used for
fifteen years with a tube type transceiver using 6146B tubes in the final and
there was never a problem. The input network in the amplifier used six simple
LC tuned resonant circuits, with a bandswitch selecting one tuned for desired
HF band. Fixed mica capacitors and 3/8" diameter coils wound on wooden-dowels
were used with good success.
I now have only a solid-state transceiver that's allergic to any RF power
reflections from my trusty amplifier (an SWR that was acceptable before).
So I'm currently redesigning the input circuit to my HB amplifier and have
so far come up with two possible remedies. Apply the input signal to a tap on
the inductor of the LC circuit currently used and adjust the location of the
tap for minimum SWR on each band.....my input signal's rotary bandwitch already
has an unused section for such a tap (There'd be a different coil tap for each
band of course).
However, the 1985 ARRL Handbook shows a GG amplifier in which there are no
resonant input circuits, yet also shows a spectrum analyzer display proving
that there's no problem meeting the FCC's 40-dB-down spec on harmonics. The
relatively high RF input impedance of the GG tube's filament leads to ground
was simply matched to the input coax with single 4:1-balun-wound toroid (no
bandswitch or any resonant LC circuit dedicated to each band).
I can see that if one used a toroid that had an insuffient rating for power
that it might cause harmonics, but if the proper-sized toroid was used it seems
that this would work and I could also eliminate my input circuit bandswitch.
It would also save trying to match six small micas from my junk box to six
coils that I wind by hand in order to resonate them at the 160m through 10m
band mid-frequencies.
The question is, a lot of work or not, which was is best. Using the
switched six bandpass resonators or the single toroid balun? Am trying to
"get-er-done" before SSCW next Saturday, so will need to know immediately,
except I will be probably trying the "balun method" as you are pondering this
question.
John, W8OHT (In SS with PVRC compatriots)
.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|