re: addition of a UR diode in the alpha amplifier
input "swamp" section
-
The swamp resistance value would have to be
changed. The indicated swamp R value appears
to be similar to the average dynamic tube
impedance. Both the tube impedance and the
swamp R value in parallel equate to a convenient
toroidal xmfr secondary. An amps member
reported to me (here) the secondary ratio is 2.25
to 1. Those values jive with my calculator.
-
Your suggestion taken to the next step, would
be a series R & UF Diode paralleled with
the current circuit. The R value chosen to be
similar to the tubes parallel drive impedance
value.
-
The current amplifier drive requirements well
suit the output of many common Amateur
Transceivers. Most Amateurs will probably
over drive the amplifier with the swamp
R circuit removed or revised to provide only
half cycle loading.
-
Some of the relative gratis stability
provided by the lower existing swamp R circuit
might go away. But it might not matter with this
ground grid layout.
-
Retaining the current layout with the addition
of the series R-D would provide positive half
loading.
-
I have measured removed ps equalizer resistors
and I believe their replacement with MOF types is
a good idea. Same type/value Comp-R's varied
as much as 30K from other in the same bank.
-
skipp
-
: From: 2 <2@vc.net>
> I finished restoring a two-hole 76A a few months ago, and
>it seems to be running fine. (nice little amp) My question:
>
>With the original attenuator, (resistor pack) I get roughly
>650-700 watts out for 75 watts drive. I don't really want to
>run it much harder than that, but does 75/650-700 with the
>stock pack sound about right? I would guess so, but never
>having played with an amp so heavily "swamped" on the input
>before, it's hard to know for sure.
>
-
:-If I owned one, I would put an ultrafast recovery diode in
:series with swamper-R. The diode's arrow needs to
:point toward gnd in order to provide loading on the
:positive half of the driving cycle - where the cathode
:draws virtually no current. This config. removes the
:swamper from the burden on the exciter during the
:negative half of the driving cycle. / Mouser carries such
:diodes. /
:- A useful mod on older Alphas is to replace the orig.,
:350v-max, unstable, carbon-comp, "equalizer" resistors
:across the 450V electrolytic capacitors with modern MOF
:resistors. If these capacitors have a date code prior to
:1980, they should probably be replaced.
: - For SSB service, 8874s are best tuned up with a tuning
:pulser to avoid stripping the oxide coating off of the cathode.
:[photo on p.17 in Sept., 1990 QST article, "Parasitics Revisited"]
:cheers, Jim
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