>
>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.
50-ohms might fly.
> 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.
>-
The sticky wicket is that the cathode impedance is far from constant. It
varies from virtually infinity to approximately one-half of the mfg's
spec impedance. A transformer is not going to work very well in
impedance hell.
>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.
I do not see how such a circuit would do the job. Perhaps a roadmap
would help.
>-
>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.
>
A fool and his money are soon parted.
-
>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.
>-
However, if series cathode reistors are added, things might be less
squirrelly.
>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.
>-
Impressive
cheers, 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
>
>________________________________________________________________
>GET INTERNET ACCESS FROM JUNO!
>Juno offers FREE or PREMIUM Internet access for less!
>Join Juno today! For your FREE software, visit:
>http://dl.www.juno.com/get/web/.
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|