[Central-Electronics] Today's AC Mains voltage too high for many BA Rigs

Fri, 17 Jan 2003 07:02:38 -0500

Dear Gang,

I had a long conversation with a long-time, former Collins employee a while
back. We discussed the 516F-2 power supply that goes with the KWM-2 transceiver.
Around 1960 the primary voltage was 108 VAC or so, and the AC line voltage did
"creep up" over the years.. He did recommend the use of a Variac to get the
voltage down to a level compatible with the older power supplies...

What I did was to set the Variac so the power supply voltages were within levels
specified for the circuitry. I did go to the local radio supply company (started
by Hams in the 1940's) and purchase a new Variac for between $100 and $200.
These units are found all over hamfests, basements, garages, etc.!!

Collins did make adjustments in the AC primary voltages, and "bumped up the
voltages" as time progressed.

73,

John, W4NU
Atlanta
(K4JAG 1959 to 1998)

JM/CO wrote:

> Yes, we ALL have to deal with this problem. Attempting to deal with this on
> an individual set / component basis is an  exercise in futility. The
> solution lies in being able to control one's AC. You have basically solved
> your problem by the use of a variac. Some people build a sort of bucking
> transformer circuit which reduces the AC by a percentage. This method, while
> effective, is still somewhat of a "shotgun" approach, as voltage out, is
> still dependent on voltage in. I would suggest that you obtain a 20 amp
> variac, and an accurate AC voltmeter, and run ALL your gear at some voltage
> of your choosing, say--117 volts, and worry no longer.
> Chuck N1LNH
> ----- Original Message -----
> From: "Jim Candela" <[email protected]>
> To: "[email protected]" <[email protected]>;
> <[email protected]>
> Sent: Thursday, January 16, 2003 8:32 PM
> Subject: [Central-Electronics] Today's AC Mains voltage too high for many BA
> Rigs
>
> > Follow-Up on 20A.
> >      I have an observation that is perplexing me. Some of my old AM  Boat
> > Anchor gear was designed in the days when the AC mains power was lower.
> For
> > example on  one of  my Central  Electronics  20a 's,    I measured the
> > filament voltage (non-RMS DVM), and it came out to 6.9 vac with a AC line
> > voltage of 123 volts. I had to lower my ac mains with a variac to 112 vac
> to
> > get the filaments down to the nominal 6.3 volts.
> >
> >     This brings up a couple of issues:
> >
> > 1.) The filaments running ~ 10% high may have more emission, but cathode
> > longevity  may suffer along with more likely heater to cathode shorts.
> >
> > 2.) The AC power transformers may not have adequate flux BH headroom, and
> > could start to saturate, leading to high AC input current, and hot
> > magnetics.
> >
> > 3.) The High voltage may be beyond the ratings of the components (too much
> > voltage) which may lead to higher heat dissipation. For example,
> > electrolytic capacitors may see too much voltage. This is especially true
> > when solid state power supplies are used with vacuum tube circuits. The
> > voltage soars to the peak AC value until the tube heaters warm up enough
> to
> > pull current. The higher tube element voltages may also (if unregulated)
> > move the DC operation points to dangerously unstable levels. For example,
> a
> > 10% rise in the G2 voltage (say 400 to 440) of a Beam Power tube can
> > increase the DC operating point such that the plate dissipation is too
> high
> > leading to a thermal run away condition.
> >
> >  My first thought of attack concerning my 20A was to add resistance to the
> > filament wiring. This is doable so long as the ac mains voltage is
> > reasonably constant. The DC voltage issue can be solved in other ways,
> such
> > as regulating the G1 fixed bias, and the G2 screen voltage to my 7591 beam
> > power tubes. It seems that when the ac mains increases, the G1 fixed bias
> > goes more negative, and the screen G2 goes more positive. The sensitivity
> to
> > G2 is greater than G1, so the plate current varies quite a bit with a
> small
> > change in ac mains voltage. Just thinking about this, I could add some
> gain
> > to the G1 potential VS ac mains voltage  change to cancel out the G2
> effect
> > on plate current. A little bit of cathode bias to the 7591's might also
> help
> > with stabilizing the DC operating point as well as adding a little
> > degeneration (gain, and distortion reduction). This approach might be an
> > option to regulating both element voltages.  I have plenty of headroom
> with
> > the electrolytics.
> >
> > Another option would be to add a multi-tap buck/boost transformer so that
> I
> > can accommodate a variety of AC mains voltages, and still get the nominal
> > 6.3 volts ac to the filaments. This need not be large, and could be
> > installed within the rig. A 12Vct 2A transformer could be used to add 6,
> 12,
> > or subtract 6, 12 volts from the AC mains input. It would require a fancy
> > switch (maybe 2), or network of jumpers to work however.
> >
> > What I have ended up doing was to install a 5 amp variac to a 6 plug
> outlet
> > strip. I am currently running my RCA AR88 receiver, CE 20A, BC-458 VFO,
> and
> > a rack mount Kepco 200 volt power supply all off the strip. I set the
> variac
> > to 114 volts. The AC drops to 112 with my linear set to 200 watts carrier
> > output. Now I need to find out about my Gonset GSB-201 linear (4 X 572B),
> > and determine what filament voltage is on the 572B's. Hey at $50 bucks
> each
> > (today's prices) I don't want the filaments burning at 7.0 volts (6.3
> > nominal??). I will need a 15 amp solution to dropping the voltage to the
> > Gonset should the filaments be too hot.
> >
> > Do any of you fine folk have this problem, and if so, how are you dealing
> > with it?
> >
> > Jim,
> > WD5JKO
> >
> >
> >
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