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

Thu, 16 Jan 2003 21:36:22 -0500

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
>
>
>
> --- StripMime Report -- processed MIME parts ---
> multipart/alternative
>   text/plain (text body -- kept)
>   text/html
> The reason this message is shown is because the post was in HTML
> or had an attachment.  Attachments are not allowed.  To learn how
> to post in Plain-Text go to: http://www.expita.com/nomime.html  ---
> _______________________________________________
> Central-Electronics mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/central-electronics