Hi Jim
My situation is quite different from yours, but I
thought that my experience might be useful to you.
I use an old pole pig, in the power supply, used as a
high voltage source, for a number of HF, VHF & UHF
power amplifiers. The high voltage was initially
excessive, so I used a 2 KVA varible auto transformer
to reduce the supplys' output voltage. After
experiencing problems with the vari-autoxfmr. (arcing
at the brush), I replaced it with a fixed xfmr., that
dropped the line voltage the desired amount.
This lash-up has worked flawlessly, for appx. 20
years, AND as a side benefit, the no load to full load
voltage regulation, of the HV supply, improved very
noticeably!
Why? Because, as the load, on the power supply, and
thus on the "bucking transformer" increases, the
output of the latter decreases, decreasing the amount
of "buck". As a result, the voltage delivered to the
pole pig increases, producing a sort of compensation.
Another consideration would be that it would be a lot
less effort to drop the line voltage, external to your
equipment, relative to the effort required to modify
all the individual pieces of equipment involved. A
heavy duty filament transformer should do nicely. In
my case, I had to drop my line voltage appx. 30 volts,
or so, thus I used a power xfmr., from a defunct paper
tape reader, that happened to be available.
GL & 73 de Ron n4gjv
--- Jim Candela <[EMAIL PROTECTED]> wrote:
> 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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