re: Beam "Sweep" tube amplifier circuits
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Bill Smith has been asking about the Dentron and & Sevtlana
conversion circuit related to the grid placement. A topic which
I've been doing quite a bit of homework. Just my personal
fondness for that old home brew circuit scrounged from TV
set horizontal output sections years ago...
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Sweep Tube Amplifiers are by no means new.. My collection
of ARRL and Radio Handbooks show many construction
articles on such layouts. Anyone building amplifiers can
find Handbooks for rock bottom prices on Ebay. Handbooks
are an excellent resource of tube amplifier circuits.
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The only issue I take with most of the mentioned circuits
is the "most practical" placement of required bias - grid
connections. Here's what I've found works very well...
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The cathode is DC grounded. I've found many common
Handbook choke values which work fine. It depends on
how many bands, a calculator and dip meter what values
you might end up using.
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Cathode drive is best applied through a classic pi tuned
circuit. At the high end of HF near 28MHz, the tube side
(of the pi) cap value often becomes the stray and tube input
capacitance in many circuits. I've actually used different
values of coaxial cable to adjust the circuit. The actual value
depends on the tube... No tuned input means bad news
in the output results.
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Grid #1 is rf bypassed to ground, but fed a negative control
bias through a choke. I've found this method much easier
than a series high watt zener in the cathode path (the most
common used in Dentron and Handbook layouts). Those
larger value zeners are becoming pricey and are often the
first item to fail in a "glitch event." Put a bumper sticker
on your car "glitch happens"....
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My most practical bias supply approach is to rectify, filter
then multiply the heater winding, then apply it to the grid #1
circuit through a well bypassed resistive voltage divider.
The advantage of this method is it's simplicity. It is also
a layout that can be applied to other non sweep tube
circuits should you like it enough.
The "ground" end of the negative divider is "dc lifted"
through a simple relay contact in standby which applies
the full negative supply output to cut off cathode current in
receive/standby mode.
Should a circuit require a larger negative voltage, a separate
supply can be built up with separate higher xmfr windings.
It's a very simple approach for which I wish I could claim the
source credit for... alas here's where the heat gets turned
up.
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I "liberated" the above circuit from an old Palomar 300a
CB amplifier. Although the amplifier works well and has
the 40 through 10 meter bands, it was sold as a two-stage
CB amplifier. No ones trying to kid anyone here. But a
practical circuit can be found in the most strange of places.
Why not lift the "better circuit" and apply it to a legal
application. That I did... enough said...
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Grids 2 and 3 are tied to chassis, DC and RF ground.
Zero Signal Anode current can be set by adjusting the
Grid #1 bias supply resistor voltage divider values.
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I've found this circuit layout works very well and appears
to be much more stable than other layouts I've tried. Of
course, some exciter "feed through" power appears in
the output. Drive appears to be around 15% lower than
the same circuit less grid #1 tied to ground with the G2&3
grids for similar outputs.
My previous amps mention of grid #1 dissipation concerns
are not a problem with this method.
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Output tank Z is often very low... Handbooks often use a
older method of tank coupling, but I can manage a pi tank
with proper concerns in place. Dentron Re builders might
have to "touch up" their tank circuits with the Newer
Svetlanas in operation.
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If you can stomach the trip over to www.cbtricks.com for a
copy of the Palomar 300a circuit diagram, it's well worth
a look when considering sweep tube bias methods.
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Don't forget your parasitic suppression in the anode path.
Used to be Sweep tubes were relative poor performers in
higher VHF ranges, but that's changed in newer generation
tubes with lower output capacitance. Metering in my last
Dentron Rebuild was moved over to the negative side of
the power supply, in the interest of safety. It's often well
described in Handbooks. Good luck with your conversion
or new EL-509/519 project.
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cheers
skipp
nospam4me@juno.com
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