[AMPS] amptalk

[Richard Byrd]

        Jim Henderson mentioned the folks at Svetlana were looking for
someone to donate a good amplifier for them to experiment on.  You'd think
they would look for an engineer to donate some tubes to.  My friend, Jack
Ray is a distributor right in my own town (Douglasville) and I really wanted
to put less expensive tubes in these 76 amplifiers.  But it's much easier to
use a triode and so far Svetlana is sticking to tetrodes.  Maybe you could
ground the screen and use it like a triode.  Anyway the socket for the
Svetlana 4CX800 is kinda pricey.  Eimacs' 3CX800 plugs into the same socket
as the 8874...that's handy.

        Steve Merchant asked the question about conflicting drive
requirements for certain models of ETO's Alpha 76 and 78 series.  A 'normal'
76 or 78 requires about 80 to 100 watts of drive to make the legal limit.
The 'normal' rig has five 470 ohm 2W carbon resistors from cathode to
ground.  A 2:1 toroid connects the transceiver to the amplifier input.  This
combination makes a good input VSWR for the exciter on all bands and
improves IMD products.  ETO calls this input package the 'Low Drive Kit.'
        The next version has FIFTEEN 220 ohm 2W carbon resistors from
cathode to ground with a 3:1 toroid on the input.  This version requires 200
watts drive to make the legal limit.  You could buy these kits from
'ETO'...perhaps 'Alpha Power' has them. I could supply those as well.  It's
a real pain to get to the underside of that chassis however.  If you aren't
sure about one you can pull the rear tube out and with a dental mirror and
some good light you can see the color bands on the resistors.  I have seen a
few botched jobs on that modification.  There's a high incidence of damage
on the amplifiers which require more drive.  Operators tend to drive them
harder just to make power and the toroid often flames out.

        8874 tubes seem to have alot of open filaments.  If you can only get
1 kW out, hold your hand over the exhaust holes and see if one is cold.
Many times you can rap on the tube WITH AN INSULATED TOOL and bring it back
to life till the contest ends.
One cold tube will usually upset the input match some, otherwise it's ok to
run it that way.  If you take the tube out because it's shorted or
something, be sure to tape up the hole in the chassis AND the cover before
operating.

        Someone wanted to know what my story is regarding service and
upgrades and what goodies are available.  I work full-time as Chief Engineer
for CBS radio at Z-93 in Atlanta.  I am a chronic DXer as well.  Several
years ago some ham friends who are also Airline pilots and doctors and such
(big buck guys, hi) began bringing me Alpha 77's to get their two-hole kits
installed.  I fell in love with that beast and after two years of work,
built a carbon copy of a 77.  It has been on the air for 5 years without a
single failure.  Later on when kits were no longer available I started
making my own.  Then the 'little' Alphas started coming for various problems
and I developed a QSK upgrade, WARC fix, 3rd tube addition, and finally the
3CX800 upgrade.
I never advertise because I'm afraid of success.

Alpha 77 two-hole kits are $450 (less tube).  Instructions are explicit and
simple.  Tubes can usually be found for around $250.  They are used
extensively in medical equipment such as MRI and X-ray.  The client's
exhaust plenum mustbe sent to Atlanta to get the second hole punched.

I have a WARC fix for the 77 but I don't like it...too much work.

Alpha 76 kits to upgrade to a pair of 3CX800's are $219 (less tubes).  For
the time being, that must be done in Atlanta.  It's easy to do and
completely reversable but there just aren't enough in service yet to release
user installable kits.
No 78 rigs have been upgraded yet but I don't see a real problem with that.

A very reliable WARC fix is available for the 76, 78 and 374A for $250.  On
the 76, 12 and 10 meters share a switch position.  On the 78 and 374A a new
band position is added past the old 10 meter setting.  User kits are not
offered for the WARC fix but I'm flirting with the idea mainly because some
folks outside the US are interested.  As band conditions improve there will
be more pressure to produce kits.  By the way, this is not a 'gimmick' fix.
In principle, it's the same concept used in the newer Alpha rigs.

A vacuum-relay QSK printed circuit board is available for the 76 and 374A
for $325.  This replaces the open-frame T/R relay.  Dozens of these are in
use now.  If you worked the big Togo operation in the CQWW contest, those
were 76's with my WARC and QSK upgrades.       

        


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