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Re: [Amps] FW: FW: Triode Amps / options / tetrodes

Subject: Re: [Amps] FW: FW: Triode Amps / options / tetrodes
From: "jim.thom" <>
Date: Sat, 4 Dec 2021 08:56:20 -0800
List-post: <>
I heard  last  week a 8877  from Richardson's  was reduced  from $2930.00
to $2k.  (Eimac).

A  Greenstone 3CX-3000A7  used to be $729.00.  It's now up to $925.00
A  3CX-3000A7 is instant on,  50 ohm input Z.    50  watts  of drive  =
1.5  kw  out.
IMD3 is  -53dbc (-59db pep)  @ 5 kw PEP output. NO parasitic suppressor
required. Instant on.

The  24.0 pf  anode to grid C  rises to 33.0 pf when tube is installed in
the grid ring. The extra 9 pf is from the proximity of the lower anode fins
to the chassis. The 33 pf stray C is easily dealt with on the upper HF
bands with the inclusion of a small  value  coil between plate block cap
assy..and C1  tune cap.  The  tube C  + the extra small coil forms a step
down L network.  This transforms the plate load Z down to a much lower
value, such that a practical low Q PI net can be used.

Econco  told me on the  phone that they  could  rebuild Eimac, Svetlana, +
Greenstone 3CX-3000A7....or any other Chinese  3CX-3000A7 tubes. Econco
sells Eimac  rebuilt  3CX-3000A7's for  $830.00 ( you provide the dud).

The 3CX-3000AA7 is  still used in FM broadcast. Typ life on the filament
for 5 kw FM broadcast use is 35K hours (4 years).   The cooling
are  minimal  for  1.5 kw output levels....including RTTY / FM / DATA /
FT-8.  1.5 kw output  @ 60% eff  requires 2.5 kw DC input...and  1 kw anode
diss.  Typ efficiency on  160-40m is 68-70%..and decreasing a bit as freq

RF parts  sells a  superb  chimney.... and an ( expensive $375.00)
socket....which also handles the  bigger  3CX-6000A7 tube.   makes a machined  grid ring +  mating  2 piece,
inner +outer collet  for  the a reasonable price..  Floyd
at  Eagle  made  has  a  new  5000  sq foot  machine  shop.     His
160-10m  230 uh  ferrite  loaded  plate choke  has been used up to  6kv on
all bands 160-10m.  It's simple to make too, just type  43 beads .736" OD x
.400" ID X 1.125" (or  2.0") long, slid inside a hollow  1" OD (.75" ID)
Teflon tube.   Wound with Teflon coated  22 ga ( silver stranded)  wire.1"
OD  x .75" ID  hollow teflon tubing is available at Mc Master Carr.   The
225 watt  CCS grid dissipation is actually a lot more, with the tube in GG,
and grid bonded to the (cold) chassis.

For max cooling, a Dayton 6DKY9  blower (cheap)  puts out an easy  80
cfm  @  1.2" backpressure.  Eimac specs 67 cfm  @  1.2" back pressure for 4
kw CCS anode dissipation with an inlet air temp of  104 deg F.   With lower
inlet air temps, and / or lower anode dissipation, the cooling requirements
drop through the floor.

Jim  VE7RF.
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