The Heathkit SB-230 is a linear amplifier for amateur radio use,
covering the 80, 40, 20, 15 and 10 m bands. Its maximum input power is 1200 W PEP
in SSB mode. It requires less than 100 W drive to produce full output.
The SB-230 was sold in kit form by Heathkit from 1974 to 1978. Occasionally
unbuilt kits can still be found and they sell at record prices in auctions
(more than $ 800 on Ebay).
The SB-230 uses one single 8873 EIMAC triode in grounded-grid configuration. The
8873 was designed for conduction cooling. A finned radiator is mounted on the
amplifier rear panel. A square block of BeO (Beryllium Oxide) is used
as thermal link to transfer heath from the external tube anode, which
is at RF and HV (2300 V) potential, to the radiator (at ground potential).
BeO is an excellent electrical
insulator and also has very good thermal conductivity.
A thermal silicone compound, as used for
power transistors, improves the heath contact of the two faces of the BeO block
with the anode and the heath sink respectively.
Unfortunately BeO is a highly toxic and carcinogenic material,
so the assemby manual recommends
extreme care in handling the block. With today safety standard is doubtful if
a similar kit could be sold to the public. The SB-230 was the only commercial ham
linear amplifier, together with the Henry 2K Ultra, that used this tube.
The tube anode temperature should never exceed 250C. A thermal
switch on the heath sink turns off the tube in case the temperature rises beyond
The 8873 tube is electrically equivalent to the 8875 and 8874 (3CX400A7)
and differs only for the cooling method. The 8875 uses transverse-flow
forced-air cooling whereas 8874 uses axial-flow forced-air cooling. The
8873 has the lowest anode dissipation: 200 W. The 8875 has 300 W and the
8874 400 W. Of those EIMAC tubes only the
8874/3CX400A7 is still in production and can be purchased new, albeit at a price
around 400 USD. The 8873 is rare and can occasionally be found on
the Internet auctions and at flea markets. NOS tubes are sold at high proces.
8873 8874 8875 Data Sheets
1974 list price: $ 339.95
The SB-230 is capable of 600 W PEP output at 50% duty cycle in CW mode. This corresponds to
1000 W DC input quoted above, with an amplifier efficiency of 60%. In SSB mode the PEP power is around 700 W.
At 100% duty cycle (RTTY or in key-down condition) the amplifier must not exceed 400 W input, corresponding
to the 200 W anode dissipation with some safety margin.
See here for the schematics.
See here for the Heathkit assembly manual.
(in case of broken links and if you have difficulties in locating
them I can email the files to you)
Substituting the 8873 tube
The SB-230 has many estimators because of the elegant design, the simple circuit and
the relatively roomy chassis that makes mods simple. However the scarcity
of the tube poses problems. Many projects exist to
replace the 8873 with something more readily available. They fall into two
Useful links to SB-230 retrofit projects can be found below.
- Replacing the 8873 with the 8874. This can be done without any electrical modification. Of course a blower must be installed for forced-air cooling and probably
the radiator must be removed to make room on the rear. The problem is that the 8874 tube is not cheap either.
- Replacing the 8873 with a Russian triode like the GI-7B or GI-7BT. K4POZ
discusses in details these modifications. The GI-7B(T) can be purchased for about 25-30 USD NOS and has a 350 W plate dissipation. The retrofitting can be done either with
forced-air cooling (needs to install a blower) or with conduction cooling by machining
a dedicated anode cooler. K4POZ sells a socket for the GI-7B(T). If you are mechanically inclined you could probably build your own easily.
There have been speculations on the possibility of modifying the
8874 for conduction cooling. This would require
to replace the finned radiator in the 8874
with a solid metal block (possibly a salvaged 8873 anode). Although
possible in principle, the operation requires industrial
facilities not available to the normal amateur
and the risk of damaging the (expensive) 8874 tube is high. Thanks to
Tony I0JX for this information.
I purchased my SB-230 on Ebay (USA) and the shipping to Europe cost almost as the
beast alone. The seller had no idea if the unit was working (except for lighting up)
so the risk of a DOA (Dead On Arrival) was big.
Luckily the BeO block was intact and the tube lighted up. Then I could proceed
to more serious tests. The HV supply worked, but the meter read about 22% low. The reason:
the three 1 Megohm resistor had increased value because of age. Replaced
them with "new" ones (but try to find 2 Watt Allen Bradley resistors today!)
I am grateful to PA3GOS and K4POZ who helped me with advice and support during the restoration and
the various tests.
||Initially I could not get any grid-current reading. I found the grid feed-thru capacitors
were leaking to ground. I replaced them with ceramic discs and tightened up properly the grid
contact ring (which was loose). In the process I learned that you must be very careful
in putting the thermal paste between the anode and the BeO block: too much will
squeeze out on the BeO block sides and can cause an arc between the anode and the
radiator flat face. I use Dow Corning 340 Thermal paste as suggested by EIMAC.
||Following tests indicated that the SB-230, once excited, developed terrible parasitic oscillations (tens of watts). I failed to destroy my FT-450 exciter. The 33-ohm grid resistor disintegrated. Following K4POZ's suggestion I changed the wiring of the 8873 plate circuit. The assembly manual illustrates the two possible schematics but it does not explain which to select. In my case the change resulted in stable conditions so I strongly recommend it.
||The default anode circuit wiring according to the Heathkit
assembly manual. The tuning capacitor is connected directly to the 8873 anode.
Conduction cooled tubes gained some popularity in the seventies and eighties because
of their noiseless, solid-state-like operation.
The other linear amplifier using the 8873 was the
2K Ultra (see picture) introduced in 1972. The 1980 ARRL Radio Amateur's Handbook featured
a 2 kW linear amplifier project also using a pair of 8873. The 8560A tetrode (similar to the 4CX250B)
was also conduction cooled and a project using it was presented in the same Handbook. The 8560A still
appears in the Svetlana catalog.
The SB-230++ Project
After several tests I concluded that the original 8873 tube was exhausted. I could not get more than
200-300 W out of it. I decided tentatively
to substitute a russian GI-7BT tube. The refurbished
linear became the SB-230++.
Click here to go to the
The SB-230+ Project
After some time I could get hold of a good 8873 tube (thanks Antonio, IS0EBO). I therefore decided to go
back to the original configuration, much quieter than the GI-7BT version with its two
blowers. However, to increase stability, I modified the original circuit to achieve a
true grounded-grid configuration. I also inserted a grid overcurrent protection.
The upgraded linear became the SB-230+.
Click here to go to the
SB-230+ page (under construction).
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