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Special Design Products SDP-1000L. Saying goodbye to a fine old amp.




 
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June 02, 2023, 05:36:46 AM *
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Author Topic: Special Design Products SDP-1000L. Saying goodbye to a fine old amp.  (Read 622 times)
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Patrick J. / KD5OEI
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« on: January 13, 2023, 03:36:30 AM »

This old amplifier, built for U.S.A.F. MARS use, which I've had a number of years is going to a new home. I got a great reproduction manual from Pete/Manualman.

A friend is moving to Arkansas and I'll be installing it there in the spring when I'll visit for a few days and do some ATV riding. He's got a 40 FT telescoping tower, so the antenna should be relatively easy to erect. I've already supplied an antenna matcher, FWD/REF power meter, and other accessories for it.

The SDP-1000L is a classic Eimac-approved case of a 'grounded grid' Tetrode design with all normal element voltages on a pair of 4X250Bs for AB1 operation, as referenced to the cathode voltage.

The control grids are well-grounded for RF via high quality capacitors at the socket, but are maintained at their usual negative bias voltage of negative 55 to 65 Volts.

The cathodes are at zero DC voltage during normal operation, being returned to B- through a latching overload relay, and driven by the RF input signal.

The screens are at 350VDC regulated by a VR tube string, and dropping is via a series resistor from the 2000V plate supply.

G1 cutoff bias for standby is -150VDC. For tuning, the cathodes are returned to DC ground through a 100 Ohm resistor.

This is one fine way to properly cathode-drive most small tetrodes of the type that can't tolerate much grid dissipation. The 4CX250 has a grid dissipation of 0 or 1 watt, depending on the data sheet consulted. It is not designed to sustain grid current. It is therefore very sensitive and the RF-grounded-grid setup means that no neutralization is needed.

The other usual way to set up this kind of tetrode for grounded-grid service is merely to let the high voltage supply's negative end float at the positive side of the grid bias voltage, with the negative terminal of the bias supply grounded at the grid terminal of the tube socket. It creates a few small complications but is just a choice.

The amp has a TR switch, but it is a manually operated coaxial rotary switch and was intended to be used with a small separate transmitter and receiver. It can be manually keyed by turning the HV on and off or through a set of rear panel terminals that completes the circuit for the blocking bias keying relay while the HV is left on. - Therefore it needs an external RF relay for transceiver use, and another relay to interface from the typical low voltage, low current transceiver keying output to the amplifier's 120VAC bias keying relay coil circuit.

There is also a terminal block on the rear providing -150V, 400V, and 1000V to operate a companion SDP-1S oscilloscope unit, which is probably a modulation scope but I've never seen one.

Metering is for screen current with a range of -20 to +30mA, and plate current of 0-600mA. The screen current can go negative on this one, depending on loading. The screen current meter is invaluable for proper loading of the amplifier.

It's easy to get full output with much less than the usual 100W plastic radio drive level. It's really easy to tune and load. There is no grid protection but the cathode current overload relay will trip under overdrive as well as out of resonance situations. The amplifier has a 1KW DC CCS input rating. The input VSWR was decent thoughout the amplifier's frequency range of 2.2-28MHz. The pi-coil is a slightly modified  80-10M ham radio-type unit with integral bandswitch.

There are some pictures of it here: https://bunkerofdoom.com/kd5oei/sdp1000l/index.html


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Radio Candelstein - Flagship Station of the NRK Radio Network.
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« Reply #1 on: January 14, 2023, 12:15:11 AM »

I wonder how many of these are in existence.  I've had one for several years now.  I use it mostly on 75 meters.
Mine was found at a ham swap.  I convinced a friend to buy it for $80.  He passed it on to me. 
The coil is different and ps caps have been replaced.  Brutishly heavy and works fb
ka7woc
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Bob (aka Boatyard)
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Patrick J. / KD5OEI
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« Reply #2 on: January 20, 2023, 01:12:47 AM »

It would be interesting to see pictures of the coil and other differences. I traded a DX-394 for mine at a fest.

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Radio Candelstein - Flagship Station of the NRK Radio Network.
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