A GiANNiNj SCJENTIFIC COMPANY
S3 West 23rd Street. New York 10, New York rm interested. Please send HQ470A VHF info to:
6 Meter AM and SSB
Transmitter from the
The T-51 A/ARQ-8 transmitter was one of a series of Radar Jamming units used in WWII, Exact operating details of this transmitter are not known to the writer but apparently consisted of a variable self-excited oscillator tube (6V6), driving on 832A mixer stage and this in turn driving the final 832A, or in some cases, an 829B, A noise source was injected into the 832A mixer stage via its grid circuit and mixed with the excitation from the 6V6 oscillator tube.
The frequency range was from approximately 30 mc to 100 me, and made use of roller coil inductors (rollo coils), rather than the conventional LC circuits. This enabled the unit to cover tins wide range of tuning. Some of the equipment came equipped with an 832A in the final stage and some units contained an 829B tube.
Leroy May W5AJG 9428 Hobart St, Dallas 18, Texas
Photo credit: Jim Dun gun, Dallas
Upon inspection, it was felt this transmitter could be modified into a decent 50 mc unit without too many changes. This was done some four years ago and has since rendered excellent service. Actually, the unit is easily converted into an SSB exciter as well, and this was done at the same time with a second identical unit. The SSB version will be described later.
As an AM transmitter and/or exciter, this modified unit can use either an 832A in the final with an output of about 20 watts, or an 829B, with up to about 60 to 65 watts output. This represents 100 watts or so input power, in the latter case. This is the condition which we will describe.
The original circuit was wire traced and is drawn out in Fig, 1. The modified 50 mc AM transmitter is shown in Fig. 2. On the modified schematic, the original components are left unmarked, while the added or changed parts are so indicated with their values, This modified unit may he modulated, and if so, a modulator of about 50 watts or so will be required. The 829B operates as a Class "CT amplifier, A jack is provided for CW operation—and under such operation, the input can be jacked up a bit higher still, without damage to the 829B. Should the transmitter be used as an exciter to drive something more powerful, the T-51A/ARQ-8 plate voltage may be reduced until the minimum desired power output is reached to properly drive the high powered final.
The photograph will show that the T-51A/
ARQ-8 has been mounted on a standard 7 inch rack panel by providing a square cut-out on the left and then fitting in the panel of t lie unit. It will be necessary to provide some additional enclosure shielding, since the original unit contains nothing in this line—having fitted into another type of case configuration. The rear photograph shows the works, less the aluminum back cover plate, but the shielding requirements may be gleaned nevertheless. The first step in the modification process will be to scuttle the self-excited 6V6 oscillator tube and insert in its place, a crystal-controlled oscillator-doubler using a type (>A(~:7 tube in a familiar grid-plate arrangement. The crystal used is the common 8+ mc variety, and the 6AG7 plate coil is resonated to the second harmonic of the crystal, or 16+ mc. From this point, the excitation at 16 + mc is fed into the rearranged 832A tube which formerly was a mixer, but now will function as a push-pull tripler to 50 mc. This tube will in turn drive the final 829B stage as the straight through Class "C" amplifier*
The 832A tripler tube is lightly loaded and easily provides the necessary excitation to the 829B stage, In the grid circuit of the tripler the original grid coil is changed out to another type and the link from the 6AG7 is fed into the middle of this new coil. The 832A tripler plate circuit will use the original rollo coil. About 250 volts dc is ample for this stage plate supply,
! 829B final stage comes in for a bit more of a change, especially in the plate circuitry, The grid circuit of this stage will again use the original rollo coil for its tuning but the plate circuit makes use of a new LC combination, The reason is this: when the input power to the final 829B is stepped up to around 100 watts or so and the output runs in the vicinity of 65 watts, the original ro llo coil in the plate circuit ex 1 lib its a bit too much heating and the rather small sliding contacts used in contacting the individual turns of the coil may start arcing when the unit is tuned under power. This condition cuts down on the efficiency of the final stage and prolonged transmission causes excessive heating, it was felt best to provide for something a bit more husky. This took the form of a dual-section, double-spaced variable capacitor and a new tank coil made of No. 12 enamelled wire, with a new two turn output link. If an 832A is substituted in the final, or if the S29B tube is throttled down in power input, this change in the final tank coil will not be necessarv. No decrease in
efficiency was noted when the power output was in the order of 20 watts or so, nor was heating any problem, so the removal of the original rollo coil will depend on just how far the user wishes to push the final stage.
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