Crystal Controlled

A New

MODEL LPA-t PRICE! S375.00

MODEL 6100 PRICE! $875.00

Model gtoa ti

Grid Linear Amplifier

VANIA • STillwell 8-5581

Low distortion

All Band Linear Kilowatt

Bert Green W2LPC George Phillips WA2PDI Amperex Electronic Corp. 230 Duffy Avenue Hicksville, New York

When designing a kilowatt linear amplifier that will handle a full gallon input as a sideband, C\V or AM linear, Llie selection of the tube depends oil the following:

A. The linearity of the tube without feedback,

B. The overall physical size of the tube, socket, and blower.

C. The overall cost of the tube, socket and blower.

In category A, linearity, the Amperex HI79 was rated for lowest distortion with the 3rd order I.M. products being down better than

40db in a grounded grid circuit.

Category B, si/e, showed up as a close tie between three contenders. While the Amperex 8179 was physically larger than the other tubes it was able to utilize a much smaller blower for cooling. The 8179 only requires a small How of air over the surface of the tube which means that any small centrifugal blower may be used. The external radiator type tubes required a larger type blower to de liver the required air flow against the radiator back pressure.

In category C, cost, the 8179 was the lowest

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mam priced tube by only a few dollars. However, the socket for the 8179 listed at 1/5 to 1/7 of the price of the sockets for the other two tubes. In addition the lower cost of the blower resulted in the S179 tube, socket, blower price combination being much below the other choices.

Power Supply

The plate power supply is constructed on a 17 x 17 x 4 steel chassis enclosed in a perforated aluminium cover. The use of a choke input filter combined with silicon rectifiers and a husky transformer provide a power supply with extremely good regulation. Since the silicon rectifiers are used in series, each one is shunted with a one megohm resistor to equalize the inverse voltages across each diode. Being on the cautious side a few extra diodes were used to provide a safety factor.

A shielded cable, terminated in an Amphenol type 97-3; 06-28-410 connectors, carries primary power toT and high voltage from, the power supply,

Amplifier

The amplifier unit contains the rf amplifier itself, the bias supply, the screen supply, the filament supply, and the metering and control circuits.

The amplifier is a cathode driven, double grounded grid stage, with pi network input and output. A pi network was used for the input circuit for two reasons; first, it prov ides a proper impedance match between the 50 ohm input line and the 110 ohm input impedance of the 8179, thereby reducing drive power requirements.

The second reason for using a pi network input circuit is to reduce the harmonic input to the amplifier. In a grounded grid stage, the cathode impedance of the tube varies from a very high value to a low value at different points on the rf cycle. This causes a varying load to be presented to the driver and produces considerable second harmonic distortion. This sccond harmonic drive increases the plate input to the tube, but does not appear as usable output since the plate tank is an effective short at the harmonic frequency, ' his results in low tube efficiency and high plate dissipation. By driving the amplifier through a pi network input circuit, the amount of second harmonic appearing at the amplifier grid is reduced and the plate efficiency is increased, thereby producing more usable output for the same plate input.

The pi network used on the input of the

8179 amplifier is bands witched to cover the 80, 40s 20, 15, and 10 meter bands and is designed to have a low Q in order to cover each band without the need for retuning. Trimmer capacitors on the input and output of the pi networks allows the SWR to be adjusted to close to 1:1 on each band,

The rf signal from the output of the pi is capacity coupled to the tube filament, while the 60 cycle ac power is feci to the filament by means of a B & W all-band bifilar filament choke.

The control grid is by-passed to ground by means of three Erie 1500 mmfd stand off capacitors connected in parallel to carry the if current.

The screen grid of the 8179 is brought out to two pins and each of these is bypassed to ground by 5 Centralab 1000 mmfd transmitting capacitors in parallel ¡ his was done because at 30 megacycles the output capacity of the tube is a large portion of the total tank capacity and much of the circulating tank current flows through the screen bypass capacitor.

Hie plate circuit is a bands witched pi network constructed around a modified Ilium i-tronix #P1 195-2 pi network inductor, A Radio Switch Corp. Model 86 rotary switch is xised to change taps on the coil and on certain bands to place fixed capacitors in parallel with the variable loading capacitor. The plate tuning capacitor and the loading capacitor are 250 mm Id variable vacuum capacitors. The pi dux coil is modified by removing the 10 meter section and substituting a coil of 6 turns of U" copper tubing. 2?i" long on a lis" I.D, The tapping points for the different bands were also changed. These changes were made in order to maintain the best L to C ratio for each band. The pi network is connected to the plate of the tube through a blocking capacitor made of 3 centralab 500 mmfd i \ type high voltage capacitors in parallel. The B+ is shunt fed to the tube through a Raypar all band plate choke. Several homemade and commercial chokes were measured and the Raypar unit was found to have the best impedance characteristics over the desired frequency range. A Z-28 rf choke was connected from the output side of the pi to ground to prevent high dc voltage from appearing at the antenna terminals in case of a shorted blocking capacitor.

The grid bias supply is a simple half wave rectifier with a pi section filter and an OB2 VR tube. This provides a grid bias of 105 V which is stable and has very low ripple. Since there is no grid current drawn from this supply, it was made variable merely by connecting a potentiometer across its output. A resistor is placed in series with the potentiometer to prevent the bias from accidentally being turned down to zero.

In a linear amplifier, in order to maintain low distortion, the screen grid power supply must be regulated. Ordinarily, this can be done with V.H tubes. The 8179, however, draws more current than can be obtained from normal Y\H, tubes, thereby, making necessary an electronically regulated screen supply, Electronic regulation generally requires considerable chassis space and can be quite expensive, For this transmittter, however, a regulator was designed which compares very favorably in cost and occupies less space than a

All values mmfcf unless otherwise specified

string of V.FL tubes would, even if they could handle the current. This regulator offers performance which is very much superior to the V.R. tubes plus the advantage that it is adjustable, The regulator consists of 2 tubes, 7 resistors? 2 tubular capacitors and a potentiometer, One of the tubes is an OB2 used as a voltage reference and the other is a 6GW8 tnode-power pentode used as a series regulator and feedback amplifier. Power is supplied to the regulator by a Stancor PC8420 transformer and a bridge rectifier consisting of eight BY-100 silicon diodes and a single section capacitor filter.

Four 2 inch meters allow for monitoring the operating conditions of the amplifier: Since the amplifier plate input is above 900 watts, both plate voltage and plate current meters are necessary to meet FCC regulations. The screen current meter in combination with the plate current meter allows the loading to be properly adjusted, and the grid current meter indicates if the amplifier is being driven beyond Class ABlt

Full circuit protection is provided by fuses and a modified three pole Heinemann circuit breaker on the front panel. A Rotron muffin fan on the rear ol the unit provides a small How of air past the tube and tank coil. This is necessary sincc the amplifier is completely enclosed and would overheat unless forced ventilation was provided.

The amplifier is constructed on a 17" x 17" x4" aluminum chassis with a 19' x 14" aluminum front panel The rf amplifier is at one end of the chassis and is completely enclosed in an aluminum compartment to minimize

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