No.32
Amateur Radio
EL509

The Dentron GLA-1000 was a very popular, low-cost RF linear amplifier. Many thousands were sold during the early 1980s. The amplifier has superb styling, one of the features of Dentron amplifiers during the era. The choice of the power tube was a major contributor to the low cost of the GLA-1000. It used four 6LQ6, so-called "sweep tubes," in parallel. The 6LQ6 was developed for use in horizontal sweep circuits in TV sets; hence, the name "sweep tube." The market for these tubes during the heyday of tube-type TV set production was tremendous. Virtually all tube manufacturers of the day participated in this booming market. The 6LQ6 had a few desirable characteristics for linear amplifier service. It was designed to hold off very high voltage; it had relatively high plate dissipation capability for a low-cost glass receiving-type tube; it featured high transconductance; and, most importantly, during the peak of the market, it had an incredibly low price for a tube of its power capability. The 6LQ6 was not designed for linear amplifier service and, of course, it was no match for popular high-priced power tubes, such as the 3-500Z. The application of sweep tubes in amateur radio linear amplifiers was clearly an engineering compromise dictated by economy.

Sweep tubes similar to the 6LQ6 have been manufactured in Europe and Eastern Europe. One of the most popular was the EL509, which was used in tube-type TV sets in the USSR during the Cold War. TV technology evolved from tube to solid-state circuitry very rapidly in the West, and all sweep-tube manufacturing ceased about ten years ago. The evolution occurred more slowly in the East. As a result, the only currently manufactured sweep tubes in the world come from Eastern Europe. The most significant manufacturer is Svetlana in St. Petersburg, Russia, which still has the EL509 in production. Fortunately for sweep-tube linear owners, there is still a supply of currently manufactured television sweep tubes available. The EL509 is known under the American type numbering system as the 6KG6. The Russian EL509 is more rugged than its Western counterparts, with heavy glass construction and high-temperature, high-voltage processing. The EL509 was developed and materials were chosen during a period of Russian history when manufacturing cost for consumer goods was not important.

After the demise of sweep tube manufacturing in the West, original stocks of various types, including the 6LQ6, remained on the shelves of repair shops and tube dealers. As the supply dwindled, the price of the tubes increased according to the law of supply and demand. Today, the cost of four original 6LQ6 tubes exceeds the value of the linear amplifier, so old sweep tube linears (without tubes) are available at flea markets for a song.

This article shows the procedure for modifying an old Dentron GLA-1000. This modification was developed and tested on an early GLA-1000 to show that old linear amplifiers which have lost their value, due to the cost of replacement tubes, can be rescued from the dumpster. The simple modifications shown herein should be applicable to most other linear amplifiers that use sweep tubes.

The Russian EL509 differs from Western sweep tubes in a number of ways. The base is a standard 9-pin magnoval configuration, which differs from the bases used on most Western sweep tubes. The EL509 is physically larger than the 6LQ6 and just fits within the cabinet of the GLA-1000. Further, there are small differences in the interelectrode capacitances. These differences require minor modifications to the RF circuitry. Fortunately, the GLA-1000 socket connections can be modified easily, so the EL509 can be used without replacing the sockets or the plate caps. A socket and plate cap are available from Svetlana for modification of other sweep-tube amplifiers which require them. Both sockets and plate caps are made of ceramic and carry the nomenclature SK509 and PC509, respectively.

Later versions of the GLA-1000A are similar to the early version we modified, so the same modifications shown below can be used.

PROCEDURE

(1) Remove the top and bottom covers. Be sure the power-supply filter capacitors are fully discharged. Figure 1 shows the PC board before modification. As shown in Figure 2, perform modifications to the traces on the bottom of each tube of the tube mounting PC board. Figure 3 shows the board after modification. The modification is as follows:
(a) Cut the trace to pin 3 on each socket.
(b) Attach pin 3 to pin 2 on each socket with a wire jumper. Keep these jumpers as short as possible.
(c) With jumpers, attach pin 9 of each socket to the cathode circuit (which was the trace cut away from all the pin 3s). Keep these jumpers as short as possible.

(2) Because the Svetlana EL509 has different internal capacitance and different capacitance to the cabinet from the original 6LQ6, some modifications must be made to the output tuning and loading networks, as follows:
(a) Using a very short wire jumper, short out the first turn on the output tank coil as close to its connection on the TUNE capacitor as possible. The schematic and Figure 4 shows how this was done. Note that the jumper is simply soldered to a single turn on the coil so disassembly is not required.
(b) Attach a 200 pF capacitor in parallel with the LOAD variable capacitor. This capacitor must be a silver-mica or NPO ceramic type. A 600V rating is more than adequate. Figure 6 also shows where the capacitor mounts, across the output choke RFC-7.
(c) Some GLA-1000s have been modified with an added 10-meter tap on the tuning coil and selector switch. This setting is usually not marked on the front panel of the amplifier. If 10-meter use of the GLA-1000 is desired, be sure the full modification is performed. Remove the original unmarked (10-meter) tap wire, if it exists, and substitute a longer wire. Attach the wire to the coil at a position 1/4 turn toward the input (C9) end from the original position. This will require moving the tap to the physical top of the coil, on the third turn in from the plate-tuning capacitor. See Figure 5 for the new position of the 10-meter tap.
(d) Many GLA-1000 amplifiers will contain a filter FL1, which prevents the GLA-1000 from operating on the Amateur Radio 10- and 12-meter bands and the Citizen's Band. For operation of the GLA-1000 on 10 and 12 meters, FL1 should be removed. The FCC requires this modification to be made by a licensed amateur.

(3) Carefully plug the EL509s into the tube sockets. The pin dimensions of the EL509 are compatible with those of the 6LQ6, except that the EL509 pins are larger in diameter. If care is taken, the EL509 pins will fit into the original sockets tightly, using a slow rocking motion. Use a sharp tool to open the socket receptacles slightly, if necessary, to ease the tube insertion.

This completes the modification. No bias or other modifications need to be made to the GLA-1000 to accommodate the Svetlana EL509. When ready to operate, replace the top and bottom covers.

TESTING

(1) After these modifications are performed, turn-on and initial testing should be performed with minimum RF drive-power from the exciter (transceiver). The amplifier RF output should be terminated in a 50-ohm dummy load capable of the full output power.

Commence with 10 meters, around 28.8 MHz. Verify that resonance is achievable by observing C9. Should resonance not be achieved, C9's rotor and stator plates will not mesh. In this case, it will be necessary to move the 10-meter tap closer toward the C9 connection point. If resonance is being achieved, then RF drive may be increased and the LOAD and TUNE controls adjusted until RF maximum output, in the 400-watt range, is observed. All sweep tubes are relatively fragile. Therefore, key-down condition should be limited to the minimum time required for plate tuning and plate loading adjustments. The chart in Table 1 shows the performance we achieved for the major amateur bands. A tuning chart, or a chart similar to Table 1, for each of the amateur bands will facilitate tune-up of your amplifier and minimize key down-time during full-power tuneup.

The GLA-1000 amplifier may be capable of operating on the WARC amateur bands of 30, 17, and 12 meters. However, no attempt was made to operate the GLA-1000 amplifier on these bands. Two cautions are recommended for operation on the WARC bands:

(a) On some early Dentron amplifiers, the RF plate choke was found to be resonant in/around the 17-meter band. Should resonance occur at any frequency within an amateur band, it may be moved down in frequency by adding a turn or two to the RF plate choke.
(b) In the U. S., the maximum legal RF output power on 30 meters is 200 watts.

EXCITER CONSIDERATIONS

(1) Many modern transceivers have an automatic antenna tuner built in. If your transceiver has an automatic tuner, it should be placed in the "in-line" operating condition, as the input impedance of the GLA-1000 varies considerably from 80 meters to 10 meters. Older tube-type transceivers will not require a matching network, as the transceiver tune and load controls can match this input. Early solid-state exciters do not have antenna tuning. Without a matching network at the input to the GLA-1000, it may exhibit a high VSWR, causing the solid-state exciter to self-protect, reducing power output. If this is found to be the case, an ordinary antenna tuner can be used to achieve an impedance match at the input of the GLA-1000.

Data on the Svetlana EL509 power tube and SK509 ceramic socket may be downloaded from Svetlana website at www.svetlana.com, or may be requested by mail from Svetlana Electron Devices, 8200 South Memorial Parkway, Huntsville, AL 35802, telephone (256) 882-1344.

Many thanks to Mr. George Toma of Dexter, Michigan, for providing the Dentron GLA-1000 on which these modifications were performed and tested.

Table 1 - Test Results and Tuning Chart

NOTES:
1. Equipment External to GLA-1000
(A) Drake TR-7 Transceiver
(B) Input Power Meter and Drake MN2700 Antenna Matcher
(C) Output Power Meter Drake WN7
(D) Dummy Load MFJ-0250

2. 1 turn shorted and tap moved 1/4 turn torward cold end 3. 200 PF cap across load cap 4. One position clockwise past Ò21Ó

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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