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Усилитель на лампах ГИ-7Б
Усилитель Dentron GLA-1000 Conversion
Dentron GLA-1000 Conversion, Modification
With 4 ? PL/EL-519 or 2 ? GI-7B
Partly finished conversion with a doubling power supply, a 12.6 VAC filament transformer and 2 ? GI-7B.
Some bare Dentron GLA-1000 (Plane) series amplifiers (
A 27 kΩ resistor cuts off plate current flow when the amplifier is not being used. Relay K1A contact short the resistor out when the amplifier is keyed allowing the operating bias (D1) to hold the idling plate current via RFC1. The latter isolates the RF driving signal from ground (via C7). Two paralleled capacitors were installed in the relay lead to the "RF in" input preventing DC current flowing into the transmitter. Choke RFC1 is wired from that relay contact to the bypassed end of the zener diode. With this mod there is no need for a 3rd change over relay contact.
There is no failure mode of "glitch" protection and it is a matter of time before the amplifier fails for a number of reasons or shortcomings. Update the PA with the marked components. The extra 4.7 nF capacitor parallel to C8 improves the decoupling of the anode choke RFC6 on 80 m.
TEST WITH 4 ? PL519
WIDEBAND INPUT CIRCUIT & EL519's
The static input impedance of the cathode in a grounded grid circuit can be calculated if the valve characteristics relating to the specific circuit are known. The dynamic input impedance (during working conditions) is often higher. Furthermore it varies during a SSB transmission because the driver (the transceiver) is delivering power varying between say 0.1 W (the suppressed carrier) and about 100 W. The input- and output impedance of the amplifier are constantly varying and the driver sees a constantly varying load. If the internal controlling system (ALC) cannot handle this, a distorted signal will be generated in the transceiver. With a fast reacting SWR-indicator between TX and PA, one will see a constantly varying SWR. The input impedance of the PL519's in the circuit chosen by me was not known. With an experimental test rig, I have tried to obtain some relevant data. It turned out that these data were different for every amateur band, roughly averaging 17 Ω-27 Ω and with some guessing, I found that for 4 valves in parallel, the average real part was 22 Ω, say 25 Ω.
In most cases a tuned circuit between driver and final stage is recommended. By way of flywheel-action this tuned input-circuit will, to a degree, level out the quite variable input impedance, thereby preserving a reasonable match and thus linearity and output of the driving transceiver. The tuned input circuit also shortens the HF return path between anode and cathode by preventing this HF current to follow the longer path via the transceiver. As modern transceivers have more than sufficient power, the "flattening" of the input impedance may also be obtained by extra loading (swamping) the input-circuit with a resistance or suitable wide band combination, in which excess driving power can be absorbed. This also helps in lowering the HF return-path impedance.
In our case, matching is done with a 4 : 1 RF impedance transformer (fig>) which transforms the 25 Ω impedance to about 100 Ω. By putting a 100 Ω-swamping resistor across it, the driver will see a load with a SWR of less than 1.5. Nearly all transceivers can deliver about 100 W with such an SWR without an antenna tuner. The 100 pF trimmer is for minimising the input SWR in the 10 m band. Adjustment of the input- and output-circuits must be done with full carrier power (key down) for maximum output power and minimum input SWR.
This transformer ( This (right picture) alternative simpler resistor input circuit has a higher SWR on 10-40 m and reduced output on 10 m, however this should not be a problem with a modern transmitter with a built-in antenna tuner.
This (right picture) alternative simpler resistor input circuit has a higher SWR on 10-40 m and reduced output on 10 m, however this should not be a problem with a modern transmitter with a built-in antenna tuner.
IDLE CURRENT ADJUSTMENT
When using valves in parallel, we have to consider the individual differences. With collective bias, individual-resting currents will differ.Even if the idle currents are made equal, the HF-amplification factors are not. In our bare-bones circuit, an individual adjustment was considered, but left in favour of a simpler system, based upon DC-feedback during excitation.
This is done (
The value of the collective bias is adjusted with a string of 3-10 diodes in series. Short circuit one of more for a standing current of about 20-25 mA per valve, i.e. 80-100 mA in total for 4 valves. When using this simple bias circuit, it is advised not to wait too long with speaking after pushing the PTT button, in order to let the valves draw standing current for only a short moment. The reason for this prudence is the possibility that the individual currents deviates so much, that one or two will draw much more current and dissipate excessively. In this way, the valves will only conduct when the PTT is activated. As a rule, the individual products of standing current and anode voltage should stay below the maximum dissipation of 35 W per valve. Assuming a 10 % spread in standing currents the total dissipation, while sending without drive, will be about 130 W.
Each PL519 filament requires 40 VAC/0.3 A. In our circuit, the four filaments may be fed in series; with a capacitor of 5.6-6 μF/250 VAC added in series. The string may be connected directly to the 230 VAC (50 Hz) mains. If the chassis of the PA is earthen through the cable to the mains, this method is acceptable, with the added benefit of a gradually heating up of the filaments, i.e. without the thermal shock incurred by transformer feed. Because the cathodes are not at RF earth level, the filaments are by-passed to earth with capacitors.
This (fig>) is an example for the calculation of the capacitor in series with 0.3 A filaments.NOTE: substitute in the formula 60 for 60 Hz mains. (fig>)
After having (re) checked the newly built linear, the transceiver, a dummy-load and a power meter are connected. Don't forget the control lead for the relay! If, after applying power, the linear does stay stable, you probably did everything right. Now push the PTT-switch (in SSB mode but without speaking into the mic.) then one at a time, short as many of diodes in the cathode diode string as it takes to obtain the desired 80-100 mA anode idling current.
Warning: Be sure the power is off and the HT electrolytic discharged before touching any internal wiring!
With about 10 W drive and beginning with the 80 m band, tune the output-circuit for maximum power. Repeat the procedure, alternating between loading C and tuning C, till maximum power is reached. NB Whistling in the mike will not give a steady signal, a carrier (CW or FM) is necessary for repeatability.
Now raise the drive level to 100 W and repeat. Let the PA cool for at least 30 seconds after every 30 seconds of transmitting, to keep the valves healthy. Off-resonance, hefty currents can flow through the valves! If possible, proceed initially with lowered supply voltage. All now seems adjusted optimally but pay attention: lower the capacitance of the loading C until the needle of the power meter falls back 2 to 3 needle thickness; now re-adjust the tuning C for maximal power. Only after this last adjustment is full linearity achieved. Record capacitor settings before proceeding to the next frequency and/or antenna.
TEST OF 2 ? GI-7B WITH THE ORIGINAL 1050 V SUPPLY
Test 1 ? GI-7B with a smaller cooler for the anode. The tube was temporary mounted on a base of a defect 3-500Z for eventually testing in my FRI750 PA
Test with minor modifications of the schematic. The blue colour marks the details.
Test with 2 ? GI-7B and an external input circuit (ATU) to obtain SWR = 1 on all bands. Anode supply was about 850 V on load.
An original socket for the GI7B.
Test with 4 ? PL-519 and 2 ? GI-7B with minor modifications (see schematic) of the original GLA-1000 design.
This converted amplifier requires less drive than the 100 watts many transceivers provide. Addition of a + 10 Ω/15 W non-inductive resistor (R) is a simple way of correcting the overdrive. An RF-negative-feedback voltage will be developed across R when the GI-7B amplifies RF and reduces the driving voltage developed at the cathode. The negative feedback also improves amplifier linearity by reducing IMD to a level lower than that attainable when the tube is driven at its rated driving power without the feedback resistor, it helps to keep tube operation in its linear region. R absorbs the excess driving power as heat. The input impedance without a input circuit is + 50 Ω and SWR ≤ 1.7.
EXPERIMENTAL VARIABLE INPUT CIRCUIT WITH 1 KNOB
I did experiments with an input circuit based on myFri-match design. This matching system increased the output on the lower frequency bands and improves the input SWR. The test model (fig>) is made with a T94-6 toroid and a small twin ganging variable cap.
TEST 2 ? GI-7B WITH DOUBLING 2100 V SUPPLY
Testing the "final" circuit with10 Ω feed-back and an (fig>) temporary external HT doubling components, 6 ? 220 чF/450 V and 6 ? 1300 V/1 A. Key-down output was 700-850 W with 1500 V on load. Average CW/1850 V, SSB/2000 V and off load + 2100 V. An 24 zener diode holds the idling plate current flow to around 100 mA.
The dissipation of the tubes is overrated in this design but the transformer is not e.g. the output should be limited to 700-750 W. Therefore the feedback resistor "R" was increased to 25 Ω. Two 50 Ω/30 W non inductive "chip" resistors (
Test with temporary wiring and alternative cathode/filament connections with hose clamps and tool clips.
Standard tool clips used as connectors.
The improved wiring.
The elevating of the voltage from 1050 to 2150 V increased the plate load impedance.
Remove the 20-15-10 m coil L1 and wind another coil with the same wire size or with standard 6 mm? tinned solid copper wire (
PCB with fusing, rectifier & doubling supply, metering plate current and plate voltage.
Planned front with tuned input circuit.
Function Switch: Volts converted to grid current or rel. output converted to grid current.
To be continued
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