Dentron GLA-1000 Mod

Some (bare) Dentron GLA-1000 series amplifiers did not have a "cut-off" circuit to reduce the cathode current in reveive mode. This very simple standby mode modification circuit is a remedy to the design oversight. 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 respective RFC1 and RFC. The latter (>20 µH) isolates the RF driving signal from ground (via C7). The bypassed end of choke RFC1 is wired to the input change over relay contacts. Two paralleled capacitors were installed in the relay lead to the "RF in" input preventing DC current flowing into the transmitter. Choke RFC is wired from that relay contact to the bypassed end of the zener diode. With this mod there is no need for a 3th change over relay contact.

GLITCH PROTECTION

 

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 MOD WITH EL519 = PL519 (filament: PL519 = 40 VAC/300 mA, EL519 = 6.3 VAC/~2 A)

PL519

Drive

Output

80 m

80 W

500 W

40 m

80 W

425 W

30 m

85 W

450 W

20 m

80 W

375 W

17 m

85 W

400 W

15 m

80 W

375 W

12 m

70 W

350 W

10 m

85 W

250 W

An FRI-match ATU was used for matching the PA's input with a lenght of 2 m coaxcable between ATU and amplifier.