Sure enough an Eagle 500 base amplifier got carried into the shop. Complaint was that the main relay would go "click" as soon as the standby switch was moved to "operate" position. Good thing this killed the owner's receive signal and alerted him to the fault. It could have caused damage if it stayed keyed with no drive power for too long. Sounds like the keying transistor has shorted. A quick check confirmed this.
But this amplifier has suicidal tendencies, so fixing the keying circuit was not the whole job.
I use this shortcut to achieve that. The red wire goes from our (shameless plug) keying circuit module to the pc board hole where the original keying transistor's emitter lead was soldered.
Easy peasy.
But the real problem with this amplifier comes from the choice of tubes. It was designed for a tube called the "27LF6". Yes, that's "twenty seven", and not "twenty". This was a tube used in european televisions alone, best I could tell. Made them hard to find on this side of the pond. But five of these in series add up to 135 Volts. Since the 125-Volt line is feeding directly into a series string of the five final-tube heaters, this works out just fine. Each 27-Volt heater gets 25 Volts on it, more or less.
The two driver tubes' heaters are powered by a 30-Volt winding on the power transformer.
Thirty Volts.
Install two 20LF6 tubes as the drivers and now you have two driver tubes on a nitrous bottle with no "Off" switch. The result I have seen over and over is two dead-flat or melted driver tubes and a flamed-out plate-choke coil.
The solution is a resistor that drops ten Volts to take that thirty down to twenty. Ohms law says we need 8.3 ohms since the two heaters draw 0.6 Amps apiece for a parallel total of 1.2 Amps. Ten volts divided by 1.2 Amps is 8.3 ohms and change. Ten Volts times 1.2 Amps says this resistor will throw 12 Watts of heat.
Overkill in defense of reliability is no vice.
These 25 ohm 30 Watt wirewound resistors were cheap. Cleaned up when a local distributor got stiffed and stuck with a special order that the bankrupt customer never paid to receive. Three of them in parallel get me 8.3 ohms. This dropped the heater voltage measured at the tube sockets to 20 Volts. Bingo!
Three thirty Watt resistors each dissipating four Watts each won't get hot enough to degrade any of the thermoplastic wire laying up against them.
Much easier than re-routing a bunch of wires away from a hot power resistor.
The arithmetic for the final tubes is a little worrisome in this regard. Five of them in series add up to 100 Volts, not 125. We'll see how long those last. At least the driver tubes won't be going Chernobyl.
73
But this amplifier has suicidal tendencies, so fixing the keying circuit was not the whole job.
I use this shortcut to achieve that. The red wire goes from our (shameless plug) keying circuit module to the pc board hole where the original keying transistor's emitter lead was soldered.
Easy peasy.
But the real problem with this amplifier comes from the choice of tubes. It was designed for a tube called the "27LF6". Yes, that's "twenty seven", and not "twenty". This was a tube used in european televisions alone, best I could tell. Made them hard to find on this side of the pond. But five of these in series add up to 135 Volts. Since the 125-Volt line is feeding directly into a series string of the five final-tube heaters, this works out just fine. Each 27-Volt heater gets 25 Volts on it, more or less.
The two driver tubes' heaters are powered by a 30-Volt winding on the power transformer.
Thirty Volts.
Install two 20LF6 tubes as the drivers and now you have two driver tubes on a nitrous bottle with no "Off" switch. The result I have seen over and over is two dead-flat or melted driver tubes and a flamed-out plate-choke coil.
The solution is a resistor that drops ten Volts to take that thirty down to twenty. Ohms law says we need 8.3 ohms since the two heaters draw 0.6 Amps apiece for a parallel total of 1.2 Amps. Ten volts divided by 1.2 Amps is 8.3 ohms and change. Ten Volts times 1.2 Amps says this resistor will throw 12 Watts of heat.
Overkill in defense of reliability is no vice.
These 25 ohm 30 Watt wirewound resistors were cheap. Cleaned up when a local distributor got stiffed and stuck with a special order that the bankrupt customer never paid to receive. Three of them in parallel get me 8.3 ohms. This dropped the heater voltage measured at the tube sockets to 20 Volts. Bingo!
Three thirty Watt resistors each dissipating four Watts each won't get hot enough to degrade any of the thermoplastic wire laying up against them.
Much easier than re-routing a bunch of wires away from a hot power resistor.
The arithmetic for the final tubes is a little worrisome in this regard. Five of them in series add up to 100 Volts, not 125. We'll see how long those last. At least the driver tubes won't be going Chernobyl.
73
