Maco 300 power lag

[Cable Guy]

I have 2x Maco 300 amps. One is a 7 tube, the other has 6 tubes, with the 5th final tube removed.

They both exhibit a strange issue where dead key, after keying for a short time, starts to drop from about 250W down to 100W. After a little while, it climbs back up to regular output and stays for a while. This lag in power also affects peak output. I have tried other radios, antennas and dummy loads.

Thinking it could be the 'lytic caps, I have replaced all. It still occurs.

Has anyone experienced this and if so, what caused it?

 

[Shockwave]

I have 2x Maco 300 amps. One is a 7 tube, the other has 6 tubes, with the 5th final tube removed.

They both exhibit a strange issue where dead key, after keying for a short time, starts to drop from about 250W down to 100W. After a little while, it climbs back up to regular output and stays for a while. This lag in power also affects peak output. I have tried other radios, antennas and dummy loads.

Thinking it could be the 'lytic caps, I have replaced all. It still occurs.

Has anyone experienced this and if so, what caused it?

Neither the screen Grid or control grid voltages are regulated on this amplifier. The screen grid looks to have the same positive voltage that feeds the relays and is not likely to be moving around. However that method of a resistor and diode or just the resistor on the final stage to establish a control grid voltage is most likely the one that's moving around on you. As the tubes age they tend to develop more secondary emissions and that moves these voltages around as the tube gets hotter.

The control grid is self biased through a resistor in the final stage and dependent on how much current is being drawn to establish the voltage. The driver seems to have a diode rectifying RF to produce its voltage. A much better method would be to have a power supply at a zener diode to regulate it's bias voltage.

 

[nomadradio]

All good advice, but I'd be looking for a heat-related fault. As in, a capacitor that gets hot and stops performing.

The blocking cap between the plate choke where the tube-cap clips all connect on one side, and the Plate Tune control on the other side can do this.

Too much voltage there to just lay your fingertip on it to see if it's hot enough to burn you.

If that part is causing the problem, you'll see that the peak position of the Plate Tune knob has shifted when the power falls off. If the Tune control's peak position does NOT change when the power falls, this part is not likely the culprit. An infrared video camera would reveal this, but only if you have one.

73