Maco 300 power lag

[nomadradio]

That looks like the blocking cap for the driver tubes, under the chassis deck. If it's bad, it will probably heat up as the power drops. But it's not safe to put your finger on it to find out. Not with 900 Volts on it. A picture with a wider view would tell us what it's connected to. More than one of those caps in this model as a rule.

73

 

[Monk]

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?

I have run across the same problem in Palomar 300 2x4 tube amps. Instead of using a proper inductor/resistor to the plates of the tubes they used a ferrite bead. Over time the bead changes properties. It acts the same way you are describing.
Take the bead off and install a ~~47-56 ohm 2 watt resistor with 2~~3 turns of wire around it for a choke. No more weird power fluctuations.

Monkradio

 

[AudioShockwav]

I have run across the same problem in Palomar 300 2x4 tube amps. Instead of using a proper inductor/resistor to the plates of the tubes they used a ferrite bead. Over time the bead changes properties. It acts the same way you are describing.
Take the bead off and install a ~~47-56 ohm 2 watt resistor with 2~~3 turns of wire around it for a choke. No more weird power fluctuations.

Monkradio

If you open the cover on many of these old 300's the ferrite bead is no longer there, they get hot, break and fall off in pieces never to be seen again.
I did like that model ( 300A) running with just the four final tubes that were a decent amp for what they were.

73
Jeff