When is it time for fresh AMP tubes?

[WA2MGB]

My old faithful Clipperton L running (4) 572B tubes presently has an output of 1KW  key down on 20m and 1.2KW key down on 80m...

Last I read this is a bit below the manufacturer specs.  The tubes are the origninals from 1978.

Question:  How low do you go, before replacing the tubes?

Tnx
Ted

[WB2WIK]

That's about the power output the Clipperton-L ran, depending on line voltage and if you could drive it adequately*.  Sounds like you don't have any problem at all.

I only replace tubes when they fail!

WB2WIK/6

*This amp wasn't so easy to drive.  Many modern solid-state rigs probably cannot deliver adequate drive for the Clipperton-L, although many of the older tube-type rigs could.

[WA2MGB]

Steve:

I'm driving the Clipperton with my FT-102...which is capable of pumping out 150w without a problem... It's turned out to be a nice combo for SSB and AM

[W9GB]

Ted -

The area that I watch with the Clipperton-L is the filament voltage on the 572B tubes!  This filament transformer is NOT center tapped - which I prefer for a multiple tube amplifiers.

The 572-B tubes recommended filament voltage is 6.3 VAC +/- .3 VAC. This means that the tubes should put out full rated power with as little as 6.0 volts on the filaments.

I have seen Clipperton-L with voltages as high as 6.76 volts on the filaments. This was more than the maximum recommended voltage. Eimac did a study and found that for every 3% above the minimum filament voltage required for full output reduced tube life by half. This may be a issue with your supply voltages from local power company.

If you need to reduce this voltage - It's hard to find resistors to do the job since each tube draws 4 amps filament current.

Bob Norgaard has a good explaination for his SB-200 (which also uses 572-B tubes).
http://home.gci.net/~rnorgard/ampnotes.html

He made a coil of wire about 30 inches long. They're made of 22 ga. teflon wire. These resistive coils drop the extra voltage. He started with two 36 inch lengths and kept shortening them until he measured about 6.05 to 6.1 volts at the tube sockets. When he got the length right, he just coiled them up and hold them down in the corner with a stick-on and a ty-rap.

w9gb