[Amps] FL-21200B

[Alek Petkovic]

G'day all,

Here's an interesting one.

I have recently acquired an old Yaesu FL-2100B. I didn't really need it. I 
just bought it because it was cheap and nobody else wanted it.

The amplifier is in quite good condition and works very well. I have done 
nothing to it other than reduce the filament voltage from 6.8V at the tube 
bases down to a more sensible 6.2V. We have 245V AC mains here in Perth.

Today, I was fiddling with it and I noticed something strange.

With nothing plugged into the amp, I thought I would check the idling 
current. It was 80mA so I thought that was ok for the age of the tubes in 
the amp.

On close inspection of the ammeter, I noticed that the idling current was 
wavering ever so slightly. I turned the loading cap to the right and the 
idling current was definitely moving. Advancing the load cap further to the 
right to 6 or 7 on the scale brought up a nice little purple arc between 
the plates of the loading cap.

Plate current goes up to around 100mA during the arc.

This happens on the same point on the scale, regardless of bandswitch position.

Also, with setting the load cap around 5 on the scale, I can get the same 
arcing by turning the plate tune cap from left to right.

I have a number of 572B tubes of varying manufacture and vintage both 
Cetron and Chinese. The same symptoms appear with all the tubes I have tried.

To me, it sounds like this amplifier is a prime candidate for some Nichrome 
modifications.

Before I take that path, I would be interested in the hearing the thoughts 
of some of the learned gents on this forum.

Has anybody had any similar experiences?

73, Alek
VK6APK



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