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[Amps] al80a

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Subject: [Amps] al80a
From: (Tom Rauch)
Date: Sat, 3 Aug 2002 11:02:33 -0400
> >is no good. I should have a known good one tomorrow to check it out.
> >In the meantime I have seen several things that need to be done.
> >glitch diodes, glitch resistor, new suppressor unit, etc. Live and
> >learn (or re-learn??)

99% of that stuff is just needless folklore originated by people who 
sell kits.

Some people just can't face the fact that tubes fail, and so they 
"invent" odd reasons to explain a common event. For example, during 
some production runs nearly 20% of new Amperex tubes developed 
grid/filament shorts without ever applying high voltage!

You should install the glitch diode mod if your amp does not already 
have one, but do it the way the factory did it. You must have a 
couple bypass capacitors from the electrolytics to ground, a small RF 
choke to the negative end of the electrolytics over to the diode, and 
the meter lead leaving from there. A diode with a threshold higher 
than .6 volts is necessary to prevent diode conduction on current 
peaks, which is aggravated by any tiny amounts of stray RF.

> Take care about the glitch diode. In an SB-1000 (same animal) that
> someone else had "pre-repaired", I had weird meter readings on 160m
> until the glitch diode was replaced with a 1N5408. It turned out that
> his no-name diode was rectifying at 1.8MHz!

He might not have installed the RF choke or the bypass caps, or used 
a diode with a low threshold voltage.

 > While you have the cover off, it's also worthwhile to fit a step-
> relay. Rich's simple circuit is hard to beat: the relay is a 12V DC
> type, powered from the existing +12V rail. At power-on, the step-start
> resistor in series with the mains, so it takes a little time for that
> rail to come up, and this generates the delay.

AL80-A's have that in them. Heath did not choose to do that for some 
reason. Virtually all AL80A's have the glitch diode also.

73, Tom W8JI 

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