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[Amps] al80a

To: <>
Subject: [Amps] al80a
From: (2)
Date: Sat, 3 Aug 2002 04:54:08 -0700
>> >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. 

**  The factory never errs ?

>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, 

** Why ?

>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.
**   1N5400-series rectifiers begin conducting c. 0.5V @25ºC. 

>> 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.
**  Putting glitch diodes in parallel with the meter movement protects 
the meter.  If the meter is RF-sensitive, there isn't enough bypass-C on 
the anode supply.  Bypassing both sides of the HV+ glitch resistor seems 
to help.  

> > ...

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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