[Amps] Al-80a problem.

Richard 2@mail.vcnet.com
Mon, 15 Jul 2002 20:51:34 -0700

>> my al-80A hicuped tonight. I do not have the schematic for this amp.
>> It does not show HV now. After inspecting the meter, the reading was
>> open from the outside terminals. i have removed the meter from the
>> chassis and disasembled. Inside the meter is a small 1/8 or 1/16 watt
>> resistor in seris with the meter coil. This resistor reads open. The
>> meter coil reads around 400ohms and also the volt ohm meter deflects
>> the meter upward about 20%. What is the value of this resistor inside
>> the plate, HV, Power, alc meter. ? Also the plate current shunt
>> resistor is blown. I also need the value for that.
>Look in the manual. Although I designed that amp and managed the 
>company that built those meters, I can't recall the exact meter 
>resistances. I think they are 450 ohms, and if that is correct the 
>resistor would be a hand selected 39 to 68 ohm resistor that makes 
>the movement read exactly the resistance specified.
>With a 0.6 ohm shunt and a 1mA movement, that would be 450 ohms for 
>750mA full scale. Don't get ANYTHING trashy in the meter, and make 
>sure you do not splash flux beads in the meter when soldering.   
>>> Any other thoughts what went on. And how to make preventative 
>> for the future.
>Any number of things could have arced Victor. The tube could be 
>getting bad, and flashing over. The dirt could have flashed.

Or a flock of kamikaze fruit flies could have attacked.
>You must have an old AL-80A that does not have the HV fault diode 
>from the supply rail to ground. You need to add that diode, two 
>capacitors, and a small RF choke. Then a HV flashover will not hurt 
>I would not worry about adding any gimmicks to the amplifier, the 
>suppressor in it is fine. 

- Not according to N7WS' laboratory measurements of 100MHz Q on a HP 

>Amplifiers that are not stable do not last 
>perfectly for 15-20 years, and then suddenly need major modification!
>Virtually 100% of the time the problem you saw is either caused by 
>dirt from poor maintenance or a defective component, most often the 

My guess is that if you check the tube @8kV on a high-pot, you will 
measure under 10uA of anode-grid leakage.   The quickest test is to 
measure the resistance of the suppressor-R. 

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