[Amps] AL-80B questions

[2]

>
>Greetings amps gurus,
>
>A friend of mine just called me. He bought an AL-80B about 6 months ago. 
>It's now eaten it's 4th tube. He's sent the amp back once and gotten 
>replacement tubes the other times.
>
>This morning he wanted to try to work some of the contest on 10M, began 
>tuning up, said he heard a little "tick..... tick-tick" and then no 
>output. ZSAC is about 250mA.
>
Sounds like a grid-filament short [0v bias] or a bias diode short, or 
both.  If there's anode-I on standby, the tube may have a grid/fil short. 
 If the tube cuts off on standby, the tube is apparently ok.  

>The other times this happened he said Ameritron told him that they'd 
>gotten a bad batch of tubes (Amperex) and exchanged them for him.
>
There is nothing wrong with Amperex tube quality control.  I have used 
the same pair of Amperex 3-400Zs for the last 21 years and they appear to 
still have normal output.  However, like Eimac's and Penta's, if an 
Amperex 500Z oscillates at VHF, the 3-500's filament helices may bend 
enough to short against the grid-cage (as is shown in the Nov., 1990 
*QST* article). 
-  Ameritron is an excellent example of Not Invented Here syndrome.  For 
example, Eimac had a bad run of 8877s for c. 13-months.  Ameritron blamed 
 8877 failures in AL-1500s for a period of c. a decade on Eimac.  [see 
Figure 24 on my Web site].  Another example of NIH syndrome is ETO.  Dick 
E. told me that the new 8877 failures he saw in Alpha 77s must have been 
due to a manufacturing defect or Eimac would not have replaced them under 
the new tube warranty.  It seems like it's uaually the other guy's fault. 
 

>The one time that he sent the amp in to Ameritron they said they found 
>nothing wrong.

Without a high-potential tester or a C-meter, bent filament helices are 
hard to identify.
>
>Having never experienced parasitic problems first hand I can't speak from 
>experience but the last time this happened I suggested that he try one of 
>Rich Measures' retrofit kits. He says he now has it but hasn't installed 
>it yet.
>
Installing lower-Q vhf suppressors in an Ameritron amp carries risk.  
During a telephone conversation with Ameritron's amplifier "expert", he 
told me he would order their technician to "rip-out" any non-Ameritron 
parasitic suppressor he saw.  Presumably, any change automatically voids 
the warranty.

>My questions:
>
>1: Does this sound like a symptom of parasitic oscillations?

yes, unless the amp was being used aero-mobile in a stunt plane.
-   If you have access to a dipmeter, take a look at the vhf dip on 
either side of the DC blocking cap.  This is likely the freq where it 
occasionally oscillates.  Other AL-80 owners report a dip at c. 150MHz.

>  I thought the 3-500Z was pretty rugged with respect to faults.
>
Mine have lasted, but I don't use ordinary parasitic suppressors.  

>2: Does anyone know of any problems with recently built Amperex bottles?
>
No.   Amperex is the only 3-400Z/3-500Z manufacturer I know of that has 
not had a mfg defect,

>3: I'll probably help him install the parasitic supressor kit sometime 
>this week.  Are there any other things to look for or suggested 
>corrections to this thing that would make it less prone to eating another 
>tube?

One way to additionally reduce the effect of the 3-500Z's  0.15pF of 
anode/cathode feedback C is to provide a VHF termination at the cathode.  
This can be accomplished with a 25 - 35 pF cap in series with 10 or so 
ohms, connected from the cathode to chassis-gnd.  
>
>Thanks in advance all!

have phun, Randy.


-  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, 
www.vcnet.com/measures.  
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