>
>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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