[Amps] Ameritron AL-80B

Rich 2@vc.net
Fri, 6 Dec 2002 06:08:20 -0800


>I have checked the archives for problems with the AL-80 and found some great
>information.  My experience on this kit is limited and wanted to confirm
>some of the information.
>The original problem was no output and  in the standby position the valve
>glows deep red. I replaced D17 (S/C) on the negative rail of the filter
>capacitors which can be damaged by a 'gas arc'?

Stories of "gas" and "barnacle"/Rocky Point-effect arcs have not been 
supported by autopsy evidence of kaput 3-500Zs.  A seemingly more 
plausible analysis is that the valve's/tube's 0.15pF feedback C sometimes 
causes the 3-500 to regenerate/oscillate at the AL-80's anode 
self-resonant ringing freq. of c. 150MHz.  Since there is no load @ 
150MHz, grid-I and other things can run amuck and the tube can apparently 
be damaged.  [grid-fil shorted 3-500Z autopsy photo on p.33, Sept. 1990 
*QST*]  The chance of a recurrence in this amplifier can be lessened by 
lowering VHF-gain -- which is done by lowering the VHF-Q of the parasitic 
suppressor.  However, there is a trade-off:  c. 2% less power out at 27 - 
29MHz.  
-   It might be of interest to see what the anode resonant freq is with a 
dipmeter.  Either end of the DC blocker C is a good test point.   To 
check for evidence of a past VHF osc, unsolder one end of the VHF 
suppressor's resistor and measure its ohms with a DMM.  If the ohms have 
increased more than 50% or so from the original value, chances are that 
there has been an intermittent VHF osc.

>I now have about 150mA constant plate current and negative going anode
>current, all in standby position.
>The amp has had D17 changed previously and internally is in excellent visual
>condition.  It has sat in the same position since I bought it several months
>ago and has not been used /AM! (Although it has been to VP8 on a DXpedition)

'Tis well known that DXpeditions substantially increase the chance of 
parasitic oscillations.  Murphy was right. 

>Just before the failure I applied 15watts of 10m RF when the amp was set and
>tuned on 15m.  Slight burning smell then no output.
>From what I have read it looks like I need to replace the 3-500z.  

correctamundo.  A good tube will be able to stand more than 5kV from 
grid-to-fil.  A tube with a bent fil helix will not.

>Before I
>buy a new valve (and cancel the kids Christmas presents) I would appreciate
>further comments
>Thanks
>Jim  MM0BQI
>
>
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-  Rich..., 805.386.3734, www.vcnet.com/measures.