>Hi all
>
>I am chasing an AL811 that its owner says is unstable - symptoms are
>unstable standing current at rest with no drive.
>
>The amp has been sitting for 2 years unused.
>
>Could this be caps reforming, gassed tubes, or the amp turning into an
>oscillator (TPTG!)?
>
>any ideas anyone!
>
? A friend who writes magazine reviews was evaluating the prototype
AL-811 before MFJ went into production. During the first half hour of
use, an audible glitch occurred and the tungsten filaments of 2, 811s
shattered and fell to the bottom of the glass envelope. He brought the
amplifier by for me to see. My dipmeter found a sharp dip at c. 90MHz at
the DC blocker C. My guess was that the filaments broke due a high burst
of grid-current during a parasitic visitation. This puts lateral stress
on the filament due to EM force at a right angle to current flow. It is
my opinion that the long grid lead in the 811 contributes to VHF
instability -- i.e., above its self-resonant freq., the seemingly
grounded grid is not grounded - so a feedback path exists between output
and input. Gonset and Heath apparently realized this problem and they
attempted to neutralize the 811s at the parasite freq. However, the
result was even less stability so they abandoned the idea. The only way
I know of to reduce V gain at 90MHz is to reduce the Q of the anode VHF
parasitic suppressor. This can be done by (1) increasing the L of the
suppressor-inductance, L-supp, increasing the dissipation capability
plus increasing the R of the suppressor-resistor, R-supp. Another method
(2)is to make the suppressor inductance out of resistance-wire. The
latter method reduces the dissipation in R-supp. The sticky-wicket is
finding a resistor with c. 10nH of intrinsic L that will handle the
considerable dissipation on 10m, so I use method (2).
>
cheers, Simon
Murphy was right.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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