[Collins] 32S-1 Oscillations & Other Troubles

[Dr. Gerald N. Johnson, electrical engineer]

There was a change to the neutralization circuit to make it work with
the later 6146B tubes. For certain the Bs are larger and have more
capacitance so the neutralization is different. And very necessary else
they will oscillate, most strongly on 10m.

I've read about the feed through capacitor problem, but I've not run
into it personally. Since the neutralization circuit is a bridge with
that capacitor as one of the elements, its value is far more critical
than if it was just a bypass capacitor. I think sometimes the failure is
fairly obvious with the capacitor having crumbled leaving the feed
through wire dangling. Its probable that the capacitor was mounted
before much else and the radio was built around it to make replacement
most difficult once you find one. Many feed through capacitors have a
large tolerance on value and one way at the limits of that tolerance may
be too far out for successful neutralization after replacement.

Neutralization adjustment isn't optional, its required to keep the tubes
from oscillating and thus to keep them linear. Stuffing in 6146B in a
circuit designed before they were made can run into neutralization
circuit capability limitations.

I like using 6146B at 6146 ratings because they will have a very long
life time. I don't like running 6146B at 6146B ratings in the S-Line
because the output tank components have little margin for extra power
and are hard to get to when replacement is required, especially the mica
loading capacitor padders that aren't made these days with the requisite
current rating. Dipped silver mica's won't survive at those currents.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.