>
>As I was searching through the database of my new QST collection on CD-ROM,
>I found an interesting suggestion from an old acquaintance and mentor of
>mine, Barry Boothe, W9UCW of "The Minooka Special" fame.
>
>In a mid-70s issue of QST, Barry describes a unique method of implementing
>EBS to his homebrew amp. Rather than employ bias to the cathode, as is
>common in the ETO line of Alpha amps and perhaps others, he applies bias to
>the control grid in a GG configuration, by placing a pair of 0.01 uF mica
>from the grid to ground, and impresses negative bias to the control grid
>with the commonly-found darlington transistor pair who's bias is driven from
>the amp's input RF sample. From an R.F. perspective, the tube remains
>grounded, yet allows for D.C. bias. It's a single-state switch, but the
>point to be made is that EBS bias is applied to the grid rather than the
>cathode, thus eliminating the need for complicated protective measures
>between the EBS circuit and the cathode.
>
? However, some various fault possibilities.
1. A gas flashover in the tube due to a leaky seal.
2. The anode touches the grid (some 3-500Zs)
3. An intermittent vhf parasite generates a large burst in grid-current.
- - Each of these faults makes the grid dangerous to an EBS, so rugged
protective measures would still be necessary. // Another issue is
RF-input actuated transmit/receive bias switching. A friend recently
purchased such an amplifier (a 91B), and it generates a pecuilar type of
IMD as the amplifier rapidly switches from Tx-bias to Rx-bias between
syllables of speech. At the risk of being boring, I believe it is a
mistake to allow the bias to switch to Rx (cut-off) mode at any time when
the amplifier relays are in Tx mode.
- later, Paul.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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