>Rich,
>
>Thanks for your reply.
ur welcome, Paul.
>As is almost always the case, there are trade-offs
>associated with every change, some more obvious than others.
>
? Rumour has it that Ameritron/MFJ is going to add RF-actuated Tx/Rx
bias switching to supposedly improve their line of amplifiers. If they
do so and *QST* does product testing, a large pizza with 4 toppings and
salad bar says the IMD test will not use ordinary speech.
- later, Paul
>
>-----Original Message-----
>From: Rich Measures <measures@vc.net>
>To: [AMPS] submissions <amps@contesting.com>
To: <amps@contesting.com>
>Date: Monday, December 28, 1998 1:04 PM
>Subject: Re: [AMPS] Electronic Bias Switching
>
>
>
>>
>>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...
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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