>Subject: Re: [AMPS] Alpha 77DX Bias Switching
>Sent: 3/26/1997 6:37 AM
>Received: 3/26/1997 10:05 AM
>From: Vincent Fedele, Vince.Fedele@VSTTECH.COM
>To: Dick Byrd, n4uq@mindspring.com
>CC: amps@contesting.com
>
>Re: [AMPS] Alpha 77DX Bias Switching
>
>Several comments on the pros and cons of the 77 EBS system have been
>included in recent discussions here on the Amps forum, and have provided
>interesting and informative feedback.
>
...snip...
>Dick Byrd, and others have commented on the downsides of the EBS circuit
>being correctly stated as a crackling or popping sound between syllables
>of speech, and this is indeed a weak point of the standard (i.e., stock)
>Alpha 77 biasing system. Also stated was the fact that it is a
>tri-state system, with:
> 1) the tube cut off at 60V;
> 2) partially on at 20V when the relay (PTT) line is keyed but with
>no RF present from the exciter; and
> 3) final bias of 8.2V which occurs with the presence of about
>200mW of RF from the exciter.
>
>The crackling or popping sound between syllables of speech is heard when
>the tube is switched from partial cutoff at 20V abruptly to 8.2V full-on
>as RF is detected from the exciter.
Dick Byrd has heard it--and so have I. ......The fix is to NOT ALLOW the
RF input to determine the bias. Whenever the RF relays are in the
transmit position, the tube should be biased in the linear region.
Whenever the RF relays are in the receive position, the tube should be
biased to cutoff anode current flow.
Using a modern high-speed T/R amplifier switching circuit, when the
operator stops speaking, the VOX drops and 2mS later the amplifier is in
receive mode. What could be simpler or more logical? ..........Allowing
a Linear amplifier to intermitently switch into nonlinear bias during
transmit is hardly good engineering practice, Mr. Erhorn.
In other words, tri-state bias with bi-state relays was Looneytunes. A
better mousetrap, it wasn't..
Another problem with the stock Alpha 77 is that there is not enough
voltage available to allow the Jennings RJ-1A vacuum relay to promptly
overcome its 0.3H coil inductance The fix is operate the two RF relay
coils in series from a current limited 100v to 150v source. Such a
circuit appears in "The Nearly Perfect Amplifier" in the 1/94 issue of
QST magazine. If you don't have this issue of QST, try Figure 7B at:
http://www.vcnet.com/measures
Vince: It looks to me like the stock Alpha relay supply could be
modified to produce 100V by using a fullwave doubler.
R. L. Measures, [805]386-3734
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