At 10:37 AM 3/26/97 -0400, you wrote:
>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.
>
>The original goal of the EBS system, as I understand the history, was
>twofold. First, it is an attempt to increase the dynamic range of the
>users audio by keeping the linear amplifier cut-off during non-speech
>portions of the users transmission, rather than amplifying all of the
>background noise - which is typically blower (white) noise, and
>therefore not desirable. Second, since the tube is cut off during
>non-speech (and non-CW) portions of the cycle, no plate current flows at
>all, and thus the effective duty factor is reduced, and the tube runs
>with less power dissipation. These are the supported benefits of EBS
>systems, and while I agree that these goals may be desirable, they must
>be properly implemented in order to achieve the desired results without
>introducing any negatives along the way.
>
>A better solution to the dynamic range issue might be a simple noise
>gate on the mic input ahead of the mic jack. I have heard a very good
>implementation of this method by W2IHY -?- (or perhaps WB2IHY-someone
>help me out here) that works wonderfully well. The idea here is to
>simply mute the mic input to the exciter during non-speech portions of
>the users transmission. Also, with respect to plate dissipation, the
>8877 has plenty--in a properly cooled application, and thus any real
>benefit here is minimal--unless proper cooling (i.e., air flow) can not
>be achieved.
Vince if you'll recall in one of our telephone conversations, I described
the noise gate used on my rig. This problem with the Electronic Bias
circuit is made worse by background noise in the shack. Actually if you use
alot of processing and your shack is next to the washing machine like mine
is, you should defeat the EBS circuit because it can't do what is designed
to do. By the way, W2IHY's circuit generates more popcorn (we call it the
'snare drum effect' in broadcasting) than the Alpha Electronic Bias Circuit.
>
>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.
My comment was not meant to exploit any downside to the popcorn problem but
was meant to be a simple tutorial on how to make the noise go away. The
solution is to turn a little blue potentiometer on the CB-2 control board
until the tube conducts a little.
No test equipment necessary.
>
>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. I believe the spike or crackling
>that is being heard in this case is distortion caused by operating the
>tube from a non-linear region (20V) to its' linear, full-bias region
>(8.2V). If the threshold of the bias switch is set at the background
>noise level, then this switching will occur routinely (with noise)
>whenever the PTT is keyed, effectively negating the purpose/function of
>the EBS circuit. Allowing the tube to switch into nonlinear bias at any
>time when the RF relays are in the transmit position is not good
>engineering practice.
>
>Given the Alpha design as it is, there are several adjustments that must
>be performed with a monitor scope, and which may alleviate some or all
>of what is being heard. First, use of improperly set up speech
>processing may be responsible for processing background and input mic
>ckt noise if too much processing is used. In almost all cases, this is
>more than enough to provide 200mW of RF which will switch the bias
>transistor (i.e., 20V to 8.2 V) on/off with noise and or between
>speech.
>
>The second adjustment [not provided by Alpha] which would also help is
>an RF detector threshold adjustment from 100mW to about 600mW to better
>control when the EBS bias transistor turns on. This along with proper
>adjustment of mic gain (and/or PROC setup) will preclude unwanted random
>EBS switching of the EBS with noise at the threshold--by moving the
>threshold above the noise level so that it is tripped with speech,
>rather than background noise.
>
>It should be a trivial change to simply disable the EBS, and cause the
>biasing to revert to the traditional on/off system. However, this can
>not be modified without taking into account the QSK sequencing system
>that the EBS is interconnected with. For example, the 77 uses
>pre/post-RF detectors on the input relay which prevents the T/R relays
>from operating if RF is present from the exciter while the amp is in the
>standby mode,
The pre and post detectors are disabled in the standby mode. In the
stand-by mode there is no 'operate' voltage present on the QSK board......no
operate voltage means 'no operate'!
>I will post shortly how to disable the EBS while retaining all of the
>QSK, keying and hot-switching protection already inherent in the 77
>design.
All you need to do to 'disable' the Electronic bias is to keep turning the
pot till the tubes conduct at their normal 8.2 volts. It ain't that
complicated. If you have an older Alpha which doesn't have a pot, you may
want to install one or play around with some fixed resistors....again, no
test equipment necessary.
>
>Hope this has been helpful.
>
>Vincent Fedele
>
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