Dick Byrd wrote:
>
> 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.
Hello Dick: I think we are saying the same thing here, which is that a
noise gate is probably preferred to the EBS since noise causes the
unwanted EBS switching--as both of us have described. I was not
suggesting to use the noise gate in addition to, but rather - instead of
- the EBS circuit. I am pointing out, however, that with un-processed
audio AND with an EBS threshold adjustment on the input RF detector, the
EBS circuit works just fine as I have used that setup successfully here
for 8 yrs.//
> >
> >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.
Dick Byrd wrote:
> 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.
Dick: My comment WAS meant to mention the downside of the EBS switching.
I thought that was the point and original question that started this
thread. Didn't mean to be speaking for you personally. I am not sure how
one would tell if the EBS circuit is switching on/off with background
noise without a scope or fast DMM on the EBS bias switch? In my
experience [with serial number 3419], the adjustment you mention
provides almost no idle current at minimum resistance, reading only 20mA
on the plate meter. In my 77dx, there is a fixed resistor in series with
the pot you mention that prevents more than about 20mA of plate current
at idle. Different tubes will conduct slightly differently with a fixed
adjustment, but in my 77dx, the tube was still cut off and at 20Vdc
cathode bias, and thus certainly not in a linear region of operation.
There was no change in the preceived level of the 'popcorn crackle' at
the receive end. I do agree with you Dick that the EBS is probably best
defeated, as long as it's done in such a way as to not affect the keying
and t/r switching.//
>
> >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, and also will prevent the amp from dropping from operate
> >to stby mode if RF is present and the T/R line is dropped. These analog
> >circuits are used to create the correct ?logic? to protect the relays
> >from hot-switching and they work very well.
Dick Byrd wrote:
> 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'!
Dick: You're right, I should have been more specific with notation. If,
while in 'stby' mode, the exciter is keyed and then the user operates
the SSB/operate mode while RF is present from the exciter, the pre/post
sensors prevent the amp from operating until RF is removed. The same
ckts will prevent the amp from dropping out of 'operate mode' to 'stby'
mode if RF is present and the PTT line is dropped. Try it, you'll see it
works. I like this part of the Alpha design, as it protects the t/r
relays from hot-switching.//
>
> >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.
Dick Byrd wrote:
> 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.
> >
Dick: There's probably no way you could know, but your 77 is not the
same as mine. I have a newer (circa 1988/89) 77dx that appears to be
different than what you're describing. The only time 8.2Vdc is measured
at the cathode is with the EBS transistor ON. In the schematics I have
reviewed from 1976 - 1988, no two are the same.
As is, there is no way to adjust the pot to more than about 20mA of idle
plate current--on my 77dx because the trim pot you mention (R208 = 2.5K)
is in series with R229 (470 ohms). At R208 min, there is still 470 ohms
in ckt, and only 20mA max plate is reached. If R229 is shorted AND the
trim pot is set to 0 ohms, then the bias diode (D206) anode will be
pulled to gnd whenever the T/R relays (PTT) operates, thus shorting the
EBS transistor collector/emitter to GND. This disables the EBS, but the
timing must be controlled such that the T/R relays have both engaged and
stopped bouncing first, BEFORE the tube is biased ON. There is a "T/R
closed detect" ckt that may be used for this purpose, and thus I repeat
my previous comment: 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
sequencing system that the EBS is interconnected with.//
Peace,
Vince
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Sponsored by: Akorn Access, Inc. & N4VJ / K4AAA
|