[HBR] Re: GB> RSGB One-Tube Keyer: UPDATE

[EP Swynar]

Hi Again Walt,

Gee whiz, but I sure wish that the HANDBOOK / RSGB write-ups as to the
theory of operation of this thing were even HALF as well-written as what you
so graciously e-mailed me...! Many thanks for taking the time to share your
knowledge & wisdom with me in this way...

I've printed-out a hardcopy of your note, and shall retain it in my
reference file (I have one for each of my HB creations here).

Now I guess I'll have to find a push-pull audio output transformer some
place, & try that...

Thanks again, Walt --- it's MUCH appreciated here!

~73~ Eddy VE3CUI - VE3XZ

**********************************************


----- Original Message ----- 
From: "Walt Hutchens" <waltah at earthlink.net>
To: <hbr at mailman.qth.net>
Sent: Wednesday, May 21, 2008 1:34 PM
Subject: Re: [HBR] Re: GB> RSGB One-Tube Keyer: UPDATE


> http://www.flickr.com/photos/15888272@N05/2415370027/
>
> I had never seen this circuit and found it a bit confusing, but it's
> actually not complicated.
>
> V1a is a blocking oscillator. Initially there's no current flowing in
> the tube and the cathode and grid are at the same voltage. When the
> paddle switch is closed, the tube conducts. The cathode and grid
> initially remain at the same voltage due to the 0.22 mfd cap across
> the cathode resistor so the plate current is the zero bias current for
> the plate resistor (set by the RATIO control) and plate voltage --
> something around 150 volts.
>
> The plate current begins to charge the 0.22 mfd cap and also flows in
> T1, applying a positive voltage to the grid, relative to the cathode.
> Brad's comment that the transformer wiring should checked is the first
> thing that came to my mind as well.   I'd also check the resistance of
> the various windings.
>
> In theory, the positive voltage applied to the grid holds the tube in
> saturation until a substantial voltage has built up across the cathode
> resistor.  At that point, the current flowing in the tube (and hence
> the transformer) decreases and that causes the transformer to push the
> grid negative.  That causes even less current to flow, and the tube
> shuts off regeneratively.
>
> The voltage across the 0.22 mfd cap then decays through the SPEED
> control and series resistor until the tube conducts again and the
> cycle repeats.
>
> I can see one possible fly in this ointment and it is the reason for
> the "In theory ...," above. The time constant of the combination of
> the weight control in the plate circuit and the 0.22 mfd cap would be
> about that of (say) 100k and 0.22 mfd -- that is, about 1/50th of a
> second and that's ignoring the plate resistance of the tube. The
> response of the usual small output transformer at 50 CPS or less is
> very low. Thus, the transformer will not deliver much voltage to the
> grid. If there's not enough grid voltage the 0.22 mfd cap will simply
> charge up until the tube current equals that flowing through the speed
> control and series resistor. Then everything will stop until the
> paddle is released.
>
> You could try turning the RATIO control to near one end and pushing
> the paddle that way. If it oscillates then, the transformer low
> frequency response an issue.  But the 12AT7 plate resistance is most
> of 100k, so a negative result doesn't eliminate this possibility.
>
> One other thing I'd try is sticking a 470k resistor or so in series
> with the connection from the transformer to the tube grid. That
> eliminates the grid-cathode 'diode' as a load on the transformer,
> meaning more energy stored (in the tx) and a better negative pulse
> when the tube current starts to fall.
>
> I would check the various resistances in the circuit and substitute a
> couple of other 12AT7's -- even a slight heater-cathode short or bit
> of gas in the tube will cause a problem, and those tubes often have
> 'em. 12AT7A's are most likely designed for series string operation
> (most 'A' versions are) so they would be a better bet, but by no means
> guaranteed.
>
> Even the most minor transformer short would be fatal. Another useful
> check would be hooking in another one with clip leads. Trying a better
> quality transformer (better low frequency response) would help
> eliminate that issue.
>
> Heater-cathode issues are probably the reason for the use of a
> negative supply rather than a positive one -- if the cathode was
> positive relative to a grounded heater, you'd have a forward biased
> diode across the cathode circuit, as well.
>
> When transformer, tube, wiring, and parts value issues are eliminated,
> I'd do voltage checks to get a better idea what's happening. If the
> cathode does come up to some substantial voltage (like 50 volts or so)
> when the key is closed and then decays when it is released, then the
> problem is not enough feedback and the only question is 'why?'
>
> I wonder if there could be an error in the diagram. But other than not
> having a resistor in the blocking oscillator grid, I can't see
> anything obvious.
>
> Walt
> KJ4KV
>
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