[GreenKeys] A little help

[Duncan Brown]

Tom,

I built a "filterless" TU once. It had 4 tubes (no OA2) and might have 
been similar. I think there were three dual triodes and a power tube to 
drive the loop.  The principle was to send the audio tones through a 
limiter and then through a small value (high reactance at audio) 
capacitor - maybe about 150 pf (?).  The two tones will come out at 
different levels due to the reactance of the capacitor. The two audio 
levels are detected and then amplified to drive the loop.

Worked well with strong signals without QRM. Seems like it didn't have 
enough gain for 170 hz shift, but this was back around 1968. I'll look 
around to see if I have any documentation on it.

Have fun,

Duncan
K2OEQ

On 9/16/2019 10:22, til128 at juno.com wrote:
> Hi all:
> I am looking for some help in identifying a TU that I recently acquired
> with no paperwork. :-(
> It was built by Jack K2TTY Hart. (SK)
> It is labeled "Filterless TU"
> It has a Bias adjustment and a knob on the front labeled "shift".
> There is a loop circuit at 135v at 60 mA.
> There are 5 tubes. See layout attached.
> I have tried with multiple tones to no avail.
> I have searched RTTY magazine.
> My worst fear is that it wasn't completed as the 12AT7 (center)
> and the 12BH7 (Bottom ctr) tubes are not getting warm.
> It is a very well built item and I would love to use it.
> Thanks for your kind attention and help identifying this.
> BTW - bottom left tube is "OA2"
> Tom WB2TTY Tillson
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