04/08/2017 12:21

Polyphemus 2.0


After some funny time with the first edition of a mixed regenerative/conversion receiver, where a poor ECH83 did the Local Oscillator, the RF Amplifier and the Mixer functions together, all these tasks have now been split among separate stages.

Now a '6AK5' UHF pentode is the RF amplifier, in a Tuned Plate - Tuned Grid configuration; an ECH83/6DS8 still works as Mixer + Local Oscillator (Pierce configuration, at the triode section); a '9002' triode now is the regenerative Detector and last an ECL84/6DX8 as the usual pre+final audio Amplifier.

After some rummage in my junk-box, a nice Yaesu FT-something transceiver Preselector was detected: perfect for a broadband tuneable front-end Amplifier, that allows to tune from about 3.0 up to 30.0 MHz. The Local Oscillator is rock bound on 5 bands, and there is provision for an external Oscillator also. The regenerative Detector section is tuneable from 1.7 up to 3.5 Mhz.

 

 

In comparison to the first edition of this receiver, the RF Preamplifier allows a more comfortable listening and the audio level is now much stronger. It has been observed, besides, that the same signals are now receiveable as what could be received with the former prototype: before, the ECH83 was amplifying a bit the incoming signal, while now there is a separate stage for this function. In the former prototype, then, the audio throttle was most of the time at full steam.

RF Amplifier, Mixer & Local Oscillator are all run at 90VDC, provided by a 'VR90' gas tube stabilizer (the big ceramic resistor on its side becomes pretty hot, as it manages the voltage drop from about 320 VDC from the supply). The audio Amplifier is run at 320 VDC.

The use of the BC221 variable capacitor resulted in a very smooth tuning and practically linear. Some 35 kc/s per turn are a very comfortable way of zero-beating any station!

 

 

The preselector has not yet being optimized, but the unhearting of a 18 MHz crystal in a remote region of my depot, allowed also to listen to many stations on 21 MHz band. At higher frequencies, the L.O. needs some help by additional tuning circuit, to raise the L.O. injection level at more than 10 V peak. Up to 14 Mhz band, the peformance of this receiver resulted more than satisfying and such that it allows to make also DX traffic when needed.

Still, the 'Tuning Eye' tube, configured as audio 'VU Meter', is helpful for tuning purposes. Fluctuations of the green light depends on the Volume regulation, and for peaking the RF Preselector it is just needed to reach the greatest closure of the 'green wings', driven by the background HF noise ...

The interesting thing of the Regenerodyne concept is that you have all the controls brought to the front panel and there is no need to align any fixed-frequency circuit inside, like MF transformers or whatever. Besides, the sensitivity is equal to at least a decent modern transceiver.

I have also been given an electronic audio Filter and a strong selectivity can be added if traffic is to be handled in one of the usually crowded Amateur Radio bands. If Full Spectrum operation is preferred ... Polyphoemus 2.0 is more than an excellent performer.

What else do you need ?


Radioman, April 2017


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