Wednesday, 7 March 2012

Sams Valve Radio - Almost

Work started again today on Sams single valve regenerative set. Up to now this was dogged with troubles as it just simply didnt seem to work. Well, i sought advice from the chaps at G-QRP club, always a helpful bunch, and received loads of info back. It seemed the first task was to up the HT voltage.
So, I went out today and bought 10 PP3's from which to build a 90v B+ battery. Well, they only had six on the shelf, but the assistant was stocking the batteries up. She at first didnt think there were any more, but i spotted a box at the bottom of her trolly. A box of ten. So i bought the whole box. The end batteries, where the connections to the pack are, have had shrouded 4mm sockets soldered on, and then the whole lot taped over, so as to prevent any contact with the terminals. As new, it measured 98.8v! The smaller pack is the present heater battery, 6v, although reading a bit low, using 4x AA's. In the future i'll change this for four C or D cells.

This is the view of the front panel, the big knob, which incidentally isnt big enough with a 600pF variable capacitor, is the tuning, and very delicate it is too! The bare shaft is the regeneration control pot. The 6K7G pentode valve can be seen behind the panel. A bandspread capacitor may well have to be fitted.

It can easily be seen that there is very little to the radio underneath! The view from above shows the main components, the big 600pF air spaced variable, the valve, and the audio transformer. The ferrite rod antenna has had its main winding progressively reduced to bring the tuning onto the MW band at a sensible spot and now has only about 55 turns out of the original 80, the smaller winding is the 20t tickler coil.

In its present state, its possible to find a couple of stations during the day. I think the big problems now are that even with the transformer, the audio output into low impedence phones is poor, and the tuning is far, far too coarse.

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