On Wed, 13 Mar 2013 16:20:23 +0000, richard8123
wrote:


I have a turret drum that came from an old boatanchor (Murphy B40D).

Here is the front-end schematic:

http://www.portabletubes.co.uk/boats/schem1.jpg

I want to use this drum, but I want to employ transistors not valves.

Now today, I presume we can select a transistor to replace a valve, and
it will be as good as, if not better than the old valve.

Replacing valves with semiconductors can result in much better performance but
such an operation (optimizing for performance) rarely is a one to one
replacement.


Any recommendations for suitable transistors for the local oscillator
and the rf amplifiers? I'm trying not to have to modify the circuitry
inside the drum.
Thanks.

The two RF amps, the oscillator and the mixer could be replaced with BF981, a
dual gate mosfet that can be handled easily (no SMD). However, with a one to one
replacement, R113 & 114 have to be paralleled with an RF choke, = 10 x the
inductance of the primary of TR103. R117 will have to be changed to a much
smaller value, about 100 ohm. The second gate of the mixer BF981 must be
connected to R120 / C130. No bias required. What has to be changed too is the
supply voltage: for the BF981, Vds =20V.



Edit1: I'm thinking of using the hollow state circuitry after the
Front-End. Best way to maintain the excellent audio of this receiver.

Edit2: I probably should draw out the tuned circuits only that are in
the drum, post a link, and draw comments from that. Hopefully then a
solid-state circuit can be built up.

A better modification, based on the much higher gain from the BF981 as compared
to the RF and mixer valves, would be to omit the 2nd RF stage IOW use TR102 and
TR103 as bandpass filter instead. Instead of a BF981 as mixer, better
performance could be obtained from a Gilbert cell with jfets like (matched) J310
or preferably, single gate small-signal mosfets like the BF987, because of its
low Cgs of 35fF.

Regards,
Jan