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.

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.

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.

Here is a picture of the Front End as is and a picture with the RF valves removed, but keeping (for now) the valve oscillator and mixer.

http://homepage.ntlworld.com/richard...RF%20Unit.html

Section 1 includes drum pins 1-5, section 2 pins 6-10, section 3 pins 11-15 and oscillator pins 16-20.

Need modern transistor circuit for the two RF stages.

EDIT1: Transistors: I suppose were looking at a modern FET, that is as good as a valve WRT cross modulation/overloading. Do such "angels" exist? I do not suppose we would be using 2N3819 these days? Have we moved on? :c)

Would remove: C101, C141, C142, C143.

G'day,

I have modified valve receivers in the past by using dual gate
mosfets. In my parts drawer now, I still have a couple of MFE131s,
but any dual gate mosfet should work OK although now they are usually
only available in surface mount.

I also used single gate juunction fets for the oscillator and BFO.
The ones I have left are MPF102.

Fets work OK in valve receivers as RF ampliiers, but not well as IF
amplifiers where the primary of the IF tramsformer is shunted by the
low output impedance of the fet.

Your B40 circuit looks a little tricky to use. Probably a resitive
load for the second RF amplifier would be OK (about 1K?) and I think
some experimenting with the oscillator circuit would be necessary to
find a satifactory arrangement.

Good luck. I reckon you'll be busy for a while!

73, Ric, VK7RO.

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.

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.

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.




--
richard8123

HeY OT:

I would use cascode mosfets 'cause that's the only way to eliminate the Miller effect they used to make valve tuners that cascoded triodes then they invented valves that could get around the miller effect so no more cascode triodes
IMO you could use matched GaS fets on 7pin or 9 pin valve plugs then in that circuit your going to have to transform the low output impedance of the fets to the high impedance out of the valves with a small toroid transformer
and bias the gates properly

73 de n8zu

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