40673 substituion

[Steve G4WCS]

Im having a go at building an active antenna for use with the lowe HF-150. Dusted off my “projects in amateur radio and short wave listening “ and I already have most of the parts hoarded.

Spent an enjoyable morning winding the coils, but I cant find the pack of 40673 mosfets I distinctly remember buying from Birketts years ago.

I have found a BFY90, and a couple of 3SK88’s.

Probably overkill but could I use one of these with changes to the bias arrangements, or would I inadvertently be building a vhf oscillator ?

Also , I assume the dilecon variable cap I have is suitable for use ?

Tia

[Terry_VK5TM]

Cascode two FET's - this is part of a circuit in GQRP Sprat mag last year on exactly your question, substituting for a 40673.

[Cruisin Marine]

from memory a 40673 is exactly the same as a 3N201, dual gate Mosfet

[Synchrodyne]

The 3N187 seems to have been the parent device (in the 12 millmho group) for the 40673.

By inference from the suggested applications, the 40673 was the “full specification” device. Other in the group appears to have been assigned applications according to which way their properties leaned amongst the production scatter.

40820 VHF TV RF amplifier
40821 VHF TV Mixer
40822 FM RF amplifier
40823 FM Mixer
40841 General purpose – probably good for MF and HF, as inferred from its use as the mixer in the Eddystone EB35 Mk III.

The 408xx group came a little later than the 40673. E.g. some FM tuner makers started with a pair of 40673 in the front end, the switched to the 40822/40823 combination (probably cheaper)

For the application at interest, probably any of the above would work. Or any of the slightly earlier lower slope or slightly later higher slope dual-gate mosfets. The 40673 may have been selected simply because it was about the best known of the earlier protected dual-gate mosfets, rather than for its specific properties.


Cheers,

[Nymrod121]

A BF960 will do the job nicely:

https://www.cricklewoodelectronics.com/BF960.html

(no commercial connection etc. etc

Best wishes
Guy

[llama]

Steve, Why don't I just send you a couple of 40673s?
PM your address and I'll get them in the post, no charge.
Unless of course the question is academic and you're exploring options for the future / for the rest of us!
Graham

[Junk Box Nick]

Originally Posted by Cruisin Marine

from memory a 40673 is exactly the same as a 3N201, dual gate Mosfet

I removed the 40673 from the front end of my Liner 2 and replaced it with (I think) a 3N204. 204 rings a bell more than 201 though this was over 40 years ago now so memory may be playing tricks. No change of biasing needed. At the same time I replaced the RF tuned circuits in cans for home-wound airspaced coils with trimmer capacitors.

This arrangement put plenty of life into what was a somewhat deaf receiver. It got even livelier when I put a DJ7VY design preamplifier in the handy space ahead of the RF stage.

(Did plenty of other mods including cleaning up the Tx output and reducing the power output to a couple of watts. Had much enjoyment and worked lots of DX on SSB (and CW!). Oh happy days!)

[Restoration73]

These have worked in the original applications for some of the recent repairs;

https://www.cricklewoodelectronics.com/MFE201.html

[G0HZU_JMR]

A few years ago I measured a BF981 against the datasheet using a VNA. This was a genuine 'new old stock part' from a bag of them purchased >30 years ago. When I measure them the VNA produces a 2 port s-parameter based model. See the results when the model is configured to show admittance etc to match the datasheet. The results were quite good in terms of agreement as shown below.

I may still have some 40673 devices in the loft, I could measure those to show up any differences. I also have a couple of 3SK88 devices although these were pulls from a radio. I'm pretty sure I have 40822 and 3N187 devices somewhere but these are also pulls from old radios.

I have a much better test fixture now so could measure them more accurately.

[G0HZU_JMR]

One of the risks with the high gm parts like the BF981 and probably the 3SK88 is that they can easily go unstable up at 2GHz or so. When this happens it might not be obvious if your test gear can't display this behaviour. However, you would hear lots of hissy noise down at VHF when this happens as the noise floor of the 2GHz oscillator should be higher than thermal noise.

It is possible to protect against this oscillation mode either by including a small amount of series resistance (a tiny chip resistor is ideal here) at the drain or right at the gate 2 pin.

[Steve G4WCS]

thanks @llama I now own a couple of 40673's