[ARC5] [Milsurplus] Transistorized LM

[Bob Camp]

Hi

The “typical” Fetron ran a cascode arrangement of JFET’s to replace a Pentode and often to replace a triode. That helps the reverse isolation more than a bit *if* you can get the gate on the upper JFET well grounded. If that’s not the case, this or that gets done. The circuit suffers in some way as a result.  A cascode is not the same as a pentode in a number of respects. There will always be strange circuits in this or that piece of gear that will not be happy with that approach. 

If you are going a DIY route, is adding a ground here or there “legit”? That depends on what your goals are. The same thing applies to plug in replacements that can be taken back to tubes just by swapping them out. I would agree that some pieces of gear are worth saving in original condition. At least in my opinion, that ham fest “junker" item that’s headed into the dumpster, may well be in a different category. 

Things like gain distribution matter in a receiver. There are a range of other issues as well. Simply replacing the tubes is only the first step. To get the beast to really perform as well as it should, there is indeed a whole set of next steps. Even if it’s a junker, turning it into something that does not work well does not sound like a good use of time. 

Bob


> On Aug 30, 2025, at 11:52 PM, Hubert Miller <Kargo_cult at msn.com> wrote:
> 
> I think the Fetrons main purpose was to replace valves in Telco test equipment. So you're going to see FET-6AK5 but not FET 6BA6 and 6BE6 for example.
> Remember 'tubestors' for the Collins S-Line ? There doesn't seem to be a lot of info about for these. An ongoing subject of curiosity for me.
> I recall a few paragraphs in 'Amateur Radio Techniques' book by Pat Hawker, RSGB, where he told about a reader's conversion of a single conversion Eddystone to solidstate. Said the radio did not even need realignment when done. Of course the main benefit is those RF oscillators stay near one temp, so good bye to most of the drift.
> Years back there was a fellow selling a couple BC-611 talkies on Ebay. This was maybe, good heavens, 10 - 15 years ago. Each
>  ~ $400 and they sold. I copied what i could from various sources later ( thank you ! ) but i have nothing on the transmit side. The 'conversion' used all single gate JFETs and consequently i would not call it real smart. I wrote the seller and quite politely asked for info on the transmitter side. As you know, you do not get 300 mW input power to an AM final using small signal FETs at 12 VDC. So i wondered what he did abt that. Got no answer, so i assume #1 he was tired of the subject, didn't care to deal with my imposition ( altho i did give my ham call also ) or #2 the conversion was similar to receiver, kind of dumb, just very basic and with resultant very low transmit power, but enough to demonstrate it worked, and he realizes  that and prefers not to revisit the subject, or #3 both of above are true, or #4 none of above are true and my guesses miss entirely the mark. My money is on #2. 
> 
> I am VERY interested in transistor replacements for receiver tubes. Make that, !  Of course for single conversion receivers it's a piece of cake, but when you get into double conversion and such you want to be more careful about 'gain distribution'. I have not yet attempted this myself. But i tell you what, i hate being asked to pay $$ big bucks for some Wehrmacht equivalent of a 1T4 or 6SK7, some tube you can get for a couple bucks up, here. I also of course refuse to adapt any radio to use "more modern" tubes. That is really inappropriate in the transistor era.
> 
> I read here about a solidstated R-392. I would REALLY like to know more about that and see the schematic !!
> -Hue Miller
> 
> 
> 
> Sent from my Galaxy
> 
> 
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