[ARC5] Transistorized LM

[Tom Lee]

Hi Hue,

Yes, the initial target market for FETRONs may have been fairly narrow 
(and it didn't seem to catch on in any big way, apparently), but the 
basic FETRON circuit is broadly applicable as long as only the control 
grid is being driven with a signal. Basically, if all you need is a 
low-capacitance, high plate resistance tube, the FETRON connection does 
the job nicely. Years ago I converted an old Heathkit IG-102 sig gen to 
an all-FET design. Only one subcircuit needed the full FETRON 
configuration -- the pentode half of a 6AN8. A single JFET replacing 
each tube or tube section sufficed for the rest. I was happy that the 
dial calibration was hardly affected.

If you wanted to emulate a pentode more fully (say, a mixer where the 
screen or suppressor grid is driven in addition to the control grid), I 
imagine that you could choose the bias of the upper transistor in a JFET 
cascode and do a decent job, for some generously loose definition of 
"decent". :) After all, a dual-gate FET mixer is essentially just that.

Thanks very much for mentioning tubestors. I'd never heard the term 
before, so thanks (as usual) for the education! Now I've got a new term 
to google!

-- Cheers,
Tom

On 8/30/2025 8:52 PM, Hubert Miller 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.