General Tube to FET - FET To Tube - Question / Clarification

[finkfloyd]

First off am I allowed to speak of this, Is it some sort of secret~?

So I was about to start to build a tubedriver earlier today (previously posted here) : http://www.diystompboxes.com/smfforum/index.php?topic=43915.0
    But having no tubesocket, I got impatient and tryed to bend the pins gently into a piece of perf board to mount it and broke the tube by accident!
    So I will have to buy another tube, and make sure not to be a "scrooge" and buy a socket!

And looking at a topic regarding a tube in a Distortion + got me thinking and inspired me, about instead substituting the broken tube for 2 FETS and a 100Kish trimpot/resistor for biasing purposes.

Is it really as simple as this? Do i just hookup like the below diagram omitting the heater pins?

And if I was to substitute a FET based circuit for a Tube, say running at 9 or 12volts, would I just make sure the heater pins were connected to + power? or ground?



Thanks.

[Ge_Whiz]

In principle, yes, but check what voltage you were intending to put on the anode - don't exceed the FET's rating. Leave the heater connections unconnected.

In the earlier 1970s, there was an attempt to introduce 'FETrons' - FET or multiple FETs in metal cans with valve bases, intended as plug-in replacements for popular valve types. They never caught on.

[brad]

I remember Joe Davisson having a pedal called the "Dual Mos" that kinda looked like a Tubedriver with fets.  You may be able to find a schemo somewhere.

[finkfloyd]

Hi, Thanks for the information!, Ill try the tubedriver tommorow, with fets.


But 1 un-anwsered question  

If I was to substitute a FET based circuit for a Tube, say running at 9 or 12volts, would I just make sure the heater pins were connected to + power? and/or ground?

Surely Tubes can also replace FETS too

[brad]

You'd have to connect the heaters to the voltage specified for them in the the tube's datasheet (i.e. 12.6v or 6.3v).  I doubt you'd get the same performance as fets using tubes at low voltages, because the 12AX7 for example works best at voltages of 100+.  You could always try Space Charge tubes, but using them for overdrive in stompboxes is pretty much uncharted territory AFAIK.  Even then you'd probably be looking at using voltages near their 30v max operational range.

[finkfloyd]

After sleeping and thinking about the above, Wouldnt a "tubed" phase 45 replaced by 1 tube be possible?

I mean if a tube is like 2 FETS, whats the chances of them being perfectly matched? Does this idea sound ridiculous?

The theory & principles are there?

[rockgardenlove]

^Thats a cool idea...

I want a tube phase 45

[cd]

Repeat after me: a tube does not sound like a FET.  A FET does not sound like a tube.

If you want a tube sound, use a tube.  If you want a "something else" sound, use something else.

[finkfloyd]

Hi, I know a tube does not sound like a fet, hence the reason for swapping them for that tubesound or "something else" sound  , but scientifically & theoretically pinout wise it is. (like a FET)

And I will try it and see, but I dont know the chances of it actually phasing using 1 tube though, maybe someone else here has tried something similar!

[JimRayden]

This situation is voltage-dependant. At 9-12V, even a FET sounds more tube'ish than the tube itself.

The problem with Tube Driver is that most of the clipping occurs in the first two opamp stages. Tube is there to just "smooth" the sound, I think it's just market hype and tube is in the signal chain just to raise the price of the pedal. Just like most of the today's production pedals, only today they're smart enough to put the tube glowing through a little cute display window. Awww....

But I find the FET-replacement an interesting experiment nonetheless.

----------
Jimbo

[R.G.]

I mean if a tube is like 2 FETS, whats the chances of them being perfectly matched? Does this idea sound ridiculous?

There was a whole lot of work on that back in the Golden Age, because diffamp inputs were valued then, as now. They are however just two mechanical devices that share the same environment. What's the chances of them being perfectly matched?

Zero.

They are similar because they were made on the same line by the same techinques by the same machines and people, but the two halves of a duotriode are inherently different devices. They do not even share the similarity of same-wafer matching that parts on an IC do.

But I find the FET-replacement an interesting experiment nonetheless.

It is, very much. Each device has its own sound.

But, &deity. please deliver me from the neverending stream of AHAH! new ideas consisting of replacing triodes with JFETs in quest of the sound of tubes.

JFETs are not tubes. The do not sound "just like tubes", as you know. They sound just like... JFETs.

but scientifically & theoretically pinout wise it is. (like a FET)

It is similar only to the extent that it is a depletion mode biased device. Tubes are very much more like depletion mode MOSFETs (which do exist!) than JFETs. Every possible depletion mode device can be biased in the same manner as a tube.

Other than that, it is just a three terminal device. One of the sophmore EE classes used to make you think in terms of generic three terminal devices, making note of the similarities of plate=collector=drain, grid=base=gate, and cathode=emitter=source.

[finkfloyd]

Hello

Its very early in the morning here and im breaking a personal rule of no posting whilst drinking but its OK im not really drunk, just mellow.

Anyway, Thanks very much for the clarifaction, and "CD" I hope I didnt sound rude, I was Just pointing out what I know so far.

Thanks everyone,

But 1 more question , For you especially Mr RG Keen;) , To me it seems obvious that 2 matching tubes do and can exist, so instead of my previous idea of intregrating 1 tube to 2 fets in a phaser , ready, would using one side of a tube to replace a FET, And using the side of another tube, which is "matched" to the other "tubes side"  to make a phaser, so in principle we are using 2 tubes , which are both matched?

You must admit a phaser running with that glassy tube sound is gonna sound good, I can hear it in my head now!, I know its a dream and imaginary but it most probably works , and is worth the experimentation surely.

I hope I make sense!

[brad]

Don't worry about trying to find "tube sound".  The outcome of creating something new and different is more important than how it sounds.  Even if it sounds like garbage...there is someone out there who will think it sounds like gold!