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Driver vs Power tube

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Tubeguy777

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Joined 2014
#1
I am learning as much as I can about tube amps, and Im wondering what determines whether a tube can be a driver or power tube? For example, I came across some old tubes from 1940s Rl 15T 12. It's a triode "transmitting tube". I also read that it has been used in low power Monoblocks. If one knows the characteristics of a given tube, how does that determine the necessary driver tube?
 
#2
The characteristics of an output tube determines the condition that the driver stage must fulfil. Two basic situations exist.
In most typical case the output tube requires only signal voltage drive, not current, i.e. no power is needed.
In some case of transmitting triode like 811A it requires also grid current to be driven, i.e. power is needed.
These two cases are very different and the latter is more demanding for driver stage.
 
#3
Just a thought,

Stupid question..but I'll ask it.😀

What are the thoughts regards the following:

If we have a tube concertina type phase splitter and replace the tube with a FET how does that effect the voltage dropped across the device.

Ie voltage swing out to the power tubes, if both have the same supply voltage and headroom for the swing?

Eg if it was possible to have no voltage dropped across the PI device (set mid point of supply)how would that effect the voltage swing to drive the tubes?


Regards
M. Gregg
 
#5
M Gregg said:
Just a thought,

Stupid question..but I'll ask it.😀

What are the thoughts regards the following:

If we have a tube concertina type phase splitter and replace the tube with a FET how does that effect the voltage dropped across the device.

Ie voltage swing out to the power tubes, if both have the same supply voltage and headroom for the swing?

Eg if it was possible to have no voltage dropped across the PI device (set mid point of supply)how would that effect the voltage swing to drive the tubes?


Regards
M. Gregg
Click to expand...

It would make a very bad concertina PI. It will only do it's job when the gate goes more negative. If there's no voltage drop across the PI device, so the output's operating point would be the same, as soon as the anode output goes down and cathode output goes up (grid/gate voltage increases) they clip. They can't 'cross into each others realm' so to speak. The voltage drop across the PI sets the headroom for this part of the input signal. You can increase the drop across the MOSFET, but that would defeat the purpose of the question.
 
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#7
M Gregg said:
Yes 😀,

I was thinking about the voltage drop across the device limiting the output swing. It would have to be balanced.

Regards
M. Gregg
Click to expand...

Exactly! Most concertina PI's have 1/4 HT across anode and cathode load. The other half is across the tube to set the headroom.

But like I said. It can be done with a MOSFET. If you bias the gate asymmetrically, you get a very linear, low output impedance 'modern' concertina.
 
#9
M Gregg said:
Do you have any links to working examples?
Click to expand...

If you DC couple to the previous stage then there's nothing to it; just replace the old cathodyne triode with a MOSFET. For maximum swing, the gate should rest at about 1/4 HT (same as for a pentode when you think about it). (For a triode cathodyne the grid should rest at about 1/6 HT for maximum swing, although with a triode it is more dependent on load).
 
#10
I've got this quick-n-dirty LTSpice simulation. I've not done the math or optimize the values otherwise, but it's a point I'd be comfortable to start experimenting with. Output impedance is around 12 ohm, but could be improved I'm sure.
Thanks for the idea 😀!

mosfetpi.jpg
 
#11
Merlinb said:
If you DC couple to the previous stage then there's nothing to it; just replace the old cathodyne triode with a MOSFET. For maximum swing, the gate should rest at about 1/4 HT (same as for a pentode when you think about it). (For a triode cathodyne the grid should rest at about 1/6 HT for maximum swing, although with a triode it is more dependent on load).
Click to expand...

The gate should be below(!) 1/2 HT to get any real voltage swing from the MOSFET PI. About half the maximum peak voltage you'll expect at the output. So if you want peaks of 10V, the DC bias should be 95V or less (if HT is 200V like in my spice example).

EDIT: Sorry Merlin, I didn't read. I thought you stated 1/2 HT instead of 1/4 HT. Still, my comment gives a ballpark for the voltage the previous stage can be at for safe operating points.
 
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#17
Merlinb said:
If only there was a fairly simple phase inverter that had good inherent balance, could deliver more output swing than the cathodyne, but had less gain than a long-tailed pair (maybe unity gain) so you don't end up with excessive open-loop gain in a gNFB design. *sigh*
Click to expand...


Merlin, contact member Jeff Yourison. I'm assisting him on a build that uses an IFRBC20 MOSFET, as a DC coupled "concertina" phase splitter.

A FET can swing closer to the B+ rail voltage than a triode can. That fact may be important to you. Please remember that a "concertina" phase splitter is a tricked out voltage follower and a small insertion loss is inevitable.
 
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