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Old 16th April 2014, 02:23 AM   #1
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Default Driver vs Power tube

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?
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Old 16th April 2014, 07:29 AM   #2
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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.
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Old 16th April 2014, 07:54 AM   #3
M Gregg is offline M Gregg  United Kingdom
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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?


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Old 16th April 2014, 08:55 AM   #4
M Gregg is offline M Gregg  United Kingdom
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Just for interest,
I guess it should say new old phase splitter.

New Phase Splitter

And part of the answer is are you driving a triode or a pentode power tube.

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Last edited by M Gregg; 16th April 2014 at 09:04 AM.
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Old 16th April 2014, 09:18 AM   #5
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Quote:
Originally Posted by M Gregg View Post
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
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.

Last edited by funk1980; 16th April 2014 at 09:23 AM.
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Old 16th April 2014, 09:29 AM   #6
M Gregg is offline M Gregg  United Kingdom
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Yes ,

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

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Old 16th April 2014, 09:31 AM   #7
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Quote:
Originally Posted by M Gregg View Post
Yes ,

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

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M. Gregg
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.
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Old 16th April 2014, 09:34 AM   #8
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by funk1980 View Post
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.
Do you have any links to working examples?

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Old 16th April 2014, 09:43 AM   #9
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by M Gregg View Post
Do you have any links to working examples?
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).
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Old 16th April 2014, 09:43 AM   #10
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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
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