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Old 30th October 2012, 04:45 PM   #1
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Default Cascode driver

I will make the bold assumption that we all have Morgan Jones Valve Amplifiers 4th Edition. . OK, lets go to page 101, figure 2.27. Any thoughts on converting this into a differential circuit like the Hedge driver on pg.505 ?

Is it as straight forward as it looks or am I missing something? Also, would it be OK if all the triodes where 6SN7's or should the cascode be a 6922? I need a low distortion driver with low output impedance to drive a high capacitance.
Thanks for the help.
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Old 30th October 2012, 04:54 PM   #2
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A cascode will not have a low output impedance unless you add a couple of cathode or source followers to the output.

6SN7 is not a great choice for a cascode, at least not for the bottom- it will be too heavily loaded by the top tube and sacrifice some linearity and gain. ECC88-oids are much better in that respect.
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Old 30th October 2012, 05:00 PM   #3
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Cacodes are chosen when you want high gain but lower noise than a pentode, high output impedance, or low input capacitance (or you are constructing a VHF receiver). Generally for small signals. Not a driver circuit.
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Old 30th October 2012, 05:01 PM   #4
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Thanks Sy, the circuit in question is a cascode direct coupled to a cathode follower with it tied to the top tube to provide negative feedback to each other. I guess we don't have our MJ4e to refer to . Part 2 of your reply is about what I figured. Thank you
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Old 30th October 2012, 05:29 PM   #5
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Yeah, I need to get a second copy for my office.

I think that if you look at Morgan's driver circuits, you'll notice a distinct lack of cascodes- there's a reason for that. Dave touched on most of those points; a conventional voltage amp will work better in that application (distortion and swing).
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Old 30th October 2012, 06:10 PM   #6
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Hedge, in the June 1956 Wireless World, described using a cascode as a differential amp driver for a pair of 1625 tubes. MJ shows an uncredited design, of a cascode feeding a 6SN7 cathode follower with feedback from the top tube to the CF to provide a low output impedance for driving whatever. Later in his book, He, MJ, describes using the Hedge CF as a driver for big power tubes stating that it meets the need (with the CF output) of low distortion and high voltage swing into high capacitance.

I am confident a cascode is capable of being a fine gain stage in a power amp if you use the right tubes and have enough current. My reason for starting this thread was related to wondering if converting the circuit on pg. 101 to something like the one on pg.505 was as simple as it seems. Or am I missing something.

I think this circuit is not used much mostly because you need to bias the heaters to avoid noise and failure.
I want to use this circuit because I can get more gain than one stage of triodes and less phase shift with fewer capacitors than cascaded triodes, BTW, I don't want to use a pentode and a CF. (Pentode phobia )
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Old 30th October 2012, 06:17 PM   #7
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Bear in mind that a cascode means that the lower valve gets no anode feedback so you don't get 'triode' linearity. That is why I said it is for small signals. A differential cascode will cancel even-order distortion but you still have odd-order.

You have to be careful with some 1950s designs, as back then not everyone realised just how important it was to reduce higher order distortions. They really did believe in THD.

A cascode could, under certain circumstances, make a high gain input stage but not a driver even with a CF on the output.
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Old 30th October 2012, 06:44 PM   #8
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I wish you would explain why. MJ mentions the low anode load on the bottom tube causing a loss of linearity but feels it isn't much of a problem due to the low gain and small signal swing on it. I see nothing anyplace that describes the cascode as only suitable for small signal use, in fact I see the opposite.
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Old 30th October 2012, 07:01 PM   #9
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Quote:
Originally Posted by TUBESMAN
I wish you would explain why.
I did:
Quote:
Originally Posted by DF96
Bear in mind that a cascode means that the lower valve gets no anode feedback so you don't get 'triode' linearity.
Triodes are only as linear as they are because in normal operation they get feedback from the anode voltage. A cascode stops this. It is the price you pay for low input capacitance.

You can use a stage with poor linearity, but only for small signals. By definition, a driver stage handles large signals so distortion is worse.
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Old 30th October 2012, 07:18 PM   #10
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Worse than a pentode? At the same gain and output level? As I said, I see many examples of a cascode being used as a driver, MJ has nothing bad to say about it as a driver when it is used with a CF to lower the output impedance, in fact, he recommends it for driving a high capacitance load with a large voltage swing. What you say is true about the cascode, the question is how big a deal it really is. And I suspect the harmonic structure is more benign than a pentode.
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