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Adam Alpern
Page 1 of 3
([email protected])
10/10/2001 3:55 PM
Info/Opinions on cascode input stages?
Greetings all,
I was wondering if anyone here can offer any information or opinions about using cascode triodes
as the input stage of a guitar amp? I ask because I'm starting another amp which initially was
going to use an EF86 input stage. While searching the web for EF86 info, I found a small note
about cascode v. pentode stages in Randall Aiken's page about resistor types.
If very high gain is needed, the self-biased cascode stage is the preferred method, as it
has the same or higher gain than a pentode, but no extra noise. In addition, the
cascode connection does not suffer from the high microphonics of the most common
pentode, the EF86. The self-biased cascode approach is better than the fixed biasing of
the upper tube grid because it eliminates the direct influence of the DC supply on the
upper tube grid, and has a warmer tone.
I know that Ken Gilbert has used a cascode 6N1P in his preamp. Anyway, I'm just curious about
trying different or less-common circuits to see what kind of results they yield.
I was also wondering if there was any info available on the net on general theory of operation of
cascode stages - how to calculate total voltage gain, etc... - I really haven't been able to find
anything.
Thanks!
-Adam
From:
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Subject:
KB
([email protected])
10/10/2001 5:55 PM
Re: Info/Opinions on cascode input stages?
Try this: A= 1/ (1/gmRL + 1/mu). I would also make RL much larger than rp to keep the distortion
down.,
KB
From:
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Ray Ivers
([email protected])
10/10/2001 6:27 PM
Re: Info/Opinions on cascode input stages?
Adam,
This schematic shows both cascode types mentioned by Randall; the first stage is fixed-bias, and
the second stage is self-biased.
Ray Ivers
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=144622
... 10/10/2001
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Page 2 of 3
Adam Alpern
([email protected])
10/10/2001 6:32 PM
Re: Info/Opinions on cascode input stages?
Wow, that's a lot of gain :)
Thanks!
From:
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Shea
([email protected])
10/10/2001 7:31 PM
With low output impedance?
?
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Subject:
andy fuchs
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10/10/2001 9:06 PM
Re: With low output impedance?
I was waiting to see who'd point this out. Cascodes have notoriously high output impedances, and
can't drive much. A tube cathode follower will fix this. I'd avoid a mosfet follower, as the input
capcitance of the FET might kill some high end. The other fault of cascodes is that they will amplify
the effects of cable capacitance presented to the input.
From:
Date:
Subject:
Ray Ivers
([email protected])
10/10/2001 9:43 PM
Re: With low output impedance?
Andy, Shea,
A conventional cascode circuit with the output taken from the plate has a very high output
impedance, as you said, Andy (in this case, TubeCAD says 52K, with a gain of about 80). The
second cascode stage (V2a/b) in the schematic uses the 'mu follower' connection, with the output
taken from the cathode of the top tube. This is a lower-impedance output (14K), but lacks some of
the gain of the conventional cascode circuit of V1a/b (gain of about 64). I'd think the V2 circuit
would have a much wider range of applications.
Ray
From:
Date:
Subject:
Ray Ivers
([email protected])
10/10/2001 10:14 PM
Mistake!
Sorry guys,
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=144622
... 10/10/2001
Untitled Document
Page 3 of 3
I thought that V2 was a 12AX7 like V1 - it's actually a 12AU7 on the schematic. The Zout/mu
values I gave for V2 were for a 12AX7 in that circuit; the correct 12AU7 values are 2.3K Zout with
a gain of about 11.5. FWIW, if you use a 12AX7 in the V2 circuit and change R12 to 6.8K, TubeCAD
says you can expect a gain of 84 and an output impedance of about 5K ohms, which ain't too
shabby IMO.
Ray
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=144622
... 10/10/2001