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Untitled Document From: Date: Subject: 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: Date: 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: Date: Subject: 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 Untitled Document From: Date: Subject: 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: Date: Subject: Shea ([email protected]) 10/10/2001 7:31 PM With low output impedance? ? From: Date: Subject: andy fuchs ([email protected]) 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