Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Twanger on August 16, 2010, 08:28:41 am
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hi All,
I would like to have some input regarding cascode gain stages, i've read that it can turn 2 triode into a ''penthode like'' tube...
From what i gathered so far it seems like ithe best cascode stage would be the self biasing one, i gave it a try yesterday but it sounded muffled... i mught have wired something wrong...
did anybody ever try this ?
my goal would be to overdrive the first stage of my tweed bassman, my chassis as an extra 9 pin socket and I would like to try something different...
if you guys think im heading the wrong way im open to other suggestions
Thanks
John
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Yes they look interesting don't they. The grid of the 'lower' stage is the input grid, and the grid of the 'higher' stage is like the screen grid. Whilst it generally has more gain that a single triode stage, it typically has less overall than if you cascaded the two triodes. Also whilst it has highish gain, it doesn't reproduce the range of harmonics that a pentode produces AFAICT. I understand that some tubes (e.g.; ECC88) make way better cascode stages than others.
FWIW Merlin Blencowe covers cascode stage design in his book. There's also a bit about it on his website.
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Thanks for the heads up on the valve wizard, i re-checked my wiring and indeed i had made an error, all i have to say about it for now is that the self bias cascode sounds interesting, gainy for sure.
For now i used it on my bassman, i skiped v1 and fed the cascode straight to the cathode follower,
How is this circuit intended to be used ? ( to slam the first stage of an amp or as an input stage )
Thanks again
John
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How is this circuit intended to be used ?
For guitar amp use, it is best to think of the cascode as a "pentode-replacement" circuit. And it is easiest to understand what is happening with a cascode if you can draw parallels with a preamp pentode.
Let's say you have 1 triode (perhaps a 12AX7) with a mu of 100, but an actual in-circuit gain of 50 (this is typical).
If you had 2 triodes, and ran them in parallel, the individual gains for each section would add. Say you have a tube that gives an actual gain of 50 per section; you get 50+50=100 for parallel sections.
If you used the 2 sections in cascode, one would be run at very much less than its maximum possible gain, the other pretty near maximum possible gain. The 2 gains would multiply, but because of the section that is delivering very little gain, you get a total gain of around 200-300. Much more than a single triode or parallel triodes, and very much in line with what you might expect from a pentode. If you use a 12AU7 as shown on Merlin's page on the cascode, you would probably get a gain around 60-65, which is pretty good for a tube with a mu of 20 and a typical gain (from 1 section) of 12-16.
When you cascade gain stages, the individual gains multiply. Continuing with our example, a 12AX7 with a gain of 50 for each section yeilds 50*50 = 2500.
So a cascode stage gives a gain well over what is capable from a single triode stage, but much less than what we get from 2 sections in cascade. So you don't see it more often because it yields less overall gain than the simpler circuits we see all the time.
There are also other tube types that are more suited to cascodes than typical guitar amp tubes;, such as the 6DJ8, 6BQ7, etc. They are often listed in tube manuals as "for use in cathode-drive circuits" which refers to the essentially grounded-grid operation of the upper triode in the cascode.