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Old 7th September 2008, 12:31 PM   #1
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Default Pros and cons of Cascoded front ends

I'm not a great tube freak so can anyone tell me the pros and cons of Cascoded tube front ends? So far I have noticed lower gain than some circuits and a tendency for negative waveform clip and lower noise.

I recently purchased and put together a Cascoded 6sn7 tube driver board for a Dynaco. I noticed a lack of life in addition to sounding just so so.

Was this just a **** poor design or is this a common fault to be found with this circuit?

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Old 7th September 2008, 03:19 PM   #2
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Like a Pentode's screen, a cascode stops V1's control
grid from seeing intrinsic negative feedback from the
other plate that drives the actual load.

Like a Pentode, you can run a cascode in any number
of ultralinear configurations. Or feedback directly to
V1's grid as O.H.Shrade (of RCA fame) did in 1938.

You have a lot of options to shape the curves to your
personal liking.
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Old 7th September 2008, 03:57 PM   #3
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Cascode has poor PSSR so usually used with regulated PS.

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Old 7th September 2008, 04:13 PM   #4
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The remark about poor PSRR is SPOT ON.

Cascodes have a very high O/P impedance. The gain of a cascode is approx. = (lower element's gm) (net ZL). The 'SN7 is a superb tube, but its gm is (IMO) only marginally adequate for service in a cascode.
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Old 7th September 2008, 04:38 PM   #5
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PSRR is near perfect, as long as you like the noise
reference common to B+ rather than ground.

Fine if you intend to ultrapath your output stage
cathode back to B+, then helps PSRR that the
drive from the cascode is also common to B+.

-------------------------------------------------------

But thats not the only way to abuse a cascode.
Here we ultralinear a cascode to compute 2xMu,
and then Mu follow. PSRR common to GND.

The parallel 6dj8's merely a scheme to have
intrinsic feedback of V2 correct microphonics
of V1 and vice-versa. Probably A and B triodes
of entirely different envelopes. No attemt to
match.
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Old 7th September 2008, 05:04 PM   #6
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Default Re: Pros and cons of Cascoded front ends

Quote:
Originally posted by burnedfingers
I'm not a great tube freak so can anyone tell me the pros and cons of Cascoded tube front ends? So far I have noticed lower gain than some circuits and a tendency for negative waveform clip and lower noise.
PROS:

* Higher gains.
* Greatly reduced Cmiller

CONS:

* Worse PSRR -- Needs clean DC
* Output swing is thin compared to Vpp
* Hi-Z output -- needs to work into a Hi-Z, Low C load.

Quote:
I recently purchased and put together a Cascoded 6sn7 tube driver board for a Dynaco. I noticed a lack of life in addition to sounding just so so.

Was this just a **** poor design or is this a common fault to be found with this circuit?
Got a schemo of the circuit? Here, we get into all that subjective opinion concerning how the thing sounds. You're not gonna get much consensus on that score, and I'm sure there are others out there who say it sounds just fine, and who'll rave about the improvement it made to their Dynaco's.

The cascoded front end I did was a cascoded LTP voltage amp/phase splitter. Of course, a balanced topology is going to get rid of lots of distortion that a SE won't. The cascoded LTP worked out just great, and made for a great sounding final project.
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Old 7th September 2008, 05:14 PM   #7
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A cascode does not necessarily have to use the same device on top and bottom. You can use a lower Z tube at the base, and feed it a portion of B+ from a resistor to give it the desired op point.

The 6DJ8 is designed with cascode service in mind.
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Old 7th September 2008, 05:24 PM   #8
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Darn skippy those devices don't have to be the same.
Course, the current behavior tends to be dominated
by the lower device, and voltage behavior by the top.
Unless you impose some feedback scheme that sais
otherwise.
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Old 7th September 2008, 05:27 PM   #9
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Quote:
Was this just a **** poor design or is this a common fault to be found with this circuit?
Schematic? Maybe something obvious is there. A cascode can be quite excellent; Joe Curcio's ST70 (for example) is very highly regarded.
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Old 7th September 2008, 08:52 PM   #10
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