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?
SY?
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?
SY?
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.
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.
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.
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.
Attachments
burnedfingers said:
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.
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.
Local feedback with minimum phase shift could and should
have been utilized to linearize that first cascode stage.
620R cathode resistors serve to provide bias, but totally
insufficient by themselves to assure any sort of linearity.
Rather than allow gross non-linearity to further propagate.
Phase shift the spectra of distortion products with reactive
coupling. Distort and shift again. Then hope that a strong
global negative feedback is going to fix everything. Yeah,
right...
Also perfect example of a circuit with the poor PSRR that
others have been talking about. The phase splitter that
follows is not going to appreciate hearing any grid noise
common to B+.
have been utilized to linearize that first cascode stage.
620R cathode resistors serve to provide bias, but totally
insufficient by themselves to assure any sort of linearity.
Rather than allow gross non-linearity to further propagate.
Phase shift the spectra of distortion products with reactive
coupling. Distort and shift again. Then hope that a strong
global negative feedback is going to fix everything. Yeah,
right...
Also perfect example of a circuit with the poor PSRR that
others have been talking about. The phase splitter that
follows is not going to appreciate hearing any grid noise
common to B+.
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