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Folks,
I have just read a post that someone using cascode 6H30 has a great sonic result.
I intend to give it a shot and wonder is cascode could utilize CCS to replace the top position resistor
Looking for help
Follow Ups:
In the Morgan Jones "Valve amplifiers" book there is a chapter on the relative merits (and measured distortion) of many basic circuits, from concertina to cascode and SRPP, with and without CCS.
Omnes feriunt, ultima necat.
!
The Mind has No Firewall~ U.S. Army War College.
""CASCODE" is a contraction of "CASCaded triodes with the gain of a pentode and the low noise of a triODE." It is built up of one triode in current series with another triode. The bottom triode's cathode is grounded and its grid is fed the input signal, while its plate is connected to the cathode of the top tube, whose grid is grounded. This loading of the plate with a cathode prevents the bottom tube's plate voltage from moving very much in response to signal at its grid, as the top tube cathode functions like a voltage regulator made from a cathode follower. Nonetheless, the bottom tube will experience variations in the current flowing from its cathode to its plate, because of variations in the grid to cathode voltage. The same would also hold true if the triode were connected across a regulated power supply, as a varying grid to cathode voltage would also define a varying plate current in this arrangement. In the cascode circuit, this varying current through the bottom tube must also flow through the top tube, as they are in series. And as the top tube's plate is loaded with a plate resistor, the varying current defines a varying voltage across its plate resistor, which yields the output voltage and gain of this circuit, which can be considerable. Thus, "the gain of a pentode.""
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thansk Tre, Very detailed explanation
Just wondering if this cascode would have any mechanism to reduce the overall distortion, once Allen Wright said cascode having lower distortion than common cathode stage(may be he referred to 2 stages)，if yes, how cascode reduce distortion ?
Eric, yes you can.
Here are the results of some Pentode driver stage experiments done awhile back. The idea was to see what could be done combining Pentodes and CCS's. Generally, you can't load a Pentode with a CCS as you are effectively trying to series couple 2 CCS's. This issue is resolved by having a plate resistor in circuit so there is a defined resistance that the Pentode and CCS can work into.
In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain. A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.
To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without "calibrating" the stage on the test bench.
Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.
Having the CCS in circuit allows you to use the MU output to drive the following stage. On the stages tested the MU output impedance was in the 500 ohm range.
Here is the schematic of a driver stage using a 6BQ5 Pentode and CCS combo. Highlights of the circuit performance are gain of 125, output impedance of 500 ohms and distortion of .25% at 90 volts RMS. Not many driver stages can do .25% at 90V RMS! The operating parameters and measurements are on the schematic.
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