6SN7 LTP and CSS Question

[for_p1]

Hi,

Is it possible to use 10M45S as cathode current source in 6SN7 LTP shown on schematic? From simulation it appears that cathode bias voltage is only about 10V and I don't know for sure if this is enough for depleted mode mosfet.
Another option is a 10mA constant current (limiting current) diode like LM334Z (40V/400mW max, which seems enough). Currently have no data how LM334Z (which was designed for power supplies) will behave in audio circuits.

Thanks in advance for any suggestion(s).View attachment 2677687

To use CSS you will need a negative power supply. 10M45S requires at least 10 volts to enter operational range. In circuit you show grid is at ground level. in that case cathode is only few volts above ground. This is not enough to make 10M45S work. I would say it is not enough to make any CSS work when you consider cathode voltage swing.

 

[TriodeLuvr]

Could one use a CCS and retain the ability to adjust AC balance at the plates with an arrangement like this? Would lose some gain.

View attachment 2679376

The only purpose of the CCS in this application is to create equal and opposite output voltages to drive the output stage. If one intends to unbalance those voltages after the LTP, the CCS serves no purpose. It would be simpler, less expensive and more reliable (due to the lower parts count) to simply use an AC balance pot. This has been a common practice among high-end manufacturers since at least the '60s, and the amps that have this adjustment are not improved through the addition of a CCS in the tail.

Jack

 

[thorpej]

To use CSS you will need a negative power supply. 10M45S requires at least 10 volts to enter operational range. In circuit you show grid is at ground level. in that case cathode is only few volts above ground. This is not enough to make 10M45S work. I would say it is not enough to make any CSS work when you consider cathode voltage swing.

A negative rail or DC-couple the first stage to elevate the LTP grid. I think doing the latter is actually easier to do, and it also eliminates one phase shift in the global NFB loop.

 

[egellings]

If the cathode voltage of the diff pair is too high for the max allowable voltage for the CCS, you can place a resistor between the cathodes and the CCS to absorb some of the voltage drop, sparing the CCS of the full voltage. I am doing that in a home brewed amplifier, and it works like a charm. Since a CCS presents a very high series resistance (infinite for a perfect CCS), who cares about an extra resistor adding a few more ohms to that, unless component count is critical.

 

[egellings]

If both halves of a diff pair are equally driven out of phase, then using a CCS in the diff pair's cathode circuit gets little benefit over a resistor.

 

[thorpej]

If the cathode voltage of the diff pair is too high for the max allowable voltage for the CCS, you can place a resistor between the cathodes and the CCS to absorb some of the voltage drop, sparing the CCS of the full voltage. I am doing that in a home brewed amplifier, and it works like a charm. Since a CCS presents a very high series resistance (infinite for a perfect CCS), who cares about an extra resistor adding a few more ohms to that, unless component count is critical.

FWIW, a part like the IXYS 10M45 is rated for 450V ... obviously you need to consider both the voltage drop and the current, but at least some parts can handle the voltages we're talking about there quite easily.

Unfortunately, there's pretty long lead times on the 10M45 part at the moment, but the 900V-rated 10M90 part seems to have reasonable availability at the moment.