Help on triode

[croccodillo]

Hello,

I'm building a SE amp, now THIS is the circuit at the moment running on my workbench, completed with a SS regulated power supply (300+300V with central tap) and a SE, CCS loaded, 6N6P driver stage.
The SS CCS is a cascode mosfet design (my own design, THIS).
The output transformer has a impedance of about 1500Ohm, giving a 15-20W output power per channel.
It sound amazingly, but there's a big problem: the SS CCS draws 100mA, the KT88 output tubes work at about 300V, so each CCS must handle about 30W of continuous heat dissipation.
This means BIG heatsinks.
So, my idea was to use the two KT88 as CCS, in the top of the output stage, in place of the SS CCS; but now I need a triode (or more than one) to replace the removed KT88.
In this way I can have a pure triode amplifier.
Needed characteristics:
- Maximum operating anode voltage: 600V
- Nominal operating current: 100mA
- Maximum operating current: 200mA
- Heat dissipation: 30-40W
- Output impedance: 1500Ohm

Any advice?
Of course I can use more than one, in parallel.

Ciao,
Giovanni

[tubelab.com]

My first thought would be a pair of 300B's in parallel, but this would be expensive even with Shuguang tubes. The typical cheap regulator triodes (6AS7, 6080) would have a hard time with the voltages. A 211 or similar transmitting triode could be used, but these work best at higher voltages.

I would suggest leaving the triode wired KT88's where they are and using sweep tubes in the CCS. I did this experiment back when we first talked about this concept. Your solid state current source dissipates about 30 watts. A tube CCS would dissipate even more power for two reasons. The heater power, and the tube CCS would have a higher compliance voltage since the cathode to plate voltage even in saturation is far higher than a mosfet or BJT. I tested several tube CCS's and found that sweep tubes have the lowest compliance voltage, but this is far higher than a solid state CCS. This would require more B+ voltage, or lower output power. The data is here:

http://www.tubelab.com/CCS%20circuits.htm