KT88 Usable range

[croccodillo]

Hi all,

I have a doubt:

I'm building and trying a SE parafeed amplifier, the output tube is a triode-connected JJ KT88, driven by an hybrid CCS set at 100mA (another KT88 -in pentode mode- with a cascode mosfet).

I would like to try now a dynamic current source.
This thing is something like the ACS developed by Nelson Pass.
The CCS working mode has to be divided into CC and AC.
With DC (idle) the CCS is a true CCS, sourcing 100mA.
With AC the CCS become a variable current source: higher the load current, higher the current in the CCS, up to 150mA.
Lower the load current, lower the CCS current, down to 50mA.
Give a look to Nelson Pass site, and to his US patent 5710522, or read the Aleph30 manual at his site.

The B+ is of 300+300V (central tap), well regulated.
There's also a -120V for the bias.

The driver is made with a 6N6P, connected to the +300V (half B+), loaded with a CCS (for now set at 20mA).

The final schematic will be something like this (the driver stage will be modified, and LTP will be tested).

The other side of the output transformer will be connected to the power supply enter tap (+300V).

The output transformer has an impedance of 2500 Ohm.

I would like to set the output tube idle working point at 330-340V, that @100mA means 34-35W of heat dissipation.

Which such data, I would like to swing the output tube from a minimum of 130V@150mA to a maximum of 530V@50mA, while the upper tube (CCS) will swing from 60V to 400V.
Considering the output impedance this will weans an output power of about 8W.

My question is:
Is it possible to run the KT88 tube with an anode voltage over 500V?
As told, I would like to run the output tube up to 560V, while the curves reports data only to 500V.
Is the KT88 stable and linear up to, let's say, 550-600V?
Consider also that at at highest voltage the current is limited.

Ciao,
Giovanni

[Eli Duttman]

Is it possible to run the KT88 tube with an anode voltage over 500V?

Yes, it is possible. Dig up a data sheet for the TT21. The "guts" of a TT 21 and a KT88 are the same. The anode in the TT21 is top cap connected, which permits a HIGH B+ rail voltage. You must hold the screen grid voltage down and under no circumstances exceed the dissipation ratings for both the plate and g2.

[Giaime]

Originally posted by Eli Duttman


Yes, it is possible. Dig up a data sheet for the TT21. The "guts" of a TT 21 and a KT88 are the same. The anode in the TT21 is top cap connected, which permits a HIGH B+ rail voltage. You must hold the screen grid voltage down and under no circumstances exceed the dissipation ratings for both the plate and g2.

I'm with Eli. I have an amp here that runs Sovtek 6550 at 575V, with 310V on the screens. Very nice blue glow from the residual gas