10M45S problem

[scott17]

My first attempt using the 10M45S as a CCS.
The driver is the pentode section of a 6GH8A running at 3.5mA using LED cathode bias with a 100R resistor to ground and gNFB applied at the junction of the two.

The circuit works fine with a resistor in the anode instead of the CCS. When I switch in the CCS in place of the anode resistor I'm getting some sort of an extreme low frequency feedback or something on the output of the amp.

Before I ran a source through the amp, I had dummy loads connected to the output. I checked the voltages and current with the CCS in place and all measured normal. Correct voltage drop, correct current. I am using a 1K stopper on the gate as suggested by tubelab.

Has anyone had this problem or know where I should start looking?

Thanks,

Scott

[Tony]

motorboating? check your psu decoupling caps, earthing lay-outs....a scheme we can look at will help....

FWIW, mosfets are better used at currents of more than 10mA, cascode bjt's are better at currents less than 10mA.....this is what i gathered from postings here although i have yet to try it myself....

[scott17]

Thanks Tony.

Not sure I would call it motorboating. It's more a very loud low frequency howl. The circuit does work fine with a resistor in the anode instead of the CCS.

I'll come up with a schematic eventually, as most of it is on scratch paper.

Scott

[Miles Prower]

Can't load a pentode with a CCS. Pentodes are also CCS's, and the one will fight the other, causing the oscillation you experienced. If you're gonna do that, you need to parallel a CCS with a plate resistor.

[iko]

Turn it into a gyrator, that should work, no?

[cotdt]

Have you tried a higher current?

[scott17]

Well thanks Miles. That would explain it. Does this also hold true if the pentode is wired as a triode? The pentode has a 510R resistor from the plate to the screen.

If I wanted to parallel a CCS with the plate resistor, how do I determine the current sharing between the two devices? Is this method discussed somewhere?

Thanks,

Scott

[cotdt]

If triode-strapped, it should work fine. I've tried it with a lot of different tubes.

[scott17]

Originally Posted by cotdt

If triode-strapped, it should work fine. I've tried it with a lot of different tubes.

The 10M45 data sheet indicates that it will work down to 2mA. I think I will try a higher current and see what happens.

Thanks.

[cotdt]

Originally Posted by scott17

The 10M45 data sheet indicates that it will work down to 2mA. I think I will try a higher current and see what happens.

Thanks.

I've only used it down to 8mA myself, so I don't know how it works at lower currents. Hence my recommendation to try a higher current.