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FET Phase inverter to drive valve PA

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Hello,

I haven't been able to find anything by searching the archives or googling, I'm very much a beginner - just after an opinion on a PI circuit I threw together... have mercy...

I'm attempting to put a solid state phase inverter to feed a low power valve output stage.
I'm using low voltage rails, 110V before rectification (130 after?), as I have a trafo that will do that.
Ive used IRF840 mosfets because I have some, I'm not sure if MOSFETs are the right choice...?
I'm using LTP, because its a circuit I think i know.

An externally hosted image should be here but it was not working when we last tested it.


I havent just swapped the fet in as the Vak isnt same as Vgs(on), I tried to bais properly. But I'm not sure, have I done it right?

Thanks in advance.
 
Thanks for your reply DF96, could you recommend a better device that will be happy with the voltages/currents involved?

sorry for my stupidity, thinking about it, I was treating it as a JFET so I was bound to **** up. I havent ever designed with mosfets before.

Is this the correct form for an enhancement mosfet (ignore the values)?

An externally hosted image should be here but it was not working when we last tested it.
 
A differential splitter with MOSFETs is going to be problematic. OTOH, a high gm triode voltage amplifier (like a 6GK5) DC coupled to an enhancement MOSFET "concertina" phase splitter is reasonably straight forward. The IRFBC20 is a suitable FET, as it has a small and reasonably stable reverse transfer capacitance.

Remember, a "concertina" phase splitter is a tricked out voltage follower. As such, the usual capacitance worries don't matter. The reverse transfer capacitance is the crucial parameter, as it is what determines the high freq. limit of the circuitry.
 
I'm attempting to put a solid state phase inverter to feed a low power valve output stage.
I'm using low voltage rails, 110V before rectification (130 after?), as I have a trafo that will do that.
Ive used IRF840 mosfets because I have some, I'm not sure if MOSFETs are the right choice...?
I'm using LTP, because its a circuit I think i know.

Since your voltage is limited, using SS here is definitely a good idea. It would be better, however, to use the MOSFET as a cathodyne splitter. One less hole to drill, it has the drive capability for the finals, doesn't waste a lot of voltage, and can be DC coupled to a preamp.
 

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The small MOSFET I keep in the parts bin for tube interfacing is the ZVN0545A. You might try it.
E-Line, 450V rated, 700 milliWatts.
http://www.datasheetcatalog.org/datasheet/zetexsemiconductors/zvn0545a.pdf
If using a diffamp splitter you could also try the MPSA42 bipolar transistor or if you want to run higher current the MJE340 (both 300V rated). I have seen quite a few Guitar Amps running a bipolar transistor diff amp into push pull output tubes so its been done before. I remember it particularly since I had a guitar amp to repair fro a friend and it had this (bipolar transistor diffamp splitter) circuit. In that amp a leaky coupling capacitor into one side of the diffamp turned out to be the problem.
Cheers,
Ian
 
If using a diffamp splitter you could also try the MPSA42 bipolar transistor or if you want to run higher current the MJE340 (both 300V rated).

If going this route, I'd cascode the BJTs. Done the same with SS designs, and a cascode LTP with BJTs and an active tail load works really well there. Cascoding definitely helps in the linearity dept.

Hollow state cascoded LTPs also offer excellent sonic performance.
 
Hello,

If you plan to use the differential splitter, the difference between both signals at the end depends on the gain -- which shouldn't be a problem with a FET -- and the tail resistance. This is why the long tail inverter is, these days, used with a CCS in the tail.

Love,
greg
 
Just a quick message to thank the last few people who replied: gingertube, MIles, chanmix51, Eli.

For some reason I didnt get any notification emails after my last post, so I missed all of your good advice till now. I wasn't being rude!

I'm going to try a cathodyne splitter, based on the diff circuit on amz mosfet booster page.

I'll have a look into the BJT diff amp too... I'm not sure what the advantages are, but I'll base something on the Deuce circuit, and see what happens!

Thanks again,
 
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