From: kg 
Date: 3/2/2002 6:23 PM 
Subject: more solid state heresy 

i was turned on to a cheap P-channel mosfet, the mtp2p50e, made by on semiconductor (thanks, Kurt Strain!). it comes in a to-220ab package, it's rated at 500vdss, and is available from newark electronics for something like 3 bucks each.

i am planning on testing one out in a CCS plate load for my guitar amp input stage. right now the input stage is a sovtek 6c45pi, cathode biased by an orange LED which gives me about 2 volts bias. the plate load resistance is currently 5k. the supply voltage is 330vdc, and the quiescent plate voltage is around 160vdc. the standing plate current is 30mA, and the dynamic plate impedance is about 1k5 ohms.

the constant current plate load will maximize the voltage gain of the stage. in all tube circuits, the tube's internal plate impedance works against the external plate load resistor, forming a voltage divider. in this case, because the external resistance is less than five times greater than the internal plate resistance, the tube will lose some 20% of its gain. the equation is:

Av = (mu * Ra)/(Ra + ra)

where Ra is the external plate load, and ra is the internal plate impedance. 

with my current values in circuit that gives me:

Av = (52*5k) / (5k+1k5)
Av = 260000 / 6500
Av = 40

as the Ra is increased the gain through the tube will increase as well. if Ra could be made really big, like say 2M, then i could get an effective gain of:

Av = (52*2M) / (2M+1k5)
Av = 104000000 / 2001500
Av = 51.97

the gain will reach a limit at the rated mu of the tube, which cannot be exceeded. but in this case, i have picked up a "free" 10x of voltage gain, or 20dB. not too shabby! (note that this quick calculation does not take into account the "downstream" AC shunt load.) i expect that the dynamic impedance of the solid state CCS to be in the 2M range. if a cascoded another mosfet i am sure i could get it into the 10M range, but in this application that's overkill... i'm not going to get much more voltage gain anyway.

now i've been told that these solid state CCS's do not appreciate being driven into cutoff, so for now i will limit its inclusion to the input stage, where i know the operation is completely linear. perhaps i will try it out on other stages later. i'll be posting a schematic of the input stage soon.


From: KB @ 
Date: 3/3/2002 3:28 AM 
Subject: Re: more solid state heresy 

Did you check out the IRFPE350 ? I think it's 400volts at 25A. Not sure if it's N or P though.,


From: kg 
Date: 3/3/2002 1:05 PM 
Subject: Re: more solid state heresy 

gain as tested in circuit: 49! nice...

clipping doesn't look too bad on a scope, either--it may be suitable for other stages in the amp.

i had to heatsink it, though.. my first attempt at soldering the tab to a turret ended up melting the solder while powering it up on the bench. a small al heatsink bolted to chassis fixed that nicely.

rs is 680r, which i dialed in first with a pot.
vg is 16v lower than b+ (which is 320vdc).

this gives me about 25mA and puts the vp at 175vdc. sounds very nice. the gain has been increased and i cannot hear any increase in noise.

mo l8r


From: jeremy @ 
Date: 3/3/2002 9:41 PM 
Subject: Re: more solid state heresy 

do you have a schem of the inpout stage yer using for that tube, 6PI? i hear good things about em and happen to ahve one to play with. thanx!!


From: kg 
Date: 3/6/2002 4:10 AM 
Subject: finally... 


here you go:

been having internet troubles. sorry for the delay.


From: tim 
Date: 3/6/2002 5:41 AM 
Subject: Re: finally... 

ahhhh... now I can see.

Perty cool!

From: R.G. 
Date: 3/4/2002 12:23 AM 
Subject: Welcome to MOSFET Hell... 8-) 

Check your hollow state soul at the door... 

That's a very nice trick. I think it's going to work well for you. 

(note that this quick calculation does not take into account the "downstream" AC shunt load.) 

Here's the logical extension: just glom an N-channel MOSFET follower onto it to buffer the downstream loading out. There's an N-channel device in an extended TO-92 package from ZETEX that does 450V and 1W that ought to work fine.

Gosh, I'm glad I'm not the only lost soul 8-)


From: Dave H. @ 
Date: 3/4/2002 9:32 AM 
Subject: Re: more solid state heresy 

but in this case, I have picked up a "free" 10x of voltage gain, or 20dB. not too shabby!

Yes it is. 40 to 50 is only 2dB 


From: kg 
Date: 3/4/2002 12:20 PM 
Subject: Re: more solid state heresy 

you're right, of course. i must have miscalculated it as 10x more gain, when in reality, it is only 50/40, or 1.25x.

you keep me honest, dave!