DN2540 MOSFET in a cathodyne

[bigugly]

I've been thinking about using the DN2540 MOSFET in my next amp as a split load phase inverter. I think I can just sub it in where a 12AX7 would normally go but I have very little experience working with discrete SS devices. Any advice would be appreciated.

thanks,
James

[FYL]

I've never used a depletion MF in a cathodyne but it should work OK.

[DummyLoad]

I've never used a depletion MF in a cathodyne but it should work OK.

i didn't know they still made them! i thought that enhanced is just about all that's being mfg.   that's what i get for making assumptions.   

--DL

[FYL]

Depletion MFs are quite rare and expensive but they are still being manufactured.

[kagliostro]

Interesting matter

I'm interested to know your further experiment in this solution

from my part the interest comes because of the use of one less tube (one less filament consumption) in converting old amps to guitar use

the solution with the two cathode bypass caps on the power tubes comes from the Geezer idea as to obtain a variable output power ?

(http://www.el34world.com/Forum/index.php?action=dlattach;topic=4401.0;attach=3956)

Kagliostro

[bigugly]

Interesting matter

I'm interested to know your further experiment in this solution

from my part the interest comes because of the use of one less tube (one less filament consumption) in converting old amps to guitar use

the solution with the two cathode bypass caps on the power tubes comes from the Geezer idea as to obtain a variable output power ?

(http://www.el34world.com/Forum/index.php?action=dlattach;topic=4401.0;attach=3956)

Kagliostro

I've got a finished layout done and I'll start putting it together while I wait on the DN2540s to come in. My desire was to take my favorite amp and give it a push pull power amp without having to add another 12AX7.  The decreased filament consumption, extra socket, and corresponding hole in the already cramped chassis i have were all added benefits. I have also wanted to try Geezer's idea since I saw it. Its seems a viable alternative to a more complex VVR circuit.

[bigugly]

okay! I got all the parts in today so it looks like I'll get to build the first prototype this weekend. the only change I'm going to make to the schematic in my first post is to get rid of the dual ganged post PI MV.

Depletion MFs are quite rare and expensive but they are still being manufactured.

the only manufacturer that I know of for depletion mode MOSFETS is Supertex but at $1.60 each I wouldn't call them expensive.

[38Super]

Depletion MFs are quite rare and expensive but they are still being manufactured.

the only manufacturer that I know of for depletion mode MOSFETS is Supertex but at $1.60 each I wouldn't call them expensive.

This is an interesting project.  True, a depletion MOSFET has threshold characteristics closer to a vacuum tube (device is on with Vgs=0V), but is not required in this application.

If you look at the common source connected Q1, the voltage on the source = [V1A (DC plate voltage) - Q1 V(on)]  This voltage sets Q1's operating point current.  This same current flows in the drain resistor, since Igate(DC)=0.

A depletion mode MOSFET (same w/ JFET) has a negative threshold voltage, whereas a typical garden variety MOSFET (enhancement mode) device has a positive threshold voltage of several volts (need to apply Vgate-source to turn device on).  From the circuit's perspective, the difference in using an enhancement or depletion FET is a few volts difference in the V(on) voltage, that is the voltage gate to source when the device is conducting a few mA.  This will affect bias current in Q1, which can be dialed in by adjusting V1A cathode current or adjusting R14/R15

cheers,

rob