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More MOSFET heresy: phase inverters


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1/26/2000 4:12 PM
Mark Buckingham
More MOSFET heresy: phase inverters
With all the recent talk about using MOSFETS to drive output tubes, I was wondering...  
 
Could you use mosfets to implement either a cathodyne or long-tail phase inverter? I was thinking it might be a nice way to get a free gain stage or two in an amp with a tube based inverter...
 
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1/26/2000 6:36 PM
R.G.

quote:
"Could you use mosfets to implement either a cathodyne or long-tail phase inverter? I was thinking it might be a nice way to get a free gain stage or two in an amp with a tube based inverter..."
 
 
Could you? Certainly, either kind. I suspect that something like a 600V MOSFET would drop in with only the changes needed to get the +3V to +5V from gate to source instead of the -1 to -10V of Vgk that the tube wants. Other than that, it should work. You would probably be well advised to use low-ohms gate stopper resistors on the MOSFETs, and you might possibly have to worry about oscillation due to the much higher bandwidth of the devices, but that's worrying before reaching the bridge.  
 
The real question is - does it affect tone? Only one good way to answer that one...  
 
I suspect that the way it turns out is that it *does* affect tone to the extent that the original phase inverter generated distortion and controlled bandwidth in the original amp. There won't be any noticeable distortion from the MOSFETs, as they have a wider linear range; if your ears liked what the tubes did, the tone will change. In a gain situation, the gate capacitance of the MOSFET is a consideration, so there may be some bandwidth loss because the preceeding stages may not be able to drive the gate capacitance to high frequencies. This could be prevented by using a source follower immediately in front of the PI to drive the gate capacitance of the MOSFET. The follwer MOSFET is not subject to the same problems with Cgs and Cgd because the capacitance is reduced by the feedback factor of the follower.  
 
So - try it!! It's cheap. If you like preamp and output stage distortion rather than PI distortion, you may have a real goodie on your hands.  
 
 
1/26/2000 7:38 PM
Mark Buckingham

Thanks! I'll try it. I'm thinking I will try it in my Princeton Reverb. Freeing up half of a 12ax7 might be interesting.  
 
By the way... Is there a website that lists MOSFETs and their parameters?
 
1/26/2000 8:34 PM
anonymous
quote:
"By the way... Is there a website that lists MOSFETs and their parameters?  
"
 
 
Sure, most manufacturers have the data sheets for their devices listed on their web site.  
Here are some links.  
 
http://www.fairchildsemi.com/  
http://www.harris.com/harris/  
http://www.irf.com/  
http://www.mec.panasonic.co.jp/e-index.html  
http://www.zetex.com/  
 
Aaron
 
1/27/2000 2:25 AM
Mark Buckingham
Thanks Aaron- this helps a lot.
 
1/26/2000 7:46 PM
Mark Buckingham

quote:
"  
Could you? Certainly, either kind. I suspect that something like a 600V MOSFET would drop in with only the changes needed to get the +3V to +5V from gate to source instead of the -1 to -10V of Vgk that the tube wants. Other than that,  
"
 
 
This may sound dumb but...  
Am I correct in assuming that this means simply tying the gate to a +3 to +5 supply? If So, If I just wanted to do a quick and dirty test, could I just use a couple of batteries? (Like 3 'D' cells?)  
 
1/26/2000 11:04 PM
R.G.

Not really a dumb question at all.  
 
Phase inverters are biased different ways. Most of them use a series cathode resistor and the grid(s) are tied to the bottom of that cathode resistor to generate the bias voltage, and then there is another cathode resistor to ground. This is common to both the push-pull and split-load phase inverter. The bias is actually floated on the cathode resistor, so there isn't any other bias voltage needed.  
 
A MOSFET needs a few volts positive on the gate compared to the source, so you can't use the self-bias trick. You'll probably have to use a resistive divider from B+ to hold the gate at the proper voltage.  
 
Example: A split load phase inverter with a 12AX7 uses a 47K plate resistor, a 1.2K bias resistor on the cathode, and a 47K resistor from that 1.2K resistor to ground. The signal is coupled to the output tubes through capacitors from the plate and the junction of the 1.2K and the 47K at the cathode. The grid is connected to the junction of the 1.2K and the 47K through a 1M resistor. The 1.2K resistor generates the bias voltage for the tube.  
 
To convert this to a MOSFET, you'd measure the cathode voltage - let's say it's 95V - and make a voltage divider from B+ that is at 98V. We connect the MOSFET drain where the plate was, the source where the cathode was, and the gate where the grid was. We disconnect the 1M grid/gate resistor from the junction of the 1.2K and 47K resistors to the 98V source. The MOSFET gate is now held at +98V, and its source rises to about 95V - where the cathode it replaces was. The operation should now be the same, with the exception that the MOSFET will have a wider signal range before clipping.
 

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