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myfi 16th March 2013 12:56 AM

TSE FET / Driver Stage
 
1 Attachment(s)
In my quest to understand the TSE, Iíd like to continue with a discussion of the FET / Driver Stage.



Attachment 336322


Iím new at this so when I screw up please jump in and get me going in the right direction.



This is how I think the DC quiescent bias scheme is set up.

Assuming Iím setting a 300b tube to a Gate voltage (Vg) of -65vdc to dial in itís bias point.

The MOSFET has a very high input impedance and therefore;

IG ≈ 0.
If IG ≈ 0 then for dc current IR15 ≈ 0 as well.

If a likely B- for 300b tubes is considered to be -300vdc then the current
IR13 can be found by -300vdc/(130K + 100K +22K) = -1.2ma

And since IR15 ≈ 0, IR11 = IR13 = -1.2ma

The voltage at the top of R11 = 22k * -1.2ma = -26.4vdc

The voltage at the top of R13 = (22K + 100K) * -1.2ma = -146vdc

Those are the minimum and maximum voltage available to bias the MOSFET.

The trim pot is adjusted to set the MOSFET gate voltage. Letís say I set it to -69vdc.

Using a typically Gate to Source voltage (Vgs ) of +4vdc.
The MOSFETís Source voltage (Vs) is now, Vg + 4vdc or -69 + 4 = -65vdc

The load hanging on this stage is our 300b tube and it has a high input impedance so current flowing into the tube IG ≈ 0.
And because of this there is no voltage drop at R32 so our 300b tube has a Vg of -65vdc.

Thanks,
Steve

chip647 4th April 2013 08:41 PM

Don't over-think it.
 
The 100k pot in your negative bias supply will allow you to drag the MOSFET all over the place. Just set the bias pot to the minimum Vin/Vout resistance when you start it up and after things warm up, dial it up to get the -65 volts you need on the grid of the 300b. Much easier just to measure the actual current draw of the output tube.

Really important to start out as negative as possible wen you first fire it up.

myfi 6th April 2013 12:39 AM

Chip thanks for the heads up on start up. I have just finished a 60w P-P to satisfy my need for a tube amp. My plan with the TSE is to have fun learning how and why it does what it does. I didnít know it was a hybrid when I started. But having said that, I really like Georgeís approach to his designs.


Steve


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