• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

MOSFET follower mounting in tube amps

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GregH2

GregH2

Member
Joined 2009
Hi all,

When using a source follower to drive an output tube, e.g. tubelab's powerdrive topology, is it ok to mount the MOSFET to the chassis for cooling (insulated of course)? I ask because in Morgan Jones' book he cautions against mounting CCS devices on the chassis due to the loading caused by the capacitive coupling to ground. Interested to know if the same thing applies in follower applications, or isn't an issue due to the bootstrapping effect of the output voltage.

Appreciate your thoughts,

Greg
 
W

wg_ski

Member
Joined 2007
Depends what kind of current you’re running. At a handful of mA, maybe not - but get it up in the 10-20 mA range and you can have significant dissipation. Followers and CCS’s with mosfets might need a good 50 volts or more on them. More if you’re driving triodes where you need a lot of voltage swing AND overhead. If you’re using the follower to be able to run it into AB2, you need to supply the grid current as well as the tail current. Drain-gate capacitances tend to be lower and more linear at higher VDS. Some mosfets are better about this than others - and lower capacitance mosfets often have very poor DC SOA since they are designed for switching. If you’re trying to make a current source with only 10 or 20 volts on it to minimize dissipation you might be better off with a *bipolar*. Collector base capacitance tends to level out at 10 or so volts. The Unset followers need to handle the cathode currents so they need to be beefy. Really beefy. In short - heat sink often required.
 
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