B+ MOSFET voltage reduction question

[casey73]

I've rebuilt an old Kay 720 for a friend. It had been modified far from the original, so I told him I'd set it up kinda like a Princeton with 6L6 outputs. The original PT had been replaced by a Hammond 278X, and it had 545V on the plates when I first checked it out. I put in five 15 volt, 5 watt zeners and a 20 volt 5 watt zener in series on the PT center tap to ground, and got the plate volts down to a more reasonable 455 volts. I had the zeners around so I used them. This is my first zener/center tap B+ voltage drop attempt.

The pair of 6L6s are set up as cathode bias with two 470 ohm 5 watt resistors in parallel giving me at cathode resistance of 234 ohms. I measure 31 volts at the cathode and 413 on the plates now. If my math is correct I'm drawing 132 ma current, which comes out to about 25 watts per tube.

It sounds good, but with the chassis open still open I smelled something getting hot, saw one of the zeners smoking and shut down. I figured 18 watts of dissipation across six 5 watt zeners would be 3 watts per diode. Either my math is off or I'm pushing the zeners to hard.

I found RG Keen's MOSFET B+ drop circuit so thought I'd try that. I happen to have a couple IRF130 MOSFETS around. I will use a 100V, 5 watt zener in conjunction with the IRF130 (max 75W dissipation) and I should be okay, correct? I have just enough room for a small TO-3 heat sink in the chassis.

Am I heading in the right direction?

[printer2]

Whether you are using diodes or a MOSFET you are getting rid of the same amount of heat. The MOSFET may be rated for 80W but that is with adequate cooling, just something to keep in mind.

[Structo]

You should use a single stud mount Zener when dropping voltage on the center tap.
That way, it uses the chassis as a heat sink.

[pops]

You can also drop voltage by using a large choke before the first filter cap. Make sure it can handle the extra current of the power tubes and about 3 henrys.

[casey73]

You should use a single stud mount Zener when dropping voltage on the center tap.
That way, it uses the chassis as a heat sink.

I appreciate the reply, but the whole idea of using a MOSFET and small zener is to avoid using a fairly expensive stud mount zener.

[Structo]

Ok