• 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.

Power supply issues with Borbely style 12AU7/MOSFET hybrid

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jjbunn

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M Gregg said:
Looking forward to your progress :)
What was the heat dissipation in the FETS like?


Regards
M. Gregg
Click to expand...

OK I'm back! I received the toroid and wired it up to replace the Paeco transformer that I had been using. I got around 25-0-25 DC on the rails. The amp worked, but as I was warned, it runs far too hot - much too hot for my taste.

I don't really want to start adding huge heatsinks, so I'll try a different design.

Can someone recommend a MOSFET/tube hybrid design that produces tens of Watts and doesn't fry eggs? I'd really like to re-use the toroid and MOSFETs if possible.

Thanks for all the help.
 
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ticknpop

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Borbely used a special tube desgned for low voltage car radios and 2sj79 / 2sk216 - you changed too many things and the operating voltage is too low for the tubes you are using or if you get the voltage high enough for the tubes it will run the fets too hard or too hot. Borbely's design works becuase of a carefully chosen tube that could run on low voltages that the fets could handle. Trying these other tubes just makes a mess of it.

But it's a preamp, designed as a line amplifier, not a power amp.
 
J

jjbunn

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Joined 2008
pcbunn.caltech.edu/jjb.html
ticknpop said:
Borbely used a special tube desgned for low voltage car radios and 2sj79 / 2sk216 - you changed too many things and the operating voltage is too low for the tubes you are using or if you get the voltage high enough for the tubes it will run the fets too hard or too hot. Borbely's design works becuase of a carefully chosen tube that could run on low voltages that the fets could handle. Trying these other tubes just makes a mess of it.

But it's a preamp, designed as a line amplifier, not a power amp.
Click to expand...

The circuit was fairly close to one published in an AudioXpress article, which used a 6DJ8, and was a power amplifier - it's not my own design. The article claimed it was a Borbely design.

I'll hunt around for an alternative design, but if anyone has a suggestion for a hybrid power amp that doesn't run very hot, please let me know.
 
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