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bitrex bitrex is offline
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Default Tube mono-block amp with SIX 6L6 outut tubes

On 01/01/2019 10:27 AM, Kevin Aylward wrote:
"Big Bad Bob"Â* wrote in message
...

On 12/28/18 09:16, bitrex wrote:

All amps of that scale should have active protection
circuitry/monitoring of currents, biases, and temperatures via
microprocessor to avoid potentially catastrophic faults.


Even briefly overvolt or overcurrent on a typical transistor, and
you'll be replacing it VERY soon.Â* I made the mistake of designing a
circuit that operated close to the maximum Vceo (these 60V transistors
should be able to handle 45-50V right?), and the transistors never
lasted long under load (replaced 3 times, and 3 blown fuses that were
supposed to protect them). Replaced with transistors that had twice
the Vceo and no problem.


Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their
rating at high voltage is much lower than their power ratings would imply.


Ya, BJT amps tend to have oversized output devices as compared to what
their rated maximum "RMS power" output would imply. 75 watt-rated
devices in 25 watt amps. The problem is risk of second breakdown when
working into reaactive loads, not de-rating BJTs appropriately when
they're handing significant powers into reactive loads common newbie
mistake

I designedÂ* mosfet amp in the early 80s. I tested it by putting in a
full level signal with an output s/c. The only protection was a zener
across the gates to limit the current to the device ratings. I left it
cycling with its 90 Deg heatsink thermal cutout for 3 days. No problems.

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html