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

On 12/31/2018 11:32 PM, Tim Williams wrote:
"Big Bad Bob" wrote in message
...
I think it works ok without "all that".* fuses where needed of course,
to avoid things catching on fire.* But tubes are amazingly strong at
absorbing conditions that transistors would melt under.* Sure the plates
turn pink [like when half of a push-pull transformer melts away] but
if you don't push them to the edge of physics they generally take it
ok and
survive getting a new output transformer.


A lot of sweep tube datasheets contain the rating, "short term overload
duration: 220W for 60s" or something like that.* Transistors can only
dream of such abuse, gone in milliseconds!

But the purpose of that rating must be understood.* These were TV tubes,
and the sweep tube in particular often took a beating as other tubes
heated up and other signals stabilized: horizontal oscillator and sync,
and the damper diode especially -- its high cathode voltage isolation
takes a long time to warm up.

There was definitely no money in adding a protection circuit!* Burn a
tube, pop it out, take it down to the corner drugstore and buy a new one
for a buck or a few.

Nowadays, with both tubes and transformers being rather pricey, let
alone the repairman -- the balance changes, and especially with how
little hardware is involved in adding a protection circuit (if one does
not mind that it contains silicon), it's well worth it.

Tim


Yep, the OP is talking about monoblock amp with 6, 8, ten power tubes.
The power supply iron and output transformer will be large, possibly
custom, and not cheap.

$5-10 worth of microcontroller or ICs and relays to at the least monitor
tube cathode currents and grid voltages and cut the HT if things start
going tits-up compared to a melted half of an output transformer that
might cost $3-500. Big BUMMER!