Phil Allison wrote:
>
> "Raymond Koonce"
>
> > I'm open to suggestions.
>
> ** Prove it !!!
>
> By changing those puny, under-rated & unsafe AC switches switches.
>
> .... Phil
I have a 7070 VAC amp here for some repairs and "slight" upgrading.
Its got 4 x 300B in mainly class A PP for each channel, giving a max of
70W AB.
There are two mains inputs and two mains PT because its a genuine dual
monoblock
amp.
Its also a royal pain in the arse because the fuckin thing weighs maybe
60Kg.
The two mains switches are tiny toggle things and mains wiring isn't
thick like I would use. But, they have worked for over 10 years OK.
Its a complete miracle that the toggle levers have not been broken off.
Because I hate such switches, I will be using them to switch 12Vdc
from a low VA auxiliary PS and a rugged relay for the mains which also
is used to interupt the mains if excessive Ik flows in any output tube.
I am fitting active protection against bias failure and/or thermal
runaway.
The amp was bought cheap and failing tubes caused a fire to
scorch part of the circuit board.
The repair by someone else was bodgied, and a dry joint caused more
failings.....
Another saga of a high end bitta crap producing too much smoke...
VAC had the 300B running with 37 watts of idle dissipation
which was way too high, and not at all necessary to get
a huge amount of pure class A PO before the class AB threshold.
The power and output trannies seem to have wiring colour coding
that looks very like what Hammond have.
But PT is quiet, and the OPT has wide BW, and the loading is OK.
Many hi-end amps use thin wiring, teflon insulated, with silver plated
stranded copper wire.
Initially, before I changed the HT tap, the B+ went up
to +600V before the loading pulled it down a bit.
So just after turn on there is 600Vdc across a 0.2mm thickness of
teflon.
Not good, but it seems to work.
Patrick Turner.
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