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Restoring a Traynor YBA-1A: EL34's @ 560Va 535Vg2 ?!?!?!

Posted by Jim Doyle (A) on April 13, 2007 at 09:21:07

A friend of mine has one of these guys thats in desperate need
of cleanup. Yes, it works and he's even played it, but from the
looks of the electrolytic caps underneath - its a ticking timebomb.
It's a 90W bass amp, but we're going to downgrade it to guitar duty.

The Power Trannie on this thing is UNBELIEVABLY HUGE!

What do you guys think about using KT90's or modern 6550's
at these volts? Before I start lion-taming the B+ supply I'd at
least like to see what you guys think about whats out there that
might run at the extremely hot and hard operationg point. I'm
skeptical of what might happen if he tries to put new Sovtek EL34s
in this with 530V on the screen.

In my restorations efforts, besides the obvious coupling cap and
electrolytic replacements, I'd like to try to tame this beast so
that it doesnt just eat tubes for lunch. Obviously, I'm considering dropping down the B+ to maybe 500V by rebuilding the supply for slightly more kinder CLC operation (using no more than 8uF input-C likely a film/foil/oil Industrial Motor run cap to take the ripple). There's not much room for nor filament windings for tube rectifiers here. However, the transformer's huge and with diode rectification, this thing is just going gush current.

I intend on keeping the original tone stack and Marshall driver but
makes some tiny changes in the "Range Extender" circuit for the
electric guitar.

The screen grid supply is terrifying... It's just a 470 ohm shared
10W sand resistor between it and the PSU... If you were going to choose a screen grid R that's explosion worthy, what'd you be?
A Mills 10W ?

-- Jim

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