VTL 50/50 Valve Amplifier
by James Leahy
This old relic from the early eighties is
not the only good thing to come back to life from the dark side in
the new millennium after a monumental melt down of the highest
order. The basic circuit is of a simple and fundamentally sound
design which once rebuilt can produce a pleasing level definition
When this unit arrived on my doorstep I was
unaware of the ominous rebuilding task that would await. Total
failure of tubes, capacitors, and output transformer was brought
about by the owner leaving the amplifier on for over 4 years
straight! Yes, you read that right folks! Don't ask why this was
done, as the answer is stranger then the outcome.
First off the bat a new set of perfectly
matched quad Electro-Harmonix EL34 tubes were ordered for the output
to replace the worn set of four 6CA7 power tubes.
Next on the agenda was the
task of ordering four replacement 12AT7 input tubes with matching
Russian Electro-Harmonix preferred, so as to be the same as the
Further trouble was encountered with leaky
capacitors being present. As a precautionary measure all of the
electrolytic capacitors were replaced with 105°C Military Spec items
from Evox Rifa in Sweden. Not cheap but they are made with excellent
sonic qualities and are durable as hell.
Putting everything back together and hitting
the power switch proved to be a further disappointment with the
Power Transformer testing open circuit. What started out as a simple re-tube was quickly turning into a very time consuming and
costly repair. Already past the point of no return the tranny was
ripped out the original transformer had to be
rewound by hand. Being sure to count ever last winding exactly or
else things would go pear shaped rather quickly again.
The transformer was successfully repaired
after much time and patience then refitting the unit back into the
amplifier's chassis was again a tedious job because of all the
windings coming off the unit. Anything can be done if you try hard
enough but with most repairers unwilling to invest their time and
most customers more often than not equally unwilling to spend
anymore then a few dollars, these amplifiers are generally found on
the rubbish dump today.
This one was saved and will live to fight
There was a mysterious intermittent isolating
pulse coming from the pre-amplifier that was later traced back to a
ground loop problem.
The matching VTL Maximal pre-amplifier was re-tubed
at the same time with four Electro-Harmonix 12AX7 tubes for a great
sounding budget combination.