Resurrecting a 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 and soundstage.

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 output tubes.

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 another day.

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.