Tube Power Amp Reviews

Manufacturer's Comment

Thank you everyone at Stereophile for all their support. We are so pleased with Ken Micallef 's review and all his comments on the musical performance of our amp.

I would like to point out to your readers important conditions regarding John Atkinson's technical measurements on this amp. Tubes are the only active component in a tube amplifier, so all tube amplifier measurements are sensitive to the condition of the tubes. Tubes need to be tested separately before testing the unit. In this case, the amplifier was used to test the tubes, so the measurement results show the performance of the tubes, not the amplifier.

Zesto's technical specs are based on new matched tubes with 50 hours of burn-in. The Bia 200 tubes in the review have approximately 500 hours of use and are exhibiting normal tube decay, which will affect the measurements and overall power rating.

The tubes may not be optimal for measurements but certainly are optimal for listening. It was important to Zesto to give Ken the very best listening experience for his review with a well burnt-in amplifier.

We feel it was misleading to compare the measurements between our Leto preamp and the Bia 200 amp because while 500 or 1000 hours on a small-signal tube has very little effect on its condition, similar hours on an output power tube will result in significantly more decay because it is working so much harder. The nature of tubes is always changing, which only matures and enhances the musical experience.

Barring any component failure, Zesto Audio makes no apologies for any measurement differences due to tube decay from 50 hours to 500 hours of use. It is clear from Ken's review he thinks the amp sounds great!—George Counnas, President Zesto Audio