We use US made Sylvania NOS 6GH8A tubes in our Ultravalve tube amp find them to be uniformly quiet, low microphonic, uniformly high gain, reliable, and durable.
They test and perform very well and it is very rare to have a failure in service.
We just got a shipment of new Chinese unbranded 6GH8A tube in, from Richardson, and a spot test of several says they also are just fine Some Japanese versions that we tested a while back were uniformly bad.
Frank Van Alstine
