The 5Y3GT was originally used in early low to medium power Fender amps with tube rectifiers. Tubes are imprinted without the Groove Tubes logo as depicted. These tubes are from the same batch the same as the other Groove Tubes 5Y3GT with the GT logo we're selling in our Reverb store. Manufactured in Russia 2014-2016. Audibly and operationally tested by Angela Instruments. We want the tubes to work in your amp so we test 'em in ours. Not just once, not just twice but three times. Pair of two tubes is matched to exactly the same plate voltage (e.g. 389VDC) in a real guitar amp, not in an emission 'tube checker'. If either of the tubes fails in normal service within 12 months send it back to us and we'll replace it once. Why so cheap? We bought a big bunch of these from overstocked/downsizing Fender Groove Tubes 'bulk packed' in crates of 100 tubes.