These are wonderful tubes with a great clean and detailed sound. I find they worked great as a replacement for the 12AX7’s in my McIntosh MC2000 and were my tube of choice along with a pair of Valvo blue label pinched waist 6201’s and original Tungsol 6550’s. I purchased three quad matched sets of these tubes years ago and put approximately 100 hours on each set before I put two sets away as back-up tubes for my Mac. All these tubes were in the same tube box as pictured and are military circa 1957, and bought on the same date. I’ve tried all the variants of the Sylvania 5751’s over the past two decades and these sound better than the rest including the gold pin grey plate 5751’s which I also tried. They are the rare black plate triple mica square getter version with the gold printing . I don’t have all the original boxes as they were in poor condition and tossed. The test values are as follows from my Hickok 533a tester. *NOTE: I revised the test values after I discovered the set screw on the tester bias knob had moved and the readings were low. Tubes are guaranteed to be as listed.
Tube 1: 1350/1400
Tube 2: 1350/1400
Tube 3: 1350/1400
Tube 4: 1350/1350
The price is for the matching quad set. Shipping is $5.35 via priority mail US only please, insurance is optional. I will be listing the pair of used valvo 6201 blue label pinched waist shortly along with matching quad sets of the Mullard box plate CV4004 (12AX7 equivalent and another great match for McIntosh amps).