Whilst cleaning out my "audio closet", I came across these. I bought them as NOS several years ago and never used them. Will sell for
$125 $100 shipped for the pair.
They were described as:
Matched pair of rare NOS Sylvania Gold Brand FAA 5751.
Special quality, lower gain than 12ax7, low noise ECC83/12AX7's.
These are vintage Gold Brand with the gold color printing.
FAA designates a government FAA contract.
They have grey ribbed plates, and I believe triple mica spacers for enhanced quietness and extra support rods for vibration susceptibility reduction.
Note, they are Gold Brand Sylvanias but have steel, not gold pins. This is common for military and other (in this case FAA) contracts.
They are in excellent condition and said to be one of the best 5751 tubes that were ever made.
Please see pictures:
