Alternative hi-mu triodes in place of my favorite octals
The type 75 is the grandaddy of all hi-mu audio triodes culminating with the ECC83/12AX7.
|loctal base + socket|
Loctal or Loktal (as trademarked by Sylvannia) tubes were developed concurrently with octal based tubes in the 1930s. But loctal tubes were aimed at the then emerging car and portable radio market - hence, the need for a locking-ability in the tube socket, which required a metal base that gave the additional benefit of better shielding.
The 7B6 is a loctal version of the 75
I originally built the je2A3 mono integrated amp with 7B6s in the phono stage. Right now I am running 7B4s.
The 7B4 has slightly lower Rp than the 7B6. NOS samples are still very affordable so try them both ;)