hi-mu triodes: 75, 7B4, 7B6 + 7F7

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.

Grid-capped 75s in the JE Labs Simple 45/2A3 with Noguchi PMF10WS OPTs.

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 ;)

I built this stereo preamp for the November Hi-Fi Show 2016. It uses 2 x 7F7s for the phono section which are direct loctal equivalents of the 6SL7. The line stage sports 2 x grid-capped 6F8Gs.

7F7 vs 6SL7

JEL preamp at the November Hi-Fi Show 2016 dressed in Harana Audio woodwork.