As Joseph Esmilla says, this was inspired by Bruce Berman’s Homebrew Type 76 Preamplifier described in Sound Practices #13. This design, in essence I believe, was being built by Jeff Korneff and was given what amounted to a rave review by Peter Breuniger in Art Dudley’s now-defunct Listener magazine.
As I was having good results with a 76 input tube on my 2A3PP power amp, I became interested. Besides, I like the look of the bigger ST (Shouldered Tube) and Octal tubes. (Pic above: 5687 – 76 – 6SN7 tubes.) Bruce enthused about the ‘sense of refinement, pace and excitement’ of the 76 compared with the usual 9-pin preamp tubes.
A clue to any differences in sound might lie in the construction of the tubes. The 5687 is constructed with rectangular plates and cathodes. The 76 (and some 6SN7s) have larger, oval, cylindrical plate, grid wires and cathode.
The type 76 is a radio tube from the 1930s/40s and the 56 is the 2.5v heater version of the 6.3v 76. This is JEL’s schematic and power supply: Just a single tube (76) direct-coupled to a cathode-follower (one section 6SN7) for low output impedance drive.
Bruce’s original design differed mainly in using vintage paper-in-oil PSU caps extensively and also choke decoupling of the HT for each channel. The JEL version ‘lifts’ the heater supply potential, which is good practice.
This is my build, re-using the wood chassis from my previous 5687 WOT linestage and with outboard PSU for all the AC components. The volume control is the TVC detailed in an earlier post & in the Volume Controls post. A close up below of the signal section (a rebuild and not one of my tidiest jobs!).
I usually go for Mills ‘non-inductive’ wirewounds for B+ dropper and plate load resistors. The cathode Rs are Shinkoh tantalum (which have given me good results in my phonostage). Output coupling caps are Ampohm (sadly now out of production). Panasonic TSUP PSU decoupling caps (located near the tubes) replaced Elna Cerafines and later experimentally swapped out for Obbligato film/oil (more on this later). Wiring is a mixture of solid core 0.7mm silver or copper (the white wires are unused inputs from the old WOT preamp). The Sound – This preamp sounds open and clear. Music ‘breathes’ with plenty of ambience, harmonic richness, decay and low-level detail. Dynamics and bandwidth seem good – I will measure frequency response on the bench soon. Thank you Joseph and Bruce.
Tweaks – Tube choices, deleting the cathode bypass cap, Shinkoh vs Kiwame vs Mills cathode Rs, iron choke + paper-in-oil cap final decoupling for the PSU – more in a follow-up post soon.
Note – Neutrik Speakon SPX series connectors are not specifically intended for high voltage DC PSU connection, but are rated for 250Vac and 40 Amps. A beautifully designed locking connector. I used the 4-pole version – for L/R ch B+. heaters, Gnd.Sound Practices – a CD containing all issues is still available on eBay from editor Joe Roberts, I believe.