Lower zener voltage = clipping happens at a smaller signal swing = more compression and a smoother, "vocal" sustain. Higher zener voltage = clipping kicks in only at louder signal = looser, more dynamic, more Plexi-like.
Which voltage was Jose's "default"? Two public-record stories, neither fully reconciled:
- The widely-circulated story (1980s–2020s online): 16V was Jose's canonical voltage and "the Brown Sound voltage." This appears in most kit instructions, recreate threads, and forum FAQs.
- Dave Friedman's direct recollection (Headfirst Amps Panel #11, Dec 2025): *"I got the part number once, but it was a 20-volt Zener."* Friedman has serviced ~60 Jose-modded amps over decades and recalls the 20V part as typical. He has also previously stated #12301 specifically may have had a 15V or mismatched pair.
JMIL ships 20V as the default on the strength of Friedman's direct recollection — he is the most-authoritative living source on Jose's bench practice — while keeping 16V as a one-click alternative because the older documentation is too widespread to ignore. The lab is honest about which framing it leans on.
By voltage, the players who reach for each: - 12V and 14V — bedroom-volume Brown Sound chasers (most-compressed end). - 16V — long-cited public canon; sits in the middle. - 18V — Lynch territory (diode squash on long notes, Plexi snap on the front edge). - 20V (1N4747) — Friedman-recalled Jose default; loosest of the canonical set, closest to feeling a real amp's dynamics through the modification.
Prevalence reality check: before agonizing over the voltage, remember that diodes were absent on most Jose amps (see diode-prevalence). The lower-stakes choice may be "diodes or no diodes," not "12V or 20V."