Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on October 04, 2022, 05:40:46 pm

Title: 5E3 with Pre and PI series heater wiring
Post by: scstill on October 04, 2022, 05:40:46 pm

I just discovered that on one of my better sounding builds, the preamp 12ay7 and PI 12ax7 have their heaters wired in series. 6.3vac is applied to pin 4 and 5 and pin 9 is unused. Yikes....

Amp sounds really good, is loud and low noise, lots of compliments when its played. 12ay7 and 12ax7 are (as expected) cool to the touch. Should I leave it (any risk)? or wire it correctly? Should I expect more preamp and PI gain?

FYI - schematic is standard 5E3 except that coupling caps from Pre and PI are changed from 0.1 to 0.022uf and Pre and PI cathode caps are changed from 25 to 4.7uf  supposedly to tame the lows (which it did).

Title: Re: 5E3 with Pre and PI series heater wiring
Post by: pdf64 on October 04, 2022, 06:11:05 pm

I suggest to rewire it so that the valves are supplied with their intended heater voltage.
New valves with high emission may seem to be working ok with heater voltage halved, but may be wearing more quickly (due to the electron cloud surrounding the cathode being depleted).
Even if not, when they eventually lose a bit of emission, they may cease to function. Whereas they’d be usable for longer with the proper voltage.