I currently use two Vox Lil Night Trains, one "clean" and one "dirty".
The "clean" head is all stock (for now), except for the tubes. I replaced the Stock Chinese 12AX7s (V3 Pre-amp, V2 PI) and 12AU7 (V1 output) with an EH 12AY7 in V3, TS 12AT7 in V2 and an EH 12AU7. This gives the amp a nice Vox jangly clean and a slightly warmer tone.
The "dirty" amp has had the tubes replaced with a TS 12AX7 in V3, a TS 12AT7 in V2 and an EH 12BH7 in V1. The C19 "bright cap" was placed on a switch to turn it on and off as needed. The R10 slope resistor, was replaced with a 0-100 KΩ potentiometer, to adjust the bright/dark slope. The OT was replaced with a Musical Power Supplies OT5PP
for added depth. This head had decent depth and can go from Bright jangly Vox Clean (barely!) to Vox's overdriven crunch.
I know with a low wattage amps it can be more difficult to pull a lot of bass, with smaller OTs and the output tubes usually used in such amps, let alone the typical smaller speaker. All of the mods to the "dirty" amp added a bit more bass response, as did the speaker swap in the V110NT cab and the Kustom Defender 112 cab (still debating what speaker will go in this one).
As you have seen, I have used a 12BH7 in place of the 12AU7 for the output tube (as has been done by many others numerous times), but have just found only a handful that are using ECC99s, with very little information available as to compatibility in amps designed for the 12AU7/12BH7s. The LNTs handle the increased heater amperage draw of the 12BH7's, is there much of a change going to the ECC99s? How about the OTs? Not the stock ones, but aftermarket manufacturer ones, such as the MPS OT5PP, Hammond 125c
, or the Mojo Clone of the Wright 1580
? The MPS is the correct input impedance, the Hammond 125c is adjustable, but the Mojo Clone is fixed at 10 KΩ, instead of the 22.5 KΩ of the original and MPS OT5PP. Would this be a problem, If I find I want to go with the Mojo?
Theoretically, if my math is correct, going from the 2watts of the 12AU7 to the 12BH7 should bump my output to 2.5 watts. Going to the ECC99 should put me to 2.75 watts. I know these are negligible gains, volume wise, but should assist in bumping up the drive for better bass response. Does this sound correct? (My experience with tube/valves comes from the early '80s on military radars, not civilian audio and we used only certain valves, tube rolling not allowed. I trying to recall nearly forgotten information.) What kind of output is available with these tubes?
Although the LNT is the platform I'm working with, How does this apply to other amps that use the 12AU7/12BH7 valves. Any information concerning these tubes/valves and/or OTs or any other compatible models please feel free to chime in.