musicman mystery tap

[woodyc]

I have a 2475-130 here (the one with the tube splitter) and on a whim I decided to check out the extra tap that only goes to the pilot lite. I thought this was interesting...

lo mode: heater 4.5VAC plate 478 screen 237 output 40W
pilot tap: heater 5.8VAC plate 629 screen 310 output 86W
hi mode: heater 6.5VAC plate 704 screen 346 output 113W

I'm thinking about making the hi mode 86W because if there is any difference volume or sound-wise its pretty subtle, and it may help with tube life. (The hi mode is really rough on tubes.)

Anybody else ever done this?

[g1]

Never tried it, can't see any reason it is not a viable option.
As an aside, seeing that they run the heaters at 4.5V in lo mode, I'll have to rethink what I believe about low heater voltages causing problems with tubes working properly and also hum.

[woodyc]

Never tried it, can't see any reason it is not a viable option.
As an aside, seeing that they run the heaters at 4.5V in lo mode, I'll have to rethink what I believe about low heater voltages causing problems with tubes working properly and also hum.

Yeah, as you go down in heater voltage, tubes continue to work at lower power, but 4.5V is getting close to where things get kind of unpredictable, i.e. you don't have enough space charge for the tube to work right, or something.

I went ahead and modded the amp. Hopefully the guy won't notice anything.

Here's a schematic of that amp.

[woodyc]

I worked on a Vox VR30R this morning - the heaters on that one are 4.3V (and the B+ is 30V.)

[g1]

So the starved cathode operation means you can get away with such starved filaments??? I wonder if there would be certain 12AX7's that just wouldn't work there?