Is One 'Side " of a 12AX7 Less Noisy than the Other ?

[PCollen]

I've seen some high gain amps using the 'b' (6-7-8) side of the 12AX7 as the input , with the "a" (1-2-3) side following. And today I read someone suggesting that this may be the case. Is there anything to this ?

[demonufo]

It is possible that one side of a triode may be noisier than the other due to manufacturing anomalies, but as for any specific side always being the noisier side I find impossible to believe.

It may have been more of a case of the amps being easier to lay out with tidier, less noisy lead dress (wiring b into a as opposed to the other way around), rather than the sides of the tubes themselves being a reason.

[PCollen]

It is possible that one side of a triode may be noisier than the other due to manufacturing anomalies, but as for any specific side always being the noisier side I find impossible to believe.

It may have been more of a case of the amps being easier to lay out with tidier, less noisy lead dress (wiring b into a as opposed to the other way around), rather than the sides of the tubes themselves being a reason.

My initial thoughts exactly, but thought I would ask around .

[rip]

really? I thought it was to keep the cascaded half (higher signal) further from the rest of the tubes to keep noise/cross talk down. If you put the higher signal at V1B then you might see problems as its close to V2. Like trying to keep the side with similar signal strengths together.

V1A second stage (high)
V1B first stage (low)
V2A more (closer in strength to V1B than V1A)

Or I'm completely off base.

Any one know for sure though?

Rip

[HTH]

If you look at the data sheet for an ECC83, you will see that the manufacturer actually states that the 2nd triode (pins 6, 7, is the more favourable with respect to hum... http://www.mif.pg.gda.pl/homepages/fran ... /ECC83.pdf" onclick="window.open(this.href);return false; (see bottom of page 1).

Whether this still stands for ECC83s made today, I can't say, but Philips were obviously doing something with the construction of their ECC83 that made the 2nd triode have lower hum. Maybe its the proximity of the heater CT (pin 9) to the 2nd triode that makes the difference ???

[paulster]

HTH is spot on. The specification relates to the potential hum level rather than white noise, so it's not going to make much of a difference for us in guitar amp world!

[demonufo]

That explains a lot. At least any hum that is present should in theory be lower by using this as the first stage, and then amplifying further.