5751 vs. 12ax7 voltage difference???

[playonit]

I am using the lower gain 5751 instead of 12ax7's in my DC30 clone ..since the 5751 is a lower gain tube would I expect to see lower plate pin voltages than if using a 12ax7?????

Thanks .... I sure am learning a lot this week

[Ripthorn]

Lower gain does not mean you need to see lower voltages, as I understand it. The Rp is going to be different for the 5751 and the 12ax7, but usually the 5751 can almost be dropped in as a replacement for a 12ax7.

[David Root]

In the V1A input position, a normal common-or-garden variety grounded cathode stage will generally show slightly lower plate voltage with the 5751 but not anything significant. Gain of course is 30% less.

It will extend the -3dB high end rolloff point from 20kHz to about 40kHz, and increases the stage's input impedance, compared with the 12AX7, neither of which is going to make much difference in a guitar amp.

Some say the 5751 is "cleaner" but it is just a little harder to overdrive because it biases slightly to the right of the 12AX7 under the same conditions. It has a flatter more extended high end but beyond the capability of a guitar amp or speaker, so I don't know about flatter within the guitar amp's range.

It is a pretty safe sub for a 12AX7 just lower gain and maybe a little "cleaner" depending on what follows it and how you play.