tube amp help ot

[doug deeper]

ok a few questions....
true or false....i can change 12ax7a...w/12ax7w/ with no problem....
true or false...i need to rebias if i change power tubes....
doug

[Peter Snowberg]

true or false....i can change 12ax7a...w/12ax7w/ with no problem....

No worries... it's plug and play with and play with any 12AX7__ type. Just don't go outside the AX type. Other types like AT or AU (or V or Y or Z) will require changing the cathode resistors and possibly the plate resistor as well. True.

true or false...i need to rebias if i change power tubes....

Mostly true. You can get away without it in most cases, but your tone could (will almost certainly) suffer be sub-par and tube life will suffer.

Take care,
-Peter

[puretube]

some (!) don`t require biasing at all;
e.g. "autobiasing" amps (class A) like VOX AC30...



//www.puretube.com

[Eb7+9]

ok a few questions....
true or false....i can change 12ax7a...w/12ax7w/ with no problem....

Those are generally manufacturer or packager numbers to indicate noise or other selected characteristics, not only can you substitute a 12ax7 with one suffix for another 12ax7 with a different suffix but you can generally substitute safely a 12ax7 for a 12ay7, 12au7 or 12at7 and vice versa in most linear circuit that uses them except the gain and headroom curves will be altered.  

Tube swapping is the basis for many tube amp "rock" mods - the classic example being a 12at7 - 12ax7 swap in the phase driver of a class-AB tube amp to give it an edgier sound (this idea comes from comparing a '58 Twin phase-driver to a '63 Twin phase-driver) ... Spice sims running RCA-data derived models show that using a 12ax7 tube in the Fender AB763 phase-driver circuit (that was designed for a 12at7) brings down the headroom response of the circuit to 2volts (pk) from 6volts (pk) and the small-signal gain of the circuit up by maybe 10% to 15% ... the circuit overdrives a lot earlier mainly because of the reduction of linear range and not so much the gain increase ... Try this tweak in a Garnet Deputy guitar amp where a hi-fi'ish 12au7 is used in the phase-driver position!

true or false...i need to rebias if i change power tubes....
doug

... depends, some tube power circuits will totally outshine themselves if you adjust the bias carefully - especially after inserting new tubes that have been match-selected ... there's also a safety issue here, if your power tubes are running too hot that could lead to an unecessary failure and a waste of tubes possibly ... there are several ways of biasing power tubes, the most accurate is called Tranny Shunting - but it's potentially dangerous to neophytes so it's not discussed openly ... might be a good idea to get an experienced tech to do it for you, or buy a book to learn how.

good luck !
jcm

[Jered]

some (!) don`t require biasing at all;
e.g. "autobiasing" amps (class A) like VOX AC30...

and Boogie dual recto's and with a flip of a switch you can use 6L6's or EL-34's, or tube rectifier or silicon, or 75 watts or 100 watts, but no biasing.
  Jered

//www.puretube.com

[R.G.]

Go read the "Tube Amp FAQ" at GEO. All of that, and much more is in it.