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In Reply to: RE: Lower gain (than 5751) direct replacement for 12AX7 tubes ? posted by Old SteveA on June 19, 2015 at 11:16:52
A circuit designed for a 12AX7 is designed for a 12AX7, not a 12AU7, not a 12AT7, but a 12AX7.
If you need more volume control wiggle room, pad down your volume pot.
While you can get away with a 5751, the other tubes mentioned will need very different operating points to operate properly compared to a 12AX7. Your amp may also employ direct coupling, in which case you may end up causing significant damage by even attempting this.
Follow Ups:
I don't if I would call using the 5751s in the Fosgate Sig tube headphone
amp as "getting away" with using them,as the 12AX7s tended to still add
a tube coloration (through my no Ring Radiator,direct wired,Sennheiser HD800 headphones)that the 5751s do not!
While this amp was "supplied" with 12AX7s there really isn't much comparison to how it now sounds with this singular headphone (which also
has no comparison to Sennheiser's "version" of a transparent sounding headphone; which is compromised by having that Ring Radiator in it)
Having tubes that will measure up to how the 12AX7s perform in this instance was actually a bit humorous as far as I'm concerned. Their gain
structure just doesn't appear to work in passing what my Metrum Hex DAC
is sending to them without adding some completely unneeded frequency contouring to things(in this circuit,of course)
I posted some info some time back that may be useful. Check out the link.
For the umpteenth time - tubes DO NOT have gain. We use the term gain sloppily when we substitute it for Mu , but tubes really don't have gain. They have mu, essentially the most gain a circuit using that tube could possibly have. But in the real world you rarely if ever find a circuit with the gain anywhere near the tube Mu. Usually 50% or less is what you find.
Tubes don't really have mu either.Pentodes can take on a very wide variety of amplification factors, as can triodes.
A circuit setup for a 12AX7 that has a 12AU7 plugged in will exhibit much lower gain (provided it works at all), and will likely have such piddly grid bias that grid current will be a serious issue.
Edits: 06/21/15
Tubes sure DO have mu - it is listed as a specification on virtually all tubes capable of being used as voltage amplifiers.
Mu is defined as the factor by which the plate voltage changes in proportion to a change in the grid voltage, where all other voltages and the plate current are held constant. Along with transconductance and plate resistance, it is one of the three characteristics that defines a tube's ability to perform as a voltage amplifier:
mu = transconductance x plate resistance
I do agree that amplification factor and mu can be used interchangeably.
Several triodes have many amplification factors listed, given for different operating points. The ones that don't are simply choosing to ignore that the mu changes with loading and the operating point.
If we assume a 12ax7 circuit running at 1ma. with a 180k plate resistor, 330vdc supply, 150vdc plate with a 1150 ohm cathode resistor....
Dropping in a 12au7, the cathode resistor would have o be changed to a 7.5k ohm.
Then we would have a 12au7 running at 1ma....yuk!
A 12at7 would need a 2k ohm cathode resistor and then we would have a 12at7 running at 1ma....yuk again.
Why would anyone want to do this?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Right. Overall volume may not change much. The character of over-drive & harmonics may change. IME, interchanging 12A(_)7 types have not caused any damage to any pre-amps or amps.Putting a 12AX7 into a position that had a 12AT7 or 12AU7 which set to high bias idle point, may case the 12AX7 to get noisy or die sooner. Eg: putting a 12AX7 into a Fender amp 12AT7 reverb driver position. But, no damage to other components around this reverb area.
Edits: 06/20/15
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