is EH12ax7 OK for 12at7 spot?

[kirksflooring]

I have a MB studio preamp that has a EH12ax7 in the spot above V1-V4. I've heard it called the phase shifter? splitter? What does it do and is it ok to have a different kind of tube in there? Also, what is the corresponding tube for V1- v3 or v4?

 

[fatboy135]

guitar amps are usually running on a push pull ( class B, class AB ) mode, that mode splits the signals in positive and negative. Think a sinusoidal waveform it has half positive and half negative for every period, so if you splits signal into two signals you achieve to waste less power due the tube that amplifies negative half is not working during the half positive for example and you also achieve at least the double of power rating, due every phase tube only needs to amplify the half. ok the class of an amp is usually fixed by the bias point, but you every class of amp has its owns topology.
So the preamp tubes are builded on a single ended topology ( you could think it is quite similar to a class A ) so they amplify both half on the same tube ( the entirely waveform ), but power tubes amplifies halfs, positive or negative but not the entirely wave on the same tube, so for that reason we need that someone splits our sinus into two half waveforms, well that is the phase splitter, for that reason it is on the last chain of the preamps tubes and just before the power tubes amps, easy isn´t it???

 

[kirksflooring]

i appreciate your comments. That explains a lot. I guess my last question is, If a 12ax7 is balanced properly, is it as effective as a balanced 12at7(don't 12at7s have a higher gain?).

 

[fatboy135]

ok about balanced and not balanced issue is only a taste matter, for example regarding power tubes I ever recomend well balanced tubes better than a high tech bias adjustment.
balanced triodes are for attitudes, a non balanced triodes gets a phase of the waveform a bit stronger than the other phase, that little difference is amplified by the tubes but not too much the necesarly to just ear a curious distortion, that distortion is totally allowed on guitar amps due the distortion increments the first odd harmonics getting your amp a bit better for soloing, thing that for example instruments like clarinet has the odd harmonics stronger than even and clarinets are using as solo instruments on the orchestrals.
Well now regarding about substitute 12at7 for 12ax7 tubes I´m a real total hater of that kind of substitutions. I consider the 12at7 out of the family of the 12a*7 due all the tubes from that family could dissipate 1 watt and the 12at7 2 watts, ok what does it means, well it means that 12at7 is able to flow the double of current of the all other tubes, for that reason when a designer draw a circuit he is thinking about the tube properties he goes to the datasheet and fixed all the parameters, if a circuit is designed to flow 10 mA with a 12at7 and you replace it with a 12ax7 it will be totally overloaded, for that reason I ever recomend ever to replace tubes with the same kind of tube, I know that there are people that try it and the tube works, ok my car tires are 205 r15, I could replace it with all the r15 wheels, so if the car designer designs the stability of the car he was thinking about a 205 r15 if I replace it with a 175 r15 did you think that the car will be working as the designer thought???? even both tires are r15 and they fit well on my car???

 

[stokes]

To try to put it simpler,and what fatboy says is absolutely correct,he does know what he is talking about,and from a technical standpoint,he can talk circles around the rest of us, the 12AX7 in a PI designed for the 12AT7 is going to change the distortion quality,not always for the better,you wont hurt the amp with the 12AX7,but you may or may not like the sound.The AT7 has an amplification factor of 60 where the AX7 has an amplification factor of 100.There is also a difference in the output impedance of the two tubes,but I will leave that to fatboy to explain if you want to know.Try them both and decide for yourself what sounds better.But I agree with him in that the AX7 is not going to sound better in a circuit designed for an AT7.

 

[edgarallanpoe]

i appreciate your comments. That explains a lot. I guess my last question is, If a 12ax7 is balanced properly, is it as effective as a balanced 12at7(don't 12at7s have a higher gain?).

No, you have that reversed, the 12AX7 is higher gain. Read the above posters, they know what they are talking about. I have yet to hear an amp that suggested an AT7 sound better when a 12AX7 was used instead. Every single time it sounded really brash and harsh...and not in a good way. Make sure to get a GOOD At7. They are cheap. You can pick up a NOS AT7 for about $20 that will run circles around any currently made AT7.