RH50T phase inverter tube - 12at7 or 12ax7?

[AlX]

Hi,

i just got a Randall RH50T Head (i think it was built in late 2005, at least that is the date on the EH power tubes), and found out that the phase inverter tube is marked as 12AT7. I wonder if this one shouldn't be a 12AX7, like the preamp tubes, too?

Thank you for your advice!

 

[glpg80]

a pre-amp tube is made up of whats called triodes. a preamp tube has two triodes, side A and a side B

theres also anodes, and cathodes. those deal with current flowing in and out of a pre-amp tube respectfully.

theres also emitters, but i wont go there. the other thing you need to know is that theres 2 more common ways of running a preamp tube. either as a gain stage or as whats called a cathode follower.

a cathode follower is exactly that, the current flowing through the preamp tube using the triodes kind of like a filter, and not a set of gain stages (triode A and triode B).

a phase inverter tube is the last preamp tube before anything is sent to the power-amp section of the board. basically, you dont want triode A and triode B to be un-equal. you want matched sides in this stage - something only a tube tester can tell you.

if the sides are matched, you will get WAY better open-string harmonics - as there is no distortion being sent to the high-voltage, low current power tubes.

the job of the output transformer is to turn the high voltage, low current into low voltage high current for speaker loads.

you can put a gain-stage preamp tube like the other tubes in the phase inverter stage and use it like a gain stage. a common mod for marshalls is to do just this and also put a volume control on either side to gate the volume like a master volume.


...hope this helps you out a bit in understanding why and not just "yes it will work or no"

 

[AlX]

Hi,

thank you for your reply. I'm not much of a tube amp expert, but as I understand it, you need to use different types of for example plate resistors for 12AX7s and 12AT7s, and since i don't know the schematic for the RH50T and bought it 2nd hand, i was just wondering what gets in there for the phase inverter stage by stock.

I believe it's a 12AT7, however if you even look up the specs at randallamplifiers.com or some reseller, they tell you that it has four 12AX7s in it, which confuses me a bit. Also, somebody told me that if i replaced the 12AT7 inverter-stage with a 12AX7, the amp might not work like it should due to oscillation issues? Thank you for any hints.

 

[glpg80]

Hi,

thank you for your reply. I'm not much of a tube amp expert, but as I understand it, you need to use different types of for example plate resistors for 12AX7s and 12AT7s, and since i don't know the schematic for the RH50T and bought it 2nd hand, i was just wondering what gets in there for the phase inverter stage by stock.

I believe it's a 12AT7, however if you even look up the specs at randallamplifiers.com or some reseller, they tell you that it has four 12AX7s in it, which confuses me a bit. Also, somebody told me that if i replaced the 12AT7 inverter-stage with a 12AX7, the amp might not work like it should due to oscillation issues? Thank you for any hints.

no sir. you do not need different plate resistors for the 12at7 or 12ax7.

the length of the emitters inside the tube and tube design is different, but it their voltage requirements dont change THAT much to require different plate resistors for each.

the phase inverter tube, like any other tube, has 2 sides. i already said this earlier =]

the randall amp you have is not a class A type amplifier, its class A/B or class B.

this means its a push/pull or simillar to a push pull. the phase inverter tube's job is to take the preamp gain, distortion, eq settings, and basically purify it so that it can be amplified in the power-amp section. you can use a 12ax7 that isnt matched in the PI stage and get more gain. but if you crank the volume up, your tone is going to change significantly as well. this is because the power tubes are going to show the imperfections of unbalanced triodes. and like i said, you can only get matched triodes with a tube tester. thats not really something you can turn a knob with.

the 12AT7 you have is not as high-gain as a 12ax7 or an ECC83. the major differences between the 12ax7 and an Ecc83 is that there is none. one is european style the other isnt.

as long as the triodes are matched then your cool. nothing else to it. if its not matched its not gonna hurt your amp or your power-amp tubes. just be aware that your tone will differ depending on your volume - which isnt the same as simply adding power tube distortion that alot of people including myself love.