# Can you functionally test a 12AX7 for balanced triodes in an amp?

#### printer2

##### Poster Extraordinaire
You can measure the balance of the tube and the circuit together, but if what you're wanting to do is work out what the actual transconductance and anode resistance is for each triode, you're not going to be able to do that within that circuit.
What goes in and what comes out are what the amp cares about the stage.

#### Phrygian77

##### Poster Extraordinaire
Gold Supporter
What about current draw for each triode? That's what I think I'm looking for, or at least the way I've heard "matched triode" 12AX7s described by retailers.

E.G., if I popped a 12AX7 into V1 of a Princeton Reverb, both triodes would have a 100k plate resistor off of the same node, and both cathodes would have a 1k5 cathode resistor. Would measuring and calculating the voltage drop across the cathode resistor not allow me to calculate the plate current draw in the same way it would with a power tube?

There are variables, i.e. the plate and cathode resistors, that put each triode in a different spot on the grid curves. Even if you know the exact resistances of those resistors, you can't accurately compare if the plate-to-cathode and grid-to-cathode voltages are different.

Edit: additionally, this would only reveal what each triode is doing in one spot on its curves.

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#### mfratus2001

##### NEW MEMBER!
I'm in need of one 12AX7 with balanced triodes and I'd prefer to find it from amongst my many on-hand tubes without needing a tube tester--hoping the more electronic engineering-inclined among us might be able to school me on whether or not there's an easy/effective way to test a 12AX7 for balanced triodes when installed in a working amplifier. Wondering if I can just calculate by measuring the voltage drop across the plate and/or cathode resistors etc., but I'm not knowledgeable enough to know quite what to look for.

I've had good results testing and matching power tubes for plate current when installed in a fixed bias amp using bias probes--when doing that I've read that it doesn't really work with cathode bias, which is how preamp tubes are typically biased, right?

For the curious, I "need" this tube for an upcoming Trinity OSD build, which has a balance trimmer on the phase inverter designed to maximize "note bloom." Supposedly it's the easiest to dial in when the PI is balanced to start with.
Measuring Pin 1 and Pin 6 (the plates) in a phase inverter circuit will get you a DC balance for that tube in that circuit. Many PI stages have unequal plate resistors for proper results in the output stage - 100K and 82K. The amp you use is your test rig, so accuracy depends on your test rig. A scope will give a little more information about AC gain.

#### Phrygian77

##### Poster Extraordinaire
Gold Supporter
@ElliotKnapp I recommend Merlin's chapter on the triode gain stage if you haven't read it.

#### Tom Kamphuys

##### Tele-Holic
I can recommend the whole book. It's easily the best 30 bucks investment in tube amps. (btw, it looks like the power supply book which he promised wouldn't come is actually coming soon.)

The long tailed pair phase inverter (which I assume you are talking about from what I found on the internet) is explained in chapter 9 of the guitar pre amp book.
The LTP PI is inherently unbalanced. That's why a lot of amps have 82k & 100k (unbalanced) plate resistors to counteract that. Some need other values but simply copy fender... In the video you can see a 110k & 120k resistor; in a schematic I found online 100k & 110k.
The longer the tail (the resistance in the cathode), the less influence the tube has on the imbalance. In hifi tube amps, the tail is often a constant current source which has a near infinite (AC) resistance. In that case the gain is *solely* determined by the plate resistors.
Sometimes this disbalance is actually sought after as it will cause second order harmonics.

See also http://valvewizard.co.uk/acltp.html That is the website of the author of the book. The website is also a great resource.

#### telemnemonics

##### Telefied
What about current draw for each triode? That's what I think I'm looking for, or at least the way I've heard "matched triode" 12AX7s described by retailers.

E.G., if I popped a 12AX7 into V1 of a Princeton Reverb, both triodes would have a 100k plate resistor off of the same node, and both cathodes would have a 1k5 cathode resistor. Would measuring and calculating the voltage drop across the cathode resistor not allow me to calculate the plate current draw in the same way it would with a power tube?
I’m not certain of all the factors and situations, but while current draw is the measured method used to both match power tubes and bias them, I think we also know that higher current draw does not universally equal higher amplification factor or output.

So if the goal is to label each preamp tube for current draw of each triode, that’s not the same goal as having a balanced sine wave in the PP output section.
Is that correct?

Even two triodes of a PI that draw the same current, plus two power tubes that draw the same current, does not guarantee a balanced sine wave?

I think it’s safe to say that current draw does not directly translate to amplification factor or output?
Helpful yes, but if the goal is a balanced sine wave there are other methods like a D style PI balance pot?

More asking than stating here, but for example an older worn tube draws more current while producing less output or amplification.

Some used preamp tubes may have a more worn out triode and a less worn out triode yet the triodes current draw is closer to “matched”.
Some circuits only use one triode, assuming you're testing used tubes.

Not sure how correct my comments are or if it’s helpful to the goal, and not sure what the goal is either!

#### loopfinding

##### Friend of Leo's
Fair enough, I can be satisfied with a practical approach for achieving the end result without a balanced tube, but my curiosity still remains for whether or not you can easily measure triode balance functionally in an amplifier circuit--maybe an o-scope is the ticket.

to be honest, if you're comfortable getting your hands dirty, then screw even matching power tubes too - install a bias adjust AND a balance. matching tubes is kind of a pain in the ass unless you like busy work...consider your balance pot for the PI a blessing.

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#### peteb

##### Friend of Leo's
E.G., if I popped a 12AX7 into V1 of a Princeton Reverb, both triodes would have a 100k plate resistor off of the same node, and both cathodes would have a 1k5 cathode resistor. Would measuring and calculating the voltage drop across the cathode resistor not allow me to calculate the plate current draw in the same way it would with a power tube?

I think that you are on the right track.

4. How are power tubes matched?

Power tubes are paired up based on their current draw, or "plate current". Measuring plate current draw under operating conditions is generally considered to be the most accurate method for rating a power tube, and it is the method we use.

#### ElliotKnapp

##### TDPRI Member
to be honest, if you're comfortable getting your hands dirty, then screw even matching power tubes too - install a bias adjust AND a balance. matching tubes is kind of a pain in the ass unless you like busy work...consider your balance pot for the PI a blessing.
Agree, and have had good results with a balance pot on a severely mismatched pair of 6V6s in a Delulxe Reverb I built, although it won't work on amps with bias vary tremolo. Dig the Keiji Haino pic!

#### ElliotKnapp

##### TDPRI Member
@ElliotKnapp I recommend Merlin's chapter on the triode gain stage if you haven't read it.

I can recommend the whole book. It's easily the best 30 bucks investment in tube amps. (btw, it looks like the power supply book which he promised wouldn't come is actually coming soon.)

Just didn't think about cracking it open it re: the practical considerations of testing preamp tubes in situ.

The LTP PI is inherently unbalanced. That's why a lot of amps have 82k & 100k (unbalanced) plate resistors to counteract that. Some need other values but simply copy fender... In the video you can see a 110k & 120k resistor; in a schematic I found online 100k & 110k.
The longer the tail (the resistance in the cathode), the less influence the tube has on the imbalance. In hifi tube amps, the tail is often a constant current source which has a near infinite (AC) resistance. In that case the gain is *solely* determined by the plate resistors.
Sometimes this disbalance is actually sought after as it will cause second order harmonics.

See also http://valvewizard.co.uk/acltp.html That is the website of the author of the book. The website is also a great resource.

This is definitely a fun opportunity to delve more into the LTP phase inverter, as probably more than half of my existing projects have been cathodyne. This amp has the 100k/110k plate resistor setup.

#### GPoint

##### TDPRI Member

From the video description:
"Using an oscilloscope, and two probes, you can perform the balance of a phase inverter via this process. It should work with any amp, Connect two inputs to each side of the output of the phase inverter, before it goes into the power amp tubes, on each half."

You'll also need a sine wave generator, I use an old android phone and free function/wave generator app from the headphones 3.5mm stereo jack.

In short form:
Summing out of phase signals at the output of signal polarity inverter triode(s) on the oscilloscope must be as close as possible to zero (including clipping distortion). But it does not mean all the following processes (passive components, power tubes, output transformer) are balanced. The polarity inverter is just one stage in the amp.
Perfectly balanced tube amps are more for hi-end audio, concerning the subjective side of the guitar amp's sound/tone/distortion, discussion can be endless...