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

ElliotKnapp

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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.
 

Bendyha

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Nothing against using balanced tubes.....but.....
A balanced tube, on paper, might let one achieve the most controlled shift from symmetric to offset, but the fact that there is an adjusting pot, to set "bloom", is acknowledgement of the fact that perfect symmetry is not always the most desirable state.

Is the idea to set the balance pot to specific measurements of numeric ratio ? I think not. The setting is done by ear, to achieve the sound that you like the best with that amp, and set of tubes, playing conditions.... In which case, I would try tube rolling on the finished amp. You have a selection of tubes, then use the one that sounds best, don't bother that it might be slightly unbalanced. Chances are that the coupling caps, plate resistors, power tube control-grid resistors, bias supply resistors, screen-grid resistors, power tubes and output transformer are not all perfectly matched and balanced either.

Balanced tubes are a hifi thing in my mind, where magic does not play a role. Too much scientific control, and the magic is stifled.
 

ElliotKnapp

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Is the idea to set the balance pot to specific measurements of numeric ratio ? I think not. The setting is done by ear, to achieve the sound that you like the best with that amp, and set of tubes, playing conditions....
Thanks for responding, but I'm not really deliberating about whether or not the amp can still sound good or be adjusted by ear for "note bloom" without a balanced PI tube (of course it can). The designer describes a method to make the adjustment that calls for a balanced-triode 12AX7, and I thought it would be fun to try it.

So, my question remains: is there an easy way to functionally test 12AX7 current draw in an amplifier circuit as a DIY matching method?

For reference, Trinity's instructions from the manual below:

Adjust the PI TRIM POT to get maximum “bloom’ when played.
Method:
NOTE: This may be difficult to achieve if the PI is not a matched triode tube. The objective is to
get a slight imbalance on the PI plates.
1. Set the trimmer for a 6.5 volt difference
[between the two plates] (I like the input side as the higher plate). Set the
amp clean (no boost, no mid boost, no overdrive, no bright) with all the appropriate
controls at noon. Use a neutral sounding guitar on both pickups with it's volume(s) and
tone(s) on 10.
2. Play these single notes on the 3rd,4th,5th frets of the B string - a D, E flat E. Let each ring
out before playing the next one (no finger vibrato please). In an ideal world they should all
bloom equally.
3. If the E blooms but not the D move the trimmer to make the input side lower by a half a
volt. Fine tune to taste by ear.
4. Most of the time the 6.5V is near perfect, have seen some amps do this better at a 3 volt
difference but it's less common in my experience.
 

ElliotKnapp

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Thanks for your criticism. Without knowing the voltages involved in his particular amp, I couldn’t really know those things, now could I? How did your post help?
Umm...aren't I the one asking for help here? What did you need my help with?

I know very little about oscilloscopes, but it seems logical to me that if somebody is recommending using one to measure current in a 12AX7, they would probably be able to offer advice like (as a totally uneducated example) "connect the probes across the plate resistor and use an X mV setting," but it sounds like maybe that's asking too much.
 

dougsta

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Just about as helpful as saying "digital multimeter" and walking off.

What settings on the scope? Leads connected where?



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.
 

printer2

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Just about as helpful as saying "digital multimeter" and walking off.

What settings on the scope? Leads connected where?
What have you done before, what do you have on hand to mess around with tubes? Much better knowing these details rather than throwing suggestions that are not practical.
 

ElliotKnapp

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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.

Fantastic, thank you!
 

ElliotKnapp

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What have you done before, what do you have on hand to mess around with tubes? Much better knowing these details rather than throwing suggestions that are not practical.
Thanks!
-I don't have a tube tester.
-I do have many functioning hand-wired amplifiers available to functionally test 12A_7 tubes if there's a way to do it.
-I do have accurate multimeters (my initial assumption based on biasing/measuring power tubes was that I might be able to measure current via voltage drop across cathode resistors like you can with a power tube)
-I do have a cheap digital oscilloscope (which I'll likely try out after viewing the video above).
-I've adjusted bias on ~20 fixed and cathode-biased amps using a variety of the well-known methods out there
 

Phrygian77

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Sorry, which advice did I disregard?

@Bendyha

Trinity's instructions don't say that you have to have balanced triodes. They say to adjust the plate voltage to get a 6.5 VDC difference. If that leaves the trimer in a position that doesn't let you adjust it much further, than you may need to try a different tube to follow their procedure. It's as simple as that.
 

sds1

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Not to gack this thread up anymore than it already is, but Dumble put a PI trim pot in his amps and the internet, in classic internet style, can't make up its mind on it's purpose or how to actually use it. There are several methods out there and they are all a means to a different end.

Using the trimmer to coax some sort of bloom mojo out of the PI seems to be the most popular adoption.

Based on the research I've done and a bit of my own (take with grain of salt) common sense, the method to adjusting the PI trimmer like Trinity suggests is not the one Dumble used or intended.

To me, it doesn't make much sense to adjust the PI to some arbitrary balance point when the power tubes and output transformer are just going to disrupt that balance further down the signal chain.

Rather, the PI adjustment pot can be used to balance the entire power amp, to achieve maximum clean operation.

The calibration procedure for this comes right off the Ampeg SVT schematic and takes into consideration the inherent imbalances in not only the phase inverter but also the power tubes and output transformer.

1649184937643.png


Source: https://ampeg.com/support/files/Sch...1972, 6550 tube)/SVT 1971 6550 Schematics.pdf

Essentially, bias each side of the power amp to same current dissipation, then drive the amp with test signal (40Hz in the case of SVT would be too low for a guitar amp, 100Hz would be more appropriate), then adjust PI trimmer for either a) least amount of THD at speaker jack or more practically, using just a DMM, b) equal current dissipation for each side of power amp.

Of course whatever calibration method or end game the builder prefers is fine, what sounds "best" is subjective, and we are welcome to just tweak that PI trimmer until we are satisfied... but if you want to know what the original designer intended the PI trimmer for, I believe the Ampeg SVT calibration method is the correct reference.
 

ElliotKnapp

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@Bendyha
Trinity's instructions don't say that you have to have balanced triodes. They say to adjust the plate voltage to get a 6.5 VDC difference. If that leaves the trimer in a position that doesn't let you adjust it much further, than you may need to try a different tube to follow their procedure. It's as simple as that.
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.

You completely disregarded the advice of someone who is very knowledgeable, and then asked how to use an oscilloscope to measure the output of a phase inverter.
Respectfully to you and Bendyha, I agree that balanced PIs are not necessary (or are probably even undesirable) for a good-sounding amp and that there are innumerable variables in the circuit that might make seeking "balance" an artificial exercise. I'm not disregarding the effectiveness of tube rolling and using your ears, it's just that this response didn't really address my question, which was predicated on Trinity's instructions and the key phrase "easiest to dial in when the PI is balanced to start with" and focused on learning if there's a DIY approach to measuring triode balance.

Your comment about maxing out the trimmer on an unbalanced tube seems to me to reach a similar conclusion regarding the process probably being easier with more closely matched triodes.
 

printer2

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Thanks!
-I don't have a tube tester.
-I do have many functioning hand-wired amplifiers available to functionally test 12A_7 tubes if there's a way to do it.
-I do have accurate multimeters (my initial assumption based on biasing/measuring power tubes was that I might be able to measure current via voltage drop across cathode resistors like you can with a power tube)
-I do have a cheap digital oscilloscope (which I'll likely try out after viewing the video above).
-I've adjusted bias on ~20 fixed and cathode-biased amps using a variety of the well-known methods out there
OK, you need a source for a sine wave as in the oscilliscope post (did not bother to view the video, my other computer is hooked up to speakers). You do not need a oscilloscope but could use a DMM, since you have one then it is a good time to learn to use it. You can calculate how much gain each triode section has in the circuit by setting up an input signal of 1V and then measuring the signal at the coupling capacitor. I am assuming you have a 10X probe for your scope. As said, you can take it further and include the power amp section in your balance experiment. You can check the gain of each output tube also. You could swap output tubes if it helps to get an equal amount of gain per side. Once it is balanced as well as can be done then adjust your pot while playing and use your ears. I have adjusted a paraphase inverter to the point where I find the sound pleasing and it had some imbalance in the circuit. But it depends on the style of music you are playing and your ears.
 

Phrygian77

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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.
 

ElliotKnapp

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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 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?
 

Red Ryder

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Don't know diddly squat about any of this and I don't know why I started reading any of it. But it sounds like you guys are arguing about some kind of crap ola, so I'm going to go punch my neighbor in the mouth and put an end to it.
 




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