# Preamp Tube with Two Different Plate Voltages

Discussion in 'Amp Tech Center' started by Dan_Pomykalski, Sep 3, 2020.

1. ### Dan_PomykalskiTele-Meister

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I noticed last night that one of my 12AX7s had a plate voltage of 190 volts on one side and 217 on the other side. I swapped it with a different 12AX7 and got 180 on both plates. How common is this, and does it mean anything (like my tube is about to go)?

2. ### tubegeekFriend of Leo's

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Measured at the plate itself? (Not the high voltage supply?)

Then it means that your 1st tube has two loosely matched sections and your 2nd tube has two closely matched sections. No conclusion to draw about the lifetime of either tube, though the close match does indicate good quality control at the factory.

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3. ### Dan_PomykalskiTele-Meister

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Measured at the load resistor, both of which were 99.4k.

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4. ### tubegeekFriend of Leo's

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Decimal points in tube circuits are rarely worth bothering with. As you've seen, the tubes themselves vary quite a bit. But that's a nice close match between both subcircuits.

5. ### tubegeekFriend of Leo's

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Let's compare the three measurements:

I'm going to straight up guess the supply voltage (at the other end of the loads.) 300V.

Voltage drops:

300-190=110
300-217=83
300-180=120.

Rearranging ohm's law to find current I, we have I=V/R. (Pro tip: express R in Kohms and I comes out directly in milliamps.)

110/99.4=1.106 mA
83/99.4= 0.835 mA
120/99.4=1.207 mA.

The new tube, in addition to being closer matched, biases a tiny bit hotter than either section of the older tube.

Pretty close to a +/- 20% range if the spec is 1 mA, which is (slightly) interesting.

Before we go too far down that road, it'd be nice to check my "300V" guess against reality. If it's actually higher than 300V, the actual range would be smaller %-wise, and vice versa.

Given how unpredictable preferences are, you might like the new tube better, you might like the old one better, you might not hear any difference at all.

6. ### Dan_PomykalskiTele-Meister

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I can’t remember the B+ for sure, but I think it’s like 325? I’ll check once my wife gets home and I’m off child guard duty.

I’ve been experimenting with plate voltages a lot recently, and I’ve concluded the differences are subtle enough that I don’t have a strong preference either way. The first tube with the different voltages is a Sovtek WA. The other tube is a Mullard (the long plate one). I love the WA. I just got the Mullard and so far I’m pretty indifferent towards it. The Mullard kind of just sounds like a pretty standard 12AX7. Plus the one I got was microphonic, which might be influencing how I feel about it.

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What amp is this?

8. ### Dan_PomykalskiTele-Meister

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An AB763 kind of... it’s cathode biased though. It also has a Vibro Champ tremolo, a 5U4GB rectifier, and the normal channel is wired like a Bassman, but more accurately than the Fender Custom Deluxe (or whatever it’s called).

Last edited: Sep 3, 2020
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9. ### Dan_PomykalskiTele-Meister

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B+ is 312V

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10. ### tubegeekFriend of Leo's

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122
95
132

The three currents will be just a touch higher in proportion. No meaningful difference from the guesswork value.

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11. ### Dan_PomykalskiTele-Meister

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Just realized I missed a really hood opportunity to mess with you and say something like 750 volts.

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12. ### tubegeekFriend of Leo's

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Which would be a great opportunity to run the wattage formula and see what you'd need to use for those blown 100K plate resistors!

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13. ### Paul G.Friend of Leo's

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You must remember the voltage drop is dependent on 2 things, the resistance and the current. Different tubes draw different currents. In fact, the same tube will draw different currents at different points in its life.

In general a 10% variation from "ideal" voltages is just fine. One other thing, except for the phase inverter, each triode is doing its own thing, so the rule is if it sounds good, it's right.

14. ### connucharTele-Meister

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eye have encountered this before,it's just a tube anomaly as mentioned by some of the other learned posters . but it can be rather somewhat dramatic on a 12ax7 or 12at7 used in a phase inverter position ,eye experienced notable volume loss in the amps that eye had replaced the power supply caps on, but the volume wasn't up to par yet, eye checked the phase inverter resistors for resistance drift but they were in tolerance, eye was powerfully bewildered for a day or two so eye rechecked the schematic plate voltages and they were listed as a slight difference[5 or 10 volts ] where on meye amps [3different ones 3 separate times] ,eye had variations of 40 to 60 volts between the to plates, all three times a new tube fixed the problem for me so be aware of it as eye found out from reading a lot of books and posts that the phase inverter tube is driven pretty hard, so now when eye service an amp eye always check it and meye hair is growing back now that meye head scratching has been reduced at least for that kind of problem.

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Connuchar, ewe went to a lot of trouble.

16. ### Dan_PomykalskiTele-Meister

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I know it’s a different “aye/eye/I,” but I couldn’t help but get a piratey vibe while reading his post...

17. ### tubegeekFriend of Leo's

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There was a member here "Ole Fuzzy" years back who used to spell weird for fun also. His stuff comes up in searches a lot and seems to have been well respected, and I won't say any more because I think he may no longer be among the living.

Anyway I was picking up an "Ole Fuzzy" vibe, is why I mention it.

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I enjoyed Ole Fuzzy’s post even though they were hard to read at times. He was a font of Telecaster knowledge here. Iirc, he left TDPRI because of complaints about his use of his own brand of the Language. I was a member on the Weber pages at the time as was he, and I kept up with hi: there for a while. I lost track of him not long after that.

at any rate, Ole Fuzzy came to mind when I was trying to read Connuchar’s post, too.

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19. ### Paul G.Friend of Leo's

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The plate resistors aren't just there to drop the voltage to the plates, they also convert the current coming from the plate into voltage to drive the next stage. I wouldn't stray too far from the stock values there. If you need major changes to voltages (in this case, I don't see it), you're much better off changing dropping resistors on your filter caps.

20. ### tubegeekFriend of Leo's

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My comment was in the context of an imaginary 750V plate supply, not the actual circuit, as you can see by the quote in my posting.

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