What tube for Phase Inverter?

The PI doesn't need to be matched with the preamp tubes. A 12AT7 will push less gain but more current for some extra headroom.

 

Ya 12AT types are rugged for PI slots

5751 and 12ax are a tad too much IMHO.

 

I love 12AT7 for PI in my project too.
And cathodine better than long tail pair...

 

That's exactly what I did in my Pro Junior (also 15W EL84), and I liked the results.

 

You won't notice a bit of difference but it is recommended that you use a tube for the the PI that has matched triodes. As other member stated it doesn't have to match the other pre amp tubes necessarily

 

12ΑΤ7

 

The noisiest tube you own....

 

If, after replacing the phase inverter with a 12AT7, you find the amp no longer makes a sound, remove the chassis and replace the glass fuse. The current draw of the 12AT7 is rather higher than the 12AX7.

As for matched triodes, leave that to the audiophile/hi-fi lot. A slight mismatch can sound rather nice. I don't want to go down the pretentious twazzock route of trying to describe it beyond 'more musical', so forgive me for that.
Even quite a drastic mis-match won't harm anything. Think of it like a twin cylinder motorcycle with twin carbs. You have to balance the carbs to get even power from each cylinder. with the carbs balanced, the engine thrums an even tone. Slightly out, it sounds different, but no great harm occurs.

 

Some circuits use a single triode for the phase inverter, so if it does matching is of no importance.

Also, if stock calls for a 12AX7, wouldn't it be better to use the same?

 

From the Eurotube page so you can take it for what's worth. I agree with other member saying if the circuit was designed for a 12ax7 that's what you should use. If you're putting strain on enough to blow a fuse by using a different tube that should tell you something

should I use a balanced one?A phase inverter or splitter is the tube that keeps your power tubes ( in a push pull A/B type of amp ) running 180 degrees out of phase. This is necessary because in a push pull amp one tube creates the upper half of the sine wave while the other tube creates the lower half. One of the symptoms of an unbalanced phase inverter is that some notes will sustain very well where others will fall off or sound dead. In 99% of all guitar amps the PI tube is the closest preamp tube to the power tubes and 99% of all A/B style guitar amps use a circuit that splits the phase inverter duties utilizing both triodes in this tube.We get questioned quite a bit on the subject of balanced PI tubes so we’ll elaborate a little. Most PI circuits are not designed to be perfectly balanced. Ah ha!!! Say what? So it’s snake oil then? No, we’ll explain. Most all circuits using long tail phase inverters are designed with a slight amount of asymmetry. The reason for this is because when both sides are dead on, the amp will sound a bit sterile and be missing that bit of 3D swirl that makes a tube amp sound alive. This is also the case with a way out of balance circuit, it will sound sterile and certain frequencies will suffer from phase cancellation. Logic says (we’re big fans of logic and common sense) then why balance the current on an imperfect circuit? The short answer is to get the result the designer wanted, but we’ll go further into this. When we acquired the test equipment to check for balanced triodes we started stuffing balanced tubes in everything we have to see if there was a real difference or if it was snake oil. The results? In some amps it was nothing short of amazing and in others it was noticeable but not jaw dropping.A good friend of Eurotubes who has a Marshall 6100 30th anniversary, (one of those amps Bob just loves to hate, he calls it the little Blue bastard), had fought getting a good tone with it for years so he brought it back in a few months after we started using balanced PI tubes and when we changed the PI tube for a balanced tube we were all astounded at the change. The amp simply went from so-so at best to, holy crap, is that the same amp!! So, morale of the story? It’s just like the old argument that a rectifier tube does not change the tone of an amp because it’s not directly in the tone circuit. This of course is completely false and anyone who has ever done an A/B and compared a chinese, a sovtek, a JJ and different NOS rectifier tubes will tell you there is an amazing difference in dynamics and tone. A good balanced PI tube does make a difference and it’s not snake oil. All you need is a pair of ears to hear the difference.You will still find a lot of skepticism and bad press from the "nay Sayers" on the benefits of using balanced PI tubes. The only question we ask them is "have you personally had experience in listening to the difference between balanced and unbalanced PI tubes in different amps?" Oh, and request an honest answer when posing the question.

 

I just got a Monoprice 15 watter.
Through swapping tubes; a few 12ax7, a 7025 and 5751 I have put a RCA 12AX7a in the PI and have left the original tubes in 1 and 2. The other 12AX7s were JJ, Tung Sol and other Chinese. I specifically bought the long plate JJ for the PI, but it didn't have the warmth of that RCA.

The old RCA (short grey plate) adds volume/headroom and bass/body that doesn't fart out.

With that amp I think you need to decide how much distortion you want from gain and match tubes to that.

I also replaced to stock EL84 with some JJs that sound moderately better than stock. I picked them up so I'd have another set for backup, the stock Chinese are now backup.

I think it's a pretty good amp, have fun with it!

 

1. Correct. For example, a Princeton isn't a push-pull PI design and uses only one of the two triodes in its 12Axxx for PI. A matched-triode tube is a waste of money in its shared position in the circuit.

2. There isn't a compelling reason for sticking with the same tube type, other than "it sounds right to me." For example, a Princeton uses the other triode of the same 12Axxx tube in its driver stage to add gain to the signal going to the PI circuit. I have an SCX2 with the same driver, PI, and output stages as a Princeton. It came with a 12AX7 which I switched out for a 12AT7: the PI triode may provide a little more current the output stage and the driver triode lowers the amount of gain to clean things up, a bit. One might change the tone characteristics to some degree by substituting 12AX7, 5751, 12AT7, 12AY7, and 12AU7 tubes in that position as they will provide different amounts of gain and, to some degree, current handling. One of these days, when the motivation strikes, I'll roll through those and see what my ears think.

EDIT: Ok, to address the OP's original question/issue. I've looked at the Monoprice schematic and it appears to be a push-pull PI that ***gets switched with a relay*** which is something I've never seen before. Again, roll any 12Axxx tube you want in the V3 position and leave in the one that sounds best to you. IMO a matched-triode makes a numeric difference, and may make a perceivable difference in a clean and accurate as possible stereo, but likely makes no perceivable difference in a cranked guitar amp as the distortion goes up.

 

It could make things better. Or worse. Or no difference at all. But just changing the tube will do that, regardless of whether it was matched or not. Until you see 1% or better resistors and caps all over the power amp, you're not really matching both sides of the power section anyway.

 

the neat thing about tubes is it is easy to A/B test them,

try a Mullard CV4024 in there and tell me what you think,

 

Exactly. You don't need a matched PI tube in classic Marshall circuits because they designed imbalance into the PI stage for a specific harmonic content.

Even if you had 1% resistors, caps and "matched" triodes, you would still need to have matched windings on the output transformer and you don't even want to know how mismatched those can be!

I attempt to balance out my power stage by placing the cooler running tubes on the primary winding with the highest resistance, and the hotter running tubes on the winding. It can make a difference of several watts per tube.

 

Yes. And good luck on those 1% caps, too.

 

I've got some.

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Also, FWIW, you can create a 'balancing' setup similar to what's in Dumbles and Ampeg SVT's. You can adjust the balance/imbalance in the PI and fine tune the harmonic content pretty tightly.

And lastly, even with wider tolerance caps, you can just hand choose values that are within 1% tolerance of each other by measuring with a meter

 

Let's not even think about component drift!