Help me figure out what the 12AT7 in my Mooer Tube Engine does?

[Rex_Rocker]

I have a Mooer Tube Engine. It's a 20W all-tube poweramp. It's fun and not all that loud. Probably not great, though. Cheapie Chinese stuff with kind of undersized trannies. Good for home use and small jams, though. However, last time I used it, it was having problems. There were some loud crackling noises. Sounded like a tube issue to me, but with all the recent tube price hikes, I haven't bought it new tubes. I am now visiting my home town, and I have access to my old tube stash, so I'll be bringing some back to replace the tubes.

It runs on 2 EL84's (I assume either chathode-biased or non-adjustable fixed bias because I see no bias pot anywhere inside), 1 12AX7, and 1 12AT7.

I'm 60% certain the 12AX7 is the phase inverter. It's located right between both power tubes in the circuit board.

I am however, not sure what the 12AT7 is doing. Not sure if it's an input stage or actually the phase inverter or what. Because if it is the PI, I don't have any balanced 12AT7's, and I'm going to need to buy a new one. Also, if it's an input stage, I'd prefer something with short plates (ala JJ) to avoid microphonics.

So what do you think it's doing? Thanks!

 

[Rex_Rocker]

I already put aside two nice pairs of NOS Russian 6P14P-EV's and a couple of balanced Sovtek 12AX7LPS's (just in case the 12AX7 position is indeed the PI). However, the only 12AT7 I have in my stash is a non-balanced long-plate Mullard (New Sensor, not NOS). I remember that tube being kinda microphonic in both my EVH's and Dark Terror's FX loop driver position. But I always found those positions were kinda sensitive to microphony.

Do you think the Mullard will do or will I have to get something else?

 

[Demanic]

Well, if it is the PI, I don't think that being unbalanced would make anything blow up.
It might not sound great though.
I'm sure that there's someone here who could expand upon this.

Sent from my SM-A115A using Tapatalk

 

[beaubrummels]

If you say the transformers are small, and the amp running on EL84s, I might have guessed the 12AT7 was the phase inverter. Looks like the master/presence controls' ribbon cable connects to where the 12AX7 is and the 12AT7 is near the OT and speaker out end. But looks like you can see the board they are on and potentially recognize where they are in the circuit.

How would you get a non-balanced 12A?7 tube? (I haven't heard of this. Both triodes are in the same package, how would they be unbalanced?)

 

[DankStar]

Digging the simplicity of the design. What do you run through it and does it warm up the sound?

 

[Demanic]

If you say the transformers are small, and the amp running on EL84s, I might have guessed the 12AT7 was the phase inverter. Looks like the master/presence controls' ribbon cable connects to where the 12AX7 is and the 12AT7 is near the OT and speaker out end. But looks like you can see the board they are on and potentially recognize where they are in the circuit.

How would you get a non-balanced 12A?7 tube? (I haven't heard of this. Both triodes are in the same package, how would they be unbalanced?)

I think that it's not so much that they are radically unbalanced. More that companies will sometimes test batches of tubes and label those that fail within a tighter spec range as "balanced".

Sent from my SM-A115A using Tapatalk

 

[Rex_Rocker]

Well, if it is the PI, I don't think that being unbalanced would make anything blow up.
It might not sound great though.
I'm sure that there's someone here who could expand upon this.

Sent from my SM-A115A using Tapatalk

No, yeah. I'm sure I won't blow anything up. I just want it to sound the best I can.

 

[Rex_Rocker]

If you say the transformers are small, and the amp running on EL84s, I might have guessed the 12AT7 was the phase inverter. Looks like the master/presence controls' ribbon cable connects to where the 12AX7 is and the 12AT7 is near the OT and speaker out end. But looks like you can see the board they are on and potentially recognize where they are in the circuit.

How would you get a non-balanced 12A?7 tube? (I haven't heard of this. Both triodes are in the same package, how would they be unbalanced?)

I think with tube production being nowadays being all over the place as far as quality goes, both triodes don't always have the same amplification factor. So if you have that on a PI, one of the power tubes gets more signal than the other, and that might result in crossover distortion.

I'm sure it's never THAT bad. There are plenty of people who don't think "balanced triodes" make that much of a difference. There are even people who prefer the "complexity" of unbalanced triodes.

I'm sure the difference would be subtle, but given the option, it's always like 1 or 2 extra dollars to pay for balanced triodes from tube vendors. Might as well pay the extra buck or two to have the peace of mind of at least knowing that on paper, the amp is working as it should.

 

[Rex_Rocker]

Digging the simplicity of the design. What do you run through it and does it warm up the sound?

I run my HX Stomp through it.

I don't know if it warms up the sound, TBH. I was wanting to buy a Duncan Powerstage or an Orange Pedal Baby at the time, but I found this for relatively cheap, and I'd rather have the tubes in there than the extra wattage. I rarely play outside my house. If I were jamming with a drummer, I would've gotten the Powerstage for sure. But I don't. So rather than having the loudness and headroom, I chose to have the tube-y-ness. But I never A/B'd with something solid state, TBH.

 

[GuitarDoc]

12AX7 is usually the preamp tube, and the 12AT7 is usually the PI tube. I've never had a problem with an "unbalanced" PI tube.

 

[jeremy]

ive never had an issue with unbalanced pi tubes either. i think a lot of that was marketing bs to get a few more bucks. yes, it does in theory make a difference, but whats the chances the resistors that feed each side are identical? slim at best. i dont worry about it as long as it sounds good.

 

[Ewizard]

Crackling can be a multitude of things. It can be a bad plate resistor to any one of the 4 plates of the preamp tubes. It could be a dirty socket with bad connection to the tube pins, it could be a bad cap that is leaking DC where it shouldn't. It could just simply be a bad tube.

The 12AT7 is likely the Phase inverter.

There is really no such thing as balanced tubes. Just tubes that, when new, were within a tolerance of each other. It is fairly difficult to get two tubes to be nearly identical and still have the same wear rate. The same goes for dual triode tubes such as 12A_7's. They can be close, but what difference does it really make? In the phase inverter position, it may be arguable that it makes a difference, but you can always change a component to get the value closer ( Dumble did that in some amps with a pot called balance ).

In guitar amps, this is of little benefit though. We are often looking for more disparity in order to create more complexity and harmonics. So a mismatched phase inverter could be theoretically better in that instance.

Process of elimination will be your friend. Change one tube at a time and see if the problem goes away. If it does, leave the good tube in that position, or replace it with another good tube. If that doesn't work, clean the tube sockets. If that doesn't work, you have to start getting at the amp's Kibbles-N-Bits.

 

[NegativeEase]

I always start with checking power source, visual and poke inspection of components, pot twists etc, cleaning sockets and reseating tubes and/or swapping tubes one variable at a time before anything else.

In the long run, this solves the slight majority of noise issues in a methodical way and keeps you from going crazy jumping around with guesses.

 

[Rex_Rocker]

So I just dropped in a Mullard (NS) 12AT7, a balanced Sovtek 12AX7LPS, and a pair of NOS Russian 6P14P-EV in the little poweramp, and I liked the results. Louder and slightly less brittle. I love Chinese tubes, but 6P14P-EV's are really the only EL84's that I can tolerate, LOL. And no microphonics from the long plate preamp tubes either. It worked solve the crackling too.

 

[Ewizard]

glad to hear that it worked out! Always start with the easy stuff.