The 12AX7 has a lot of gain (mu of 100 for those into technical stuff) and so is useful for voltage gain.
However the gain of the tube is traded off with a high impedance and so it is lousy for driving power tubes! Therefore a 12AT7 is employed (one could also use a 12AU7 but it has less gain). You can put a lot more current through a 12AT7 and it has a mu of 70, so you can drive your power tubes and do it with some bandwidth (back when we still used 12AT7 in our driver circuits we got well over 200KHz at full power; we since graduated to the 6SN7 which is a way better sounding tube for this sort of thing).
Phase splitters:
If your amp employs a differential phase splitter (sometimes called a 'long tail pair'; the circuit was made famous in the old Williamson Ultra-Linear design which has been copied endlessly in the world of high end audio despite the design being 60 years old...) the 12AT7 is preferred because its higher mu is desirable in such a circuit. However phase splitting can be done in other ways, such as a 'balanced phase splitter' (which is not really balanced, it just has equal resistances in the plate and cathode circuits). This circuit is very simple and so gets used a lot (the Dynaco ST-70 is a good example). If such a circuit is used quite often the 12AU7 is the tube to use (the Dynaco does not use a 12AU7 but the triode section doing the task has identical properties to the triodes in a 12AU7). Phase splitters are part of the reason that many people prefer SET amplifiers, as a phase splitter often contributes to a 5th harmonic in the amplifier design. For this reason even though we make a push-pull amplifier we don't use dedicated phase splitters.
So Wolf, if you play a guitar this also explains why your preamp sections of your guitar amplifiers are using 12AX7s, but there is usually a 12AU7 associated with the output tubes (an alternative driver tube seen in the old Marshall Plexi was the 12BH7, which is in essence a 6SN7 in a miniature envelope; as such a better sounding tube than a 12AU7...).
By comparison without feedback its hard to get a 12AX7 to go much over about 12KHz. The impedance is too high and stray capacitance is a problem. So 12AX7s are rarely used to drive power tubes because they suck too much at it; instead are used for gain in preamp sections. |
I bought a Class A single ended boutique guitar amp and the manufacturer (Burriss) swears by new Mullard 12AX7s, and they seem to work fine. |
I need some spare 12ax7s and 12at7s for my jolida 3502s. I have the mullard 6201 gold pins and the russian Gold Lion 12ax7s from Upscale. Are the new mullards ok? Thanks Ted |
My understanding is that the 12ax7s are the input tubes, have the greater influence on tone, and the 12at7s do the driving. It seems the EH 12at7s are relatively well regarded in the driver role so I'm using them (they're stock with a new JD502P), and switched the Tung Sol 12ax7s to a matched pair of Thetubestore's "preferred" ones (select Chinese it would seem) and I love 'em...they provide a little more sparkle, tonal accuracy, and aren't expensive. |
Jed,
You want to get matched gain. I'd recommend balanced triodes as well, but that one is not as essential. I wouldn't worry too much about the other options. And Tom is correct that the pre-amp tubes on that Jolida are the 12AX7s and they will make the biggest difference in terms of shaping the sound. The 12AT7s are driver tubes and you shouldn't spend as much on them. Get something quiet and reliable and don't worry about their supposed sound characteristics. Spend plenty of time listening to the amp with what's in there now & decide what you want to change regarding the sound -- post that here & you'll get loads of recommendations. I have some Jolida gear as well, they're great bang for your buck amps.
Batch |
Back to the original question, are the 12AX7's the input tubes and the 12AT7's the driver tubes? Is it true that the input tubes have a greater impact on sound quality than the driver tubes? TIA. |
Maybe Atmasphere will see this thread and post a reply;if he does you can not get any better information in my opinion;also advice so far has been pretty good. |
Hi Marakanetz and Batch68 From seeing the tubes that came with the Jolida 502P I see I don't have to have matching 12AT7 and 12AX7 tube brands. It came with EH 12AT7 tubes and Tung Sol 12AX7 tubes. I normally like having uniformity all around but hey if it is ok for Jolida it should be ok for me. I was wondering about the necessity of matching the gain of each triode of the preamp tube as well as the plate current draw of each triode being the same. Here are some examples. Please see the Options check boxes http://www.dougstubes.com/preamp-tubes/12at7-ecc81/penta-12at7.htmlhttp://www.dougstubes.com/preamp-tubes/12ax7-ecc83-7025/penta-12ax7.htmlIf matching isn't necessary I was just going to go to the music shops by me and get the Groove Tubes equivalent of these tubes. They aren't matched at all from what the associates tell me. Thanks again for the help. |
Jed,
If you are asking about matching the AT7s to the AX7s, then no. If you are asking if you should have matching AT7s (or AX7s), then I'd strongly recommend it (not absolutely required, though). |
Jedinite, you don't have to match 12AT7 or 12AX7 at all. |
Stupid questions coming up. Does this mean that it is a must for the 12AT7 and 12AX7 tubes to be matched (each triode of the preamp tube has matching gain) and balanced (when the plate current draw of each triode is the same)for the Jolida 502P? I found that some tube sellers add a surcharge for this service. |
Wolf, who knew your amp was such a 'swinger'..... |
I appreciate all of this...although I think my comprehension capabilities are being woefully overestimated. I'll look for the schematic...but bear in mind my amp is a 502P basic power amp, NOT an integrated, hence the query. |
Sorry I kept saying 12AU7 cos my personal unit is modded as a power amp not an integrated.
Let be go over this again.
Each channel has ONE 12AX7 as gain stage.
Each 12AX7 is setup in a totem pole configuration.
The output of the 12AX7 is connected to one triode of one of the triodes of the 12AT7s directly, then via a 1Meg resistor to the other triode of the 12AT7. The first one "pushes" the other one "pulls"
Each of the 12AT7s has its plate coupled using a coupling cap to a 6550, EL34, KT88 whatever you have in the amp.
Just download the schematic and spend 5 minutes looking at it and you wont think my explanation is too convoluted.
So why does it need 2 stages before the power tubes?
If the input were just the diffamp (12AT7) stage, then you will need a preamp, that swings 30-50V to drive that amp.
A CD player can only swing a few volts, so thats why the need for the 12AX7 gain stage.
The driver stage cannot have too much gain, otherwise it cannot deliver the current required to drive the power tubes and will "wimp out" and distort. |
Good answer but Johnsonwu...come on dude...you know me...I would NOT understand a schematic. I could take a basic electronics course at a local community college but it would drastically cut into my nap time. Also, the Jolida doesn't have 12AU7s but my tiny headphone amp does have one. Just one. And it's cute. Your post has only one tube I'm actually employing in my amp but I sort of get it anyway. I think your explanation, although convoluted, has dragged me kicking and screaming toward almost understanding this thing. Closer...I'm getting closer... |
Download a copy of the schematic available at many websites.
The 12AU7 is an SRPP totem pole configuration voltage gain stage where the output is the midpoint between the first half triode's plate and the 2nd half triodes cathode.
The 12AT7 is 2 triodes wired in a differential amplifier config where the signal from the 12AU7 is split into two, faciliated by the 1Meg resistot that connects between the 2 grids. one phase that is used the "push" one of the KT88s and the other half triode is uised to "pull" the othet KT88. |
What point about phase inversion? I do understand the concept of the signal level boosting my preamp does, but wondered why 2 similar tube pairs, 12AX7 and the lower gain 12AT7 are utilized in the power amp...I think I did read someplace about signal splitting...so we're close! |
Good point about the phase inversion. I believe the more common method is to use the 12AX7 for amplification and the 12AT7 for splitting the signal into the push/pull components. I haven't messed with the insides of Jolidas much so not sure if they follow that convention. |
The mentioned tubes are also dual triodes that are either used in differential connection or split the signal onto positive and negative for push-pull operation. |
The idea isn't very complicated. Amplification typically needs to be done in stages. If you try to get too much gain out of a single stage, you end up with inadequate output, distortion, non-linearity, noise and other problems.
The music sources (CD player, tuner, tape deck, etc.) you hook up to your amp provide only line level voltages, usually about 2 volts max. That's not enough to drive the output tubes directly -- they typically want several times the voltage supplied by the line level input.
Hence, one uses an interim stage with small signal tubes to boost the input level to that needed by the output tubes. (For the term "driver", think in terms of pushing along -- driving cattle -- as opposed to operating a car.) |
