12AX7t; 12AU7

[kmenard]

Are these swappable with the 12AX7 in 18watts?

[kleuck]

12AU7 are NEVER swappable with 12AX7, too much differences in current, Vpol etc.In fact, when you read the datasheets, it's obvious that the only similarities between 12AX and 12AU are the bulb and the pins (well the heater voltages and current too...)
However, your amp will not blow if you do swap one (in some amps, you can blow a pair of tubes)
You can substitute 12AX7 with 12AY7, 5751, most of the time 12AT7.

[BooBird]

12AU7's will work fine and so will a 12AY7 !
12AT7's work good for your Phase inverter or as a Driver for your reverb,
How they may sound in your 18 Watter, I don't know, but the 12AU7 will lower the gain and the AY7 will lower it a lil more, you may like it and you may not, your ear's will have to decide,
But you can truck over to Schematic Heaven and check out all the Schematic's there, you'll see where most tube's are used in each amp.
Boo

[kmenard]

I put 12ax7t, I meant 12ax7a...I just got a few 12ax7a mullards from an old amp!

[zaphod_phil]

IMO there's a measure of truth in both of these views. It is certainly true that the characteristics of 12AU7s and 12AT7s are very different to those of 12AX7s. Both the AU7 and AT7 bias differently to AX7s, and tend to draw a lot more current. They also have very low impedance on the output side - aka "anode impedance". As you probably know AU7s and AT7s can also be used as power tubes in low-power amps, especially the AU7. However, it is also true that a typical preamp stage or PI will usually still work in some fashion when you swap between these tubes. However, with 12AT7s you need to be careful that they never get overdriven, as they then tend to generate a lot of high-order odd harmonics (especially 9th), which make the tone sound pretty blah, as I discovered one time.

The original question is whether you can do that in an 18W. I think the best answer is that it depends. In a Classic/trem or Lite 18W amp, there is only a single gain stage before the PI, so the amp pretty much needs every little bit of gain that it can get, and the relatively low gain of AU7s, AY7s and AU7s will generally mean that the amp won't reach the full output power and distortion levels of an 18W amp.

In an 18W trem amp the tremolo oscillator will only work with 12AX7s of certain specific gain levels, so there's not much opportunity to try other tubes there. The Normal preamp or the preamp of a Lite can certainly use a 5751, which is like a lower gain 12AX7. Some folks have reported success with using 5751s in the PI, if they wanted a cleaner-sounding version of an 18W - what I would call a castrated 18W. However, you can get pretty much the same effect by reducing the EL84 grid resistors from 470k to 220k. 12AT7s may also work in the PI, although they will bias themselves a lot warmer, as well as having less gain. Personally, I wouldn't even both trying an AU7, due to the huge reduction in gain.

With a TMB amp, there may be more opportunity to use some of these lower gain tubes, although I have to say the idea of someone doing that to a perfectly good 18W amp appalls me.

12AX7As are equivalent to European ECC83s - they just have a different warm-up time to the older regular 12AX7s. And Mullard ECC83s are real nice tubes!!!

[Merlinb]

I put 12ax7t, I meant 12ax7a...I just got a few 12ax7a mullards from an old amp!

If it starts with 12AX7... or ECC83... then they're all exactly the same valve. The extra letters are just bonuses in old valves, and gimmicks in new ones.

[Phil_S]

IMO there's a measure of truth in both of these views. <snip>

The original question is whether you can do that in an 18W. I think the best answer is that it depends. <more snip>

With a TMB amp, there may be more opportunity to use some of these lower gain tubes, although I have to say the idea of someone doing that to a perfectly good 18W amp appalls me.

Nice post, Phil. I hope you don't mind my edit down to the bare bones on which I'd like to add a few comments.

12A?7 tubes are all pin compatible, which means you can almost always swap them without electronic consequences (like something blowing up). That doesn't mean the 12AU7 is a good sub for the 12AX7.

12AU7 and 12AT7 have low plate resistance and will draw more current. Will your power transformer support this? In most cases it will. Think before you act. Your typical 12AX7 draws under 3mA and the U or T will probably draw 10-12mA. If you swap one, OK, but 3 may be too many.

The 5751 and 12AY7 have higher plate resistance and are much closer to the 12AX7. This make them somewhat more suitable as subs from an electronics perspective, but they both have less gain. Again, think before you act.

Finally, I'd agree with Zaphod that putting these low gain tubes in an 18W raises the question, "What's the point?". If you are hankering to use these other tubes, there are other amps out there that work well with them. Get one of those.

Now, on the off chance that maybe you are looking for a less expensive sub, you might get lucky with some old stock 6EU7's. These are a 12AX7 equivalent, but the pinout is different, and requires rewiring the socket.

[zaphod_phil]

Now why I couldn't I make it simple and easy to understand, like that?

[PeterS]

Your typical 12AX7 draws under 3mA and the U or T will probably draw 10-12mA

In some circumstances that's true, but in a typical stage with a 100k plate resistor, none of those variations draw more than a couple of mA.

[Phil_S]

Now why I couldn't I make it simple and easy to understand, like that?

I, too, suffer from verbosity at times. It's easier to edit than to compose.

[kmenard]

I played with the 12au in the normal channel...I actually kind of like it...it doesn't have the grind of the trem channel, but its nice to have something voiced a bit differently.

[BooBird]

I put a 5751 in mine when i wanta change ! and i can alway's put it back org. with just a change of the tube !
Boo