Reverb and Driver tube: 12AT7 or 12AX7?

[EETStudent]

Ok Im trying to understand this better. From what Ive gathered, here are the differences(that would have contribution) in the two.

1. AT7 has lower gain(about 70) compared to 100 of an AX7.
2. AT7 has a plate current of about 10mA, compared to 1.2 of an ax7.
3. AT7 has a transconductance of 5000, vs 1600 in an AX7.

Now, how do these affect the reverb and the phase inverter stages? Mainly the phase inverter, but the reverb concerns me too(I guess for that matter tremolo stages as well). I know they 12at7 produces a cleaner signal, but what does the current and transconductance have to do with it?

[mooreamps]

From my understanding, and others may feel free to chime in on this one. It's because it takes about one watt to drive a reverb tank. Secondly, the gain of the stage is not important since the drive signal for most reverb stages come after either the first or second gain stage in the pre-amp. Therefore, you don't as much "gain" to drive the tank, but you need to source more current than what you can get from a 12ax7 to do it.

Secondly, just my approach to the whole thing, I am not picking either the 12ax nor the 12at for a reverb driver. I have seen a couple others use the 6V6, which is a better choice, but in my mind not the best choice because that tube carries too much bass for reverb. Therefore, and keep in mind I've not biult this up yet so I have no data to support this, but in my mind the most obvisous choice would be the EL-84 ; and I have an excellent tranformer to match up that power tube to the 17 inch tank I'm planning to use for my circuit. Now, as I mentioned, we only need about a watt or so to drive a tank, and the EL-84 is a 5 watt tube. Seems to me by using either a signal drive control, and/or my electronic power brake for this tube, I should be able to dial in the proper amount of drive into the tank ; no worries.

-g

[Chuck H]

I know that Traynor used an EL84 to drive a reverb tank on some models. I had one briefly. It sounded very good. I also have a transformer that would be a good match for this and I've been planning to try it.

Chuck

[tubeswell]

You can use a 6BM8 as well, and use the pentode side for the driver and the triode side for the recovery, similar to using and EL84 to drive it, but only using one tube instead of 2 for the whole thing

[Steve Conner]

Like others have said, a 12AX7 doesn't have enough balls to drive a reverb tank. I used a 12AU7 in self-split push-pull for my last reverb driver.

[defaced]

How about as a phase inverter? AX7, AT7, or AU7?

[mooreamps]

You can use a 6BM8 as well, and use the pentode side for the driver and the triode side for the recovery, similar to using and EL84 to drive it, but only using one tube instead of 2 for the whole thing

That is a very, very good point.

-g

[mooreamps]

How about as a phase inverter? AX7, AT7, or AU7?

Hmmmm. Quite honesty, I've not played with any phase inverters, but I would think the AU7 given it's low plate resistance to better drive the grids of the power tubes.

Secondly, I'm not thinking of using a pre-amp PI for my next project. I'm thinking rather, using a split power amp , using one PA tube for the dry signal, and the other PA tube for the wet signal. Just seems to me a more elegant way of doing it.

-g

[Steve Conner]

For a PI, all three tubes will work. The 12AU7 is pretty feckless in the classic LTPI guitar amp circuit due to its lack of gain, but I've got it to work in a floating paraphase PI with some tweaking. I've seen Williamson-type hi-fi amps that used nothing but 12AU7s.

[EETStudent]

Well the reverb in my amp runs off a 12AX7, but the reverb isnt very strong even at maximum level. I subbed a 12at7, and it weakened the reverb. Are we saying that a 12at7 has even smaller ball than an ax7?

[Enzo]

You are making the mistake of looking at the tube as if it were the whole circuit. It is not. The 12AT7 has lower amplification factor than the 12AX7, so if the 12AX7 has enough balls to drive a reverb pan in some circuit, then the 12AT7 will also work at lower volume. In other circuits though, the 12AX7 might not have the power to handle the circuit around it.

Transformer driven reverbs are like little power amps driving tiny speakers. But there are also higher impedance drives. AMpeg used this in many models - the reverb input end is high impedance and driven througha cap off the plate of a triode. 12AX7 might make more sense there. But note it is the circuit that tells you what kind of tube you want, not the other way around - you want the small transformer circuit, you need a small power tube. Seems to me I have also seen - for want of a better term - differential drives where the reverb transducer was essentially wired between plates of a PI sort of circuit. A little push pull stage.

[booj]

I think the T has the balls for a reverb driver, and also has the headroom, partly due to lower gain, for a driver stage.

[6267]

Just finished a 6bm8/ecl82 self-split (350 ohm shared cathode resistor, no bypass, one grid as input, the other grid grounded, 2.2k screen dropping resistor and ~290v B+)...sounds great, and it doesn't hum like the SE reverbs do from time to time. The triode halves sound really good as well, definitely a worthwhile setup!

[tubeswell]

Just finished a 6bm8/ecl82 self-split (350 ohm shared cathode resistor, no bypass, one grid as input, the other grid grounded, 2.2k screen dropping resistor and ~290v B+)...sounds great, and it doesn't hum like the SE reverbs do from time to time. The triode halves sound really good as well, definitely a worthwhile setup!

Sounds interesting. Presumably the pentode side is the driver. Are you willing (and/or able) to post a schematic?

[chipaudette]

I would suggest that you review historical amps to see the difference between 12AX7 as a reverb driver (or PI) versus 12AT7 as a reverb driver (or PI).

For example, blackface Fenders (at least my Deluxe Reverb Reissue) uses both halves of a 12AT7 single-ended, in parallel as the reverb driver. It then uses one half of a 12AX7 after the reverb as make-up gain. It's a classic design and works well. I'm not sure if all Fenders do this...it would be worth looking at some other schematics.

By contrast, the Mesa Boogie Mark series amps use a single 12AX7. Half of the tube is ued to drive the reverb and the other half is used as make-up. This has a lower tube count, which is generally good. How does it sound? Don't know...I don't have such an amp...but being pretty famous, it must certainly be acceptable.

So, I think that the best course of action would be to do a little comparision of a few different amps. I think that you'll find that Enzo's comment is true...the circuit determines the amp. If you're building your own amp, you can flip that around...you can choose a circuit to fit the tube compliment that you feel like including.

If you don't know what tube compliment you feel like including, we come to a very interesting question...how does the driver tube affect the sound or feel of the reverb? Now that raises my eyebrows. Anyone?

Chip