12AT7 vs AX7 in V1 for dynamics?

[daz]

reading about the current differences a while back it was said in the phase inverter using a T instead of a X would be more dynamic. Since a cascaded pre's V1A is a big part of the dynamics picture, do you think that if you used a T instead and upped the gain later on in the gain stages to make up for the T's lesser gain that you'd have a more dynamic tone?

[jazbo8]

Sonic difference between the X and T aside, I don't think it will make much of difference with either T or X in the V1A position, since the input level is very low and there is a lot of headroom (unless I mis-understood your definition of dynamics).

[g1]

Maybe more of an issue of current capability. For the phase splitter driving the power tubes, current is an issue. Less current capability here means more compression which means less dynamic range.
For the V1 stage, current capability is not an issue. The difference of gain here will have more of an impact. I guess if you are clipping somewhere, using a 12AT7 at V1 will reduce gain so again less compression (wherever the clipping is) will mean more dynamic range. But I would think backing off your guitar volume would have the same effect, whereas backing off your volume will not overcome the phase splitter difference.

[daz]

Maybe more of an issue of current capability. For the phase splitter driving the power tubes, current is an issue. Less current capability here means more compression which means less dynamic range.
For the V1 stage, current capability is not an issue. The difference of gain here will have more of an impact. I guess if you are clipping somewhere, using a 12AT7 at V1 will reduce gain so again less compression (wherever the clipping is) will mean more dynamic range. But I would think backing off your guitar volume would have the same effect, whereas backing off your volume will not overcome the phase splitter difference.

Thanks, seems you are quite correct. i didn't mod it for more gain, but a T7 had enough gain so i could add a couple notches on the knob and have the same amount. Dynamics were the same. Mind you, i'm not having any issues with that. But you know.....your mind is always thinking "what if". It's sorta like G.A.S., but for mods rather than gear acquisition.

[wizard333]

12AY7 would solve it to, if you can find one without a heater-cathode leak (good luck).

[Steve A.]

reading about the current differences a while back it was said in the phase inverter using a T instead of a X would be more dynamic. Since a cascaded pre's V1A is a big part of the dynamics picture, do you think that if you used a T instead and upped the gain later on in the gain stages to make up for the T's lesser gain that you'd have a more dynamic tone?

I keep hearing that 12AT7's are not really audio tubes and I really haven't heard very many of them that sound good as preamp gain stages. So for lower gain I would usually use a 5751 for ~70% of the gain of a 12AX7 or a 12AY7 for 44% of the gain. Here is a chart of approximate gain values of the various 12A_7 tubes.

MU FOR 12A_7 PREAMP TUBES

12AX7          100
5751            70
12AT7           60
12AZ7           60
12AY7/6072      44
12AV7/5965      37
12AU7/6189      20

Many current production amps have way too much distortion for me so I will often replace the 1st and/or 2nd preamp tube with a 5751. While the lower gain tubes usually offer more headroom I haven't noticed them improving the dynamics, which I associate with touch response- how you can clean up the sound by picking softer or turning down the volume control on your guitar. Is that what you are referring to by a dynamic tone?

12AY7 would solve it to, if you can find one without a heater-cathode leak (good luck).

Can you elaborate on that problem? Are you referring to current production tubes, new old stock or used old stock? Just wondering...

Thanks!

Steve Ahola

[jazbo8]

From the GE datasheet: "The 12AT7 is a miniature, high-mu, twin triode designed for use as a grounded-grid radio-frequency amplifier or as a combined oscillator and mixer at frequencies below approximately 300 megacycles." So its original use is for RF not AF. If you look at the plate characteristic charts, the curves are more evenly spaced (i.e., more linear) for Eg>0 than Eg<0, so for typical audio use with Eg<0, the response would be rather non-linear, perhaps that's good for certain effects, but not so good if linear voltage amplification is the goal.

Jaz

[deci belle]

I've been experimenting a lot with preamp and PI tubes lately because I was turned on to a bunch of old tubes and can say that the AT7 tube is much less interesting (even Bugleboy and Mullard AT7) in the PI than an AX7, but that depends on what your after, I suppose.

I always used to use the AT7 at the PI, but now i like a bright, neutral-quality AX7 tube like the Telefunken smooth-plate or a early 60s GE in that spot. Maybe for mellower tones you might want to try the 5751 over the AT7?

I only have a few preamp circuits to experiment with, but have come to prefer USA 5751 tubes (not modern Russian ones) almost exclusively in the first gain-stage as well as the cathode-follower of an old-style Bassman/Marshall circuit as well as V1 of AB763. They are really that good (to my style of playing and hearing).

I used to use a 5751 in the PI of the 5E3, but now I much prefer a bright, neutral AX7 along with an AY7 or 6072 in V1.

In the normal channel of a Blonde Bassman I still like an AX7, but a smoother sounding RCA, Mullard or Bugleboy or the Matsushita AX7~ because the 6G6 channel has a 220k plate resistor that seems to like the AX7 more than a 5751.

Some of these pulled old tubes that test strong are less than new Russian or Chinese offerings~ like only $15 on the internet. But NOS ones usually start around $30~40.

I have come to feel that an AT7 isn't such a musical tube anymore, now that I've been able to try out a palette of other tubes, in most cases.

I do use an AT7 still in the PI of the Fender Tube-reverb though.

[pdf64]

'Maybe more of an issue of current capability. For the phase splitter driving the power tubes, current is an issue. Less current capability here means more compression which means less dynamic range'
Are you thinking that a 12AT7 in the LTP (or other) phase splitter slot of a regular tube amp could enable the power tubes to operate in AB2, ie control grids driven positive?
I can't see that would be possible, as the impedance of the grid when driven positive is low, around 1-2K I think.
So in almost any voltage amp circuit driving such a load, a 12AT7 will get clipped the same as a 12AX7.
My understanding is that for a power tube grid to be driven positive in a meaningful, linear manner, some sort of intermediate power amp is probably needed, with output impedance <100ohms and plenty of voltage swing available.
Pete

[Rhodesplyr]

I keep hearing that 12AT7's are not really audio tubes and I really haven't heard very many of them that sound good as preamp gain stages.

I've installed the tubes4hifi 3 x 12AT7 driver board in a Dynaco ST70 for a friend, and it sounded really good. Maybe it's the circuit...?

[Steve A.]

I do use an AT7 still in the PI of the Fender Tube-reverb though.

The Fender 6G15 tube reverb has no PI so I suspect that you are referring to the BF/SF combo amps with reverb- right? I've never tried a 12AT7 in the PI of other amps but prefer it in the combo reverb amps which I believe are biased for the AT7 and not the AX7. Although some people like the grit and drive of the 12AX7 in there...

I have read about one NOS 12AT7 which actually worked well as an audio tube but I forget what it was. For lower gain I would stick with the audio tubes: 5751, 12AY7, etc. Does anyone actually make a 5751 these days or do they just rebrand low gain 12AX7 rejects?

I've installed the tubes4hifi 3 x 12AT7 driver board in a Dynaco ST70 for a friend, and it sounded really good. Maybe it's the circuit...?

What is a 3 x 12AT7 driver board? I thought that the ST70 uses 7199 tubes to drive the EL34 tubes... As you say the circuit is very important- in guitar amps I always think of the EL34 as being a great tube for distortion but there it is in the Dynaco ST70.

I guess I should be more specific- I don't think that the 12AT7 works well as a conventional preamp tube in a guitar amp. When I have tried them in that application they sound kinda blah and uncolored which could be a positive quality in hi fidelity audio.

Steve Ahola

[g1]

'Maybe more of an issue of current capability. For the phase splitter driving the power tubes, current is an issue. Less current capability here means more compression which means less dynamic range'
Are you thinking that a 12AT7 in the LTP (or other) phase splitter slot of a regular tube amp could enable the power tubes to operate in AB2, ie control grids driven positive?
I can't see that would be possible, as the impedance of the grid when driven positive is low, around 1-2K I think.
So in almost any voltage amp circuit driving such a load, a 12AT7 will get clipped the same as a 12AX7.
My understanding is that for a power tube grid to be driven positive in a meaningful, linear manner, some sort of intermediate power amp is probably needed, with output impedance <100ohms and plenty of voltage swing available.
Pete

Thanks Pete, don't know what I was thinking when I wrote that, maybe reverb driver or solid-state driver or something, definitely didn't have the brain in gear . Clearly power tubes don't need much current driving them otherwise we would need bias circuits that could also supply much more current than they can.

[wizard333]

I think it just depends on what the circuit was designed for. Marshalls and many other amps use 12AX7s in the driver stage and sound/feel fine. They also use 12AX7s for verb drivers and work just fine. However, if you put a 12AX7 instead of a 12AT7 into an old Fender, IMO, they sound/feel god-awful. Screechy, unpleasant, weird pick attack artifacts, and just not right. Usually when I get this call "Hey this SF/BF Fender I just got doesn't sound right, it breaks up too early and sounds thin and harsh" my first response is "see if someone stuck a 12AX7 in the driver. If so, replace it with a 12AT7." Even when modding one channel significantly on them, which I do quite a bit, I leave the 12AT7 in there as that PI/Power section just likes it a lot better, whether you're running the amp clear or dirty.

[Merlinb]

I keep hearing that 12AT7's are not really audio tubes

The 12AT7 is noisier than most triodes, and exhibits fairly high grid current. It's a good tube for hifi and cleaner applications, but for overdrven guitar preamps it is difficult to beat the 12AX7 for all-round good-guy-ness.

[deci belle]

Hi Steve~ embarrassingly, I don't know the circuit on my 6G15 RI!! It usually has an AX7, an AT7 and a 6V6.

I did a little tone-cap mod (bigger), substituted an AU7 for the AX7 and a 6K6 for the 6V6. I tried a 5751 instead of the AT7 but eventually went back to the AT7.

I'll study the schematic for it and find out what the AT7 does… haha!!

I don't know about anyone but old USA tube-makers ever producing real 5751. I get those JAN kind. I tried the Russian ones but I wasn't impressed with how they compared to the way the real 5751 tubes sound.