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?
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12AT7 vs AX7 in V1 for dynamics?
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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).
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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.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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Originally posted by g-one View PostMaybe 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.
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Originally posted by daz View Postreading 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?
Code:MU FOR 12A_7 PREAMP TUBES 12AX7 100 5751 70 12AT7 60 12AZ7 60 12AY7/6072 44 12AV7/5965 37 12AU7/6189 20
Originally posted by wizard333 View Post12AY7 would solve it to, if you can find one without a heater-cathode leak (good luck).
Thanks!
Steve AholaThe Blue Guitar
www.blueguitar.org
Some recordings:
https://soundcloud.com/sssteeve/sets...e-blue-guitar/
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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
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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.
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'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.
PeteMy band:- http://www.youtube.com/user/RedwingBand
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Originally posted by Steve A. View PostI 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.
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Originally posted by deci belle View PostI do use an AT7 still in the PI of the Fender Tube-reverb though.
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?
Originally posted by Rhodesplyr View PostI'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...?
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 AholaLast edited by Steve A.; 06-12-2013, 06:45 PM.The Blue Guitar
www.blueguitar.org
Some recordings:
https://soundcloud.com/sssteeve/sets...e-blue-guitar/
.
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Originally posted by pdf64 View Post'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. 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.
Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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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.
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Originally posted by Steve A. View PostI keep hearing that 12AT7's are not really audio tubes
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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.
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