Wish Additional 12AX7 Vintage Preamp Tubes (Mullard, Telefunken, Brimar)

Joker

Joker

Inspired
We already have some RCA and Sylvania 12AX7 tubes and, considering the fundamental impact they have on the tone (V1 magic is real), it would be nice to have a Telefunken, Mullard, Brimar, Siemens or even "newer", non military-grade brands like Tung-Sol or Electro Harmonix.

Another thing I've noticed is that there are only 12AX7 / ECC83 style preamp tubes. Woulnd't 12AT7 or 12AY7 preamp tubes present some nice opportunities for experimentation on vintage Fender amps? Considering how different 12AX7 of different brands sound, there has to be signifcant difference between 12AX7s and AT7 or AY7 apart from volume...

What do you guys think?
 
Joker said:
We already have some RCA and Sylvania 12AX7 tubes and, considering the fundamental impact they have on the tone (V1 magic is real), it would be nice to have a Telefunken, Mullard, Brimar, Siemens or even "newer", non military-grade brands like Tung-Sol or Electro Harmonix.

Another thing I've noticed is that there are only 12AX7 / ECC83 style preamp tubes. Woulnd't 12AT7 or 12AY7 preamp tubes present some nice opportunities for experimentation on vintage Fender amps? Considering how different 12AX7 of different brands sound, there has to be signifcant difference between 12AX7s and AT7 or AY7 apart from volume...

What do you guys think?
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12AY7 is correct in the Tweed Deluxe input socket (Tweed Bassman also, IIRC). The following stage and split-load phase inverter are 12AX7, though.
Not sure if the modeling algorithm is not set up to deal with multiple types in different spots or if the UI is just not set up to present those choices to the end user....
fry-not-sure.png
12AY7, 12AT7, and 12AU7 all have different gain and output drive impedances. 12AT7 and 12AU7 make decent small power output tubes in the 1W range. The old Fender reverb amps all use 12AT7, for example, to drive the reverb springs via a tiny single-ended output transformer that drives the 8 ohm load of the reverb tank's driver.
 
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One of the first mods I did on just about every amp I've owned is put a Tung Sol or Electro Harmonix 12AX7 in V1 and an EH 12AT7 in the phase inverter slot. I believe this changed how the amp responded and altered the gain structure more than it made a tonal difference.

While I wouldn't complain if there were more preamp tubes added, tweaking a couple of parameters can get you in the ballpark. The Amp block's Input Dynamics set @ 1.50, Output Compressor>Feedback type in the Dynamics page set @1.50-2.00, and the Negative Feedback in the Power Amp page. The lower the Negative Feedback value, >0, the 'looser' the amp feels. Amps that don't have Negative Feedback I'll typically leave it off and tweak the Dynamics page a bit more.

I have no clue exactly what these parameters/circuits do in the amp, just found these parameters by twisting knobs and liked the results.
 
I can hear no real difference with any of the virtual tube swaps. Wouldn’t mind more options, but don’t know how significant of change it would be.... can’t see the already excellent Fender models, et al, suddenly sounding any more “realer’ because a fancy rare preamp tube option exists.

again, just because I can’t hear it doesn’t mean it’s not there, but I think one also needs to limit options to some degree before it becomes kind of silly. How many tube types is enough ? 10, 20, 30 ? Where do we get to a point where the differences are so subtle as to make it subjectively no worth adding any more options ?
 
I wonder if changing preamp tubes in the AFX is similar to changing the power amp tubes.

i.e., you're changing a 6L6 in a perfectly biased amp with a perfectly matched output transformer to an EL34 in a perfectly biased amp with a perfectly matched output transformer when you change it in the AFX. This makes it a subtler effect than doing it in a real amp, because in a real amp unless you swapped OTs too then the transformer will be mismatched in one of the two configurations.

Maybe swapping the preamp tube types is similar: You're switching from a perfectly in spec 12AX7 to a perfectly in spec ecc83, rather than in the real world where different tubes have different imperfections and bias characteristics that can be adjusted with other advanced parameters on the AFX.
 
IronSean said:
I wonder if changing preamp tubes in the AFX is similar to changing the power amp tubes.

i.e., you're changing a 6L6 in a perfectly biased amp with a perfectly matched output transformer to an EL34 in a perfectly biased amp with a perfectly matched output transformer when you change it in the AFX. This makes it a subtler effect than doing it in a real amp, because in a real amp unless you swapped OTs too then the transformer will be mismatched in one of the two configurations.

Maybe swapping the preamp tube types is similar: You're switching from a perfectly in spec 12AX7 to a perfectly in spec ecc83, rather than in the real world where different tubes have different imperfections and bias characteristics that can be adjusted with other advanced parameters on the AFX.
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I have a feeling that it is exactly like changing power tube type is, insofar as you likely need to change not just the type, but also some of the related ancillary settings like the HF bandwidth, hardness, bias, etc., to ensure that the way they interact with the preamp circuit is correct for the new tube type.
 
In my Axe I wired up a preset to do a "diff" between the same amp type (in opposite phase) to see if I could measure and hear the "difference" between preamp or power tubes in the same amp.

Unfortunately even with identical AMP1 and AMP2 settings (after resets), they actually don't entirely cancel out. I'm not sure it's due to subtle processing timing differences or tiny level differences or something else. The net result is a ringmod kind of sound at a very low level (-40 dB? but I haven't measured it). Sometimes I think I can hear dithering as a kind of white noise. I also put an LFO'd SYNTH sine wave through them both which also gives a residual signal. Both are visible in the RTA block with sufficient level bump / compression.

I did the same with the same drive block settings and the cancelation is almost complete with a tiny residual. Changing diodes on one drive does give an audible difference (after level correction) which sounds ringmody as well.

I want to play more with this and will post a preset soon and maybe some recordings.

BTW, if you do this yourself be careful with cranking levels up as making the slightest change to one of the drives or amps, the net signal can jump + 20 - 40 dB. Another option might be to re-amp the same exact amp block (e.g. AMP1) and do the diff in a DAW, but I'm not set up to do that.
 


Nice timing on Pete's part! I agree, the changes are subtle, but, at this point in time, we shouldn't look for huge improvements anyway. The amps are practially indistinguishable from their real world counterparts, it's more about adding different flavors to the mix.

If you spend enough time, you could get every tubescreamer based pedal from the standard TS 808 model, but something tells me that people would much rather choose the jam ray instead of tweaking the bias and eq settings in the TS 808 menus.

These preamp tubes should be much less time consuming to model than all those pedals with dozens of resistors and capacitors and they add more to the complexity of the amp overdrive and distortion behavior than a normal boost or pedal would.
 
I agree that more preamp tube options would be cool.

It's hard to know if those preamp tube differences in the video were at least partly or mostly due to not adjusting the surrounding circuit values leading to differences in tone/gain/saturation. If the Axe's preamp tube + circuit modeling is 'normalized' or auto-biased (similarly to power amp modeling), the differences might be more subtle. But preamp tubes might operate in a narrower range and perhaps direct swaps w/o normalization might work virtually.

Found this for reference from http://www.aikenamps.com/index.php/technical-q-a:

Q: Can I substitute a 12AT7/12AU7/etc. for a 12AX7?
A: Yes and no. While they are the same pinout, the tubes are very different with regards to internal plate resistance, bias current, gain, etc. It won't usually hurt anything to substitute them, but it is not a fair tonal comparison if you don't also change the cathode and plate resistors to optimally bias the tube. A straight swap without regard for circuit operating conditions will lead to incorrect conclusions regarding the characteristic tone of the tube.

Q: Can I change the preamp cathode or plate resistors to change the gain or tone of my amp?
A: Yes, but you should change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing, or to the place it was originally biased. If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip very asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs. The two go hand-in-hand. If you change the cathode resistor, the plate resistor should (ideally) be changed, and if you change the plate resistor, the cathode resistor should be changed, unless you are designing for an asymmetric bias point. At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand. People tend to view preamp tubes differently from power tubes, but in reality, they are the same animal, just on a different scale. Preamp tubes need to be biased, just as power amp tubes do, and there are ranges that are more optimum than others. This is why it is not a fair test to just swap different tube types and compare tones. A 12AX7 will require different plate and cathode resistors than a 12AT7, for instance, for optimum bias point and tone.
 
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yyz67 said:
I agree that more preamp tube options would be cool.

It's hard to know if those preamp tube differences in the video were at least partly or mostly due to not adjusting the surrounding circuit values leading to differences in tone/gain/saturation. If the Axe's preamp tube + circuit modeling is 'normalized' or auto-biased (similarly to power amp modeling), the differences might be more subtle. But preamp tubes might operate in a narrower range and perhaps direct swaps w/o normalization might work virtually.

Found this for reference from http://www.aikenamps.com/index.php/technical-q-a:

Q: Can I substitute a 12AT7/12AU7/etc. for a 12AX7?
A: Yes and no. While they are the same pinout, the tubes are very different with regards to internal plate resistance, bias current, gain, etc. It won't usually hurt anything to substitute them, but it is not a fair tonal comparison if you don't also change the cathode and plate resistors to optimally bias the tube. A straight swap without regard for circuit operating conditions will lead to incorrect conclusions regarding the characteristic tone of the tube.

Q: Can I change the preamp cathode or plate resistors to change the gain or tone of my amp?
A: Yes, but you should change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing, or to the place it was originally biased. If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip very asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs. The two go hand-in-hand. If you change the cathode resistor, the plate resistor should (ideally) be changed, and if you change the plate resistor, the cathode resistor should be changed, unless you are designing for an asymmetric bias point. At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand. People tend to view preamp tubes differently from power tubes, but in reality, they are the same animal, just on a different scale. Preamp tubes need to be biased, just as power amp tubes do, and there are ranges that are more optimum than others. This is why it is not a fair test to just swap different tube types and compare tones. A 12AX7 will require different plate and cathode resistors than a 12AT7, for instance, for optimum bias point and tone.
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Lemme 'splain.

No, is too much. Lemme sum up....

For the TL;DR folks: yes you can direct swap 12AX7, 12AT7, 12AY7, and 12AU7, but there's a bit of a 'Goldilocks' thing going on with the supporting circuitry designed to work best with a particular tube type (the 'just right' case), in which other tubes will find it 'too cold' or 'too hot'.... ;)
 
Joe Bfstplk said:
Lemme 'splain.

No, is too much. Lemme sum up....

For the TL;DR folks: yes you can direct swap 12AX7, 12AT7, 12AY7, and 12AU7, but there's a bit of a 'Goldilocks' thing going on with the supporting circuitry designed to work best with a particular tube type (the 'just right' case), in which other tubes will find it 'too cold' or 'too hot'.... ;)
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And in some amps it's "way too hot" and burns out your tube that wasn't meant to deal with the voltage/current needs of that stage. ;)
 
IronSean said:
And in some amps it's "way too hot" and burns out your tube that wasn't meant to deal with the voltage/current needs of that stage. ;)
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I never had a preamp tube burns out by swapping. Tipical guitar preamp voltage (fender, mashall, mesa, ....) are all in the ballpark and current are negligible.
 
IronSean said:
And in some amps it's "way too hot" and burns out your tube that wasn't meant to deal with the voltage/current needs of that stage. ;)
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Generally only when subbing in the reverb driver spot in Fenders and similar circuits.

Most regular preamp stages and phase inverters have high enough plate and cathode resistances to not worry about damaging anything.
 
The question is if virtual preamp tube swaps in the Axe require surrounding circuit changes or if it could work fine with direct tube swaps (no "normalization").

Either way, could be fun!
 
Regarding the "difference" of two amps, both @DLC86 and I seemed to have discovered that two of the same drives in inverse polarity cancel but two of the same amps don't. (EDIT: Expected behavior and Cliff has clarified the reason which explains why turning PA modeling off results in cancelation.)

In doing some more exploration, two of the same amps with power amp modeling off (sag=0) do seem to cancel (with temporary artifacts that dissipate). So it appears that Fractal's power amp modeling has something uniquely non-linear or subject to hysteresis or other effects that are partially independent of the input signal. See this post for a preset.


EDIT:

According to Cliff amps (or power amps) in the Axe may not perfectly cancel due to different AC line phases (both in the unit and with two external recordings) but with DC supply type they should cancel. I confirmed in Scene 4 of the preset above changing both amps to DC gives almost complete cancelation with a tiny residual along with a kind of noise rumble at a very low level.

Amp difference with AC supply type (Scene 4):

rta_1.jpg

Amp difference with DC supply type (adjusted Scene 4):

rta_2.jpg
 
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