Can a 12AU7 designed clean gain stage be as loud as a 12AX7?

[GlideOn]

What remains still entirely unknown or unanswered about in this thread so far is what a 12AU7 specifically would do if it were being met with the ideal B+ voltage, ideal Anode Resistor and ideal Cathode Biasing.

By now a reasonably intelligent and aware human should have figured out that the participants in the thread can't tell you what the ideal voltage, resistors, etc. and so on for a 12AU7 are, nor specificallyat it would do.
So go do it, and tell us what and how your did it - if actually figuring it out is your objective.
It's beginning to seem to me that your objective is to keep the thread going as long as you can.

Friend, this comment is uncalled for and unwarranted.

We are all here to learn and share our experiences, insights so that we can all gain something. I can't possibly know every scenario in which this would possibly work or not, which is why I opened this up for discussion. I have more than generously explained my current circumstances and given rich context, provided detailed illustration. It's more than enough to go by.

It's beginning to seem like you're not genuinely interested in the discussion and are just "kicking the tires" so to speak. I really hope I am wrong.

If you want to humor me and add something to the discussion, go ahead and do so and I will welcome legitimate feedback good, bad or otherwise. If not, it's your time, it's my time and everyone else's time to waste freely. Either way, I haven't gotten this far without possessing a certain amount of willpower and grace in knowing that sometimes you must indeed try things despite the contrary.

[Colossal]

What remains still entirely unknown or unanswered about in this thread so far is what a 12AU7 specifically would do if it were being met with the ideal B+ voltage, ideal Anode Resistor and ideal Cathode Biasing.

By now a reasonably intelligent and aware human should have figured out that the participants in the thread can't tell you what the ideal voltage, resistors, etc. and so on for a 12AU7 are, nor specificallyat it would do.
So go do it, and tell us what and how your did it - if actually figuring it out is your objective.
It's beginning to seem to me that your objective is to keep the thread going as long as you can.

Friend, this comment is uncalled for and unwarranted.

We are all here to learn and share our experiences, insights so that we can all gain something. I can't possibly know every scenario in which this would possibly work or not, which is why I opened this up for discussion. I have more than generously explained my current circumstances and given rich context, provided detailed illustration. It's more than enough to go by.

It's beginning to seem like you're not genuinely interested in the discussion and are just "kicking the tires" so to speak. I really hope I am wrong.

If you want to humor me and add something to the discussion, go ahead and do so and I will welcome legitimate feedback good, bad or otherwise. If not, it's your time, it's my time and everyone else's time to waste freely. Either way, I haven't gotten this far without possessing a certain amount of willpower and grace in knowing that sometimes you must indeed try things despite the contrary.

[MODERATOR]

R.G.,

Again, with respect, your reply is accusatory and combative and I will ask you for a second time within a month to check yourself. I agree with GlideOn in that, this thread is about his time, his topic, and his discussion, rhetorical, useless, interesting, or otherwise. Personally, I found it interesting and it gave me something to consider which led to two interesting days of reading about the AC100 (which has a 12AU7 in V1 and the PI) and discovering some past builds posted here on the forum.

If you have reached a point in this hobby where you've developed a lack of patience with those with less experience or just in general, kindly just refrain from posting if you can't be friendly and supportive. Or maybe it's time to take a break from the forum.

I seem to need to repeat myself of late with the reminder that this forum is about friendly discourse and the civil exchange of ideas and experiences with ampbuilding.

[/MODERATOR]

Personal Foul.gif

[cdemike]

What remains still entirely unknown or unanswered about in this thread so far is what a 12AU7 specifically would do if it were being met with the ideal B+ voltage, ideal Anode Resistor and ideal Cathode Biasing.

In keeping with the reference to cathode voltage in my previous post in this thread, you’d have greater input headroom, i.e., the 12AU7 gain stage in question could cleanly amplify a signal which is comparatively large and would distort a center biased 12AX7 stage at the same B+. As to ideal B+, it really depends on what your goals are, so there’s no real “ideal” for all case uses but since the goal in this instance is clean amplification, go as high as the data sheet will allow or as is high as is reliable in known uses (see: very high B+ on 6V6 and 12AT7 in Deluxe Reverbs). In your amp, the maximum of ~410 exceeds the data sheet maximum in theoretical terms but I suspect it’d survive use if with a potentially shortened lifespan. But the data sheet will tell you how high to make the cathode resistor; I’m not near a printer or a computer where I could digitally superimpose a line over the 12AU7 data sheet, but a quick and dirty approximation gives the following with a 47k load resistor:
Anode current: ~8.7ma;
With B+ at approx 410V, center bias point is ~15v at the cathode;
Therefore, Rk should be 1719 ohms with the closest standard value lying conveniently at 1K7. This of course may require fine tuning in real world practice, but it should be enough to set up a test circuit.

After thinking more about your amp, the false PI stage is almost certainly the first to distort. I’d consequently start by shunting signal from the tone stack to the 12AU7 instead of the 12AX7 and sending the resulting amplified “false phase inverter” signal to the power stage on the premise that it would offer the greatest improvement in headroom (assuming no other changes are made in the preamp).

[GlideOn]

What remains still entirely unknown or unanswered about in this thread so far is what a 12AU7 specifically would do if it were being met with the ideal B+ voltage, ideal Anode Resistor and ideal Cathode Biasing.

In keeping with the reference to cathode voltage in my previous post in this thread, you’d have greater input headroom, i.e., the 12AU7 gain stage in question could cleanly amplify a signal which is comparatively large and would distort a center biased 12AX7 stage at the same B+. As to ideal B+, it really depends on what your goals are, so there’s no real “ideal” for all case uses but since the goal in this instance is clean amplification, go as high as the data sheet will allow or as is high as is reliable in known uses (see: very high B+ on 6V6 and 12AT7 in Deluxe Reverbs). In your amp, the maximum of ~410 exceeds the data sheet maximum in theoretical terms but I suspect it’d survive use if with a potentially shortened lifespan. But the data sheet will tell you how high to make the cathode resistor; I’m not near a printer or a computer where I could digitally superimpose a line over the 12AU7 data sheet, but a quick and dirty approximation gives the following with a 47k load resistor:
Anode current: ~8.7ma;
With B+ at approx 410V, center bias point is ~15v at the cathode;
Therefore, Rk should be 1719 ohms with the closest standard value lying conveniently at 1K7. This of course may require fine tuning in real world practice, but it should be enough to set up a test circuit.

After thinking more about your amp, the false PI stage is almost certainly the first to distort. I’d consequently start by shunting signal from the tone stack to the 12AU7 instead of the 12AX7 and sending the resulting amplified “false phase inverter” signal to the power stage on the premise that it would offer the greatest improvement in headroom (assuming no other changes are made in the preamp).

And thank you for your feedback so far. I really hope this idea bears fruition.

I echoed the same idea earlier in thread too about going around the 4.7k dropper. I could tap from the 410v (likely a few volts lower), or place an additional dropping resistor a little further up and sinply reroute the shared 5k Screen Grid resistor a bit further back to free up and extra turret or two for said dropper.

Both endeavors would allow me access to the higher B+ and consequently allow better Anode matching to the lower impedance tube. It's a balancing act here and certainly aiming towards higher voltage than lower.

You are correct in stating that the V3 stage is distorting the soonest as it does in many Marshall and Fender "Long Tail" style PI.

The Master Volume I purposely put after in hopes of emulating the effect of a PPIVMV. The gamble paid off. After adding, I noticed this healthy increase in fatness, sustain and punch that wasn't quite there around 10am on the gain pot. Adds even more without too much flub when power tubes audibly distort around 12:30-1 on the Master. It's certainly a positive change overall, but it does lean towards a more distortion DNA rather than a clean one.

I am open to giving your idea a shot of using the tamer side of the tube to act as the PI - but to be frank, I really like the effect of a 12AX7 V3 distorting and adding fatness, punch.

I've even modded a Fender Twin to have Blackface 1M/470r/1M tail config with a 12AX7 and that has such ridiculous headroom and wattage that it still remains clean until high settings. My friend still uses that amp two years later and it sounds awesome.



I did have another idea though to remedy the distorting PI while leaving the 12AX7 side as-is:

Use the Master Volume Push/Pull function to swap positions of the leads of either Treble Pot Output or V3 output.

Essentially, one setting acts like the default Post-Phase Volume.

The other setting it acts like a standard 2203/Pre-Phase Volume.



I'll rework the layout a bit and post and example(s) soon.

[cdemike]

Sorry didn't mean that to read as chastising, more to clarify by establishing continuity with the previous post. Since the input to power stage is at ground potential, it should be easy to use a SPDT switch to set whether the "false phase inverter" is taken from the 12AX7 or 12AU7 sides of the ECC832, so no real need to sacrifice the gainer side if you're liking the way it sounds. In fact, if you like the way the two sides sound as your clean and dirty sides, you could set up a relay switcher pretty easily to change channels via a footswitch.

[GlideOn]

Not at all, you offer fair points all around and explained yourself thoroughly. Thank you for keeping it civil.

The V3 is a problem. I played a bit more with it today and there's a spot where it starts to clip hard and flub some for the cascaded mode. Even a bit in Plexi mode too with boost pedal. Granted the power tubes also distort moderately being lower wattage and cathode/class A, but that shared 5k screen is needed to keep the grid dissipation in check as my tech previously pointed out. There's really plenty of gain on tap so curtailing it may only help.

I may try an 82k like on the 12AX7 side of standard Long Tail PI to tame the distortion effect some. And/or DPDT or swap the tube sides, alternatively the treble pot/V3 leads. I'm not sure if there will be a massive volume difference, but whichever has the least dramatic effect of the two I will go with.

Worse case scenario I can decrease the coupling cap on V3 to a .0047 or .0022 like in some hot rodded Marshalls and Fenders.

I think I have a plan here.

Just pulled trigger on a JJECC823 and a couple Switchcraft 14B jacks as I'm also curious as to their application on my other push/pull amps as a means of having relay-free channel switching via existing active JHS AB box and stereo cable (see other thread).

I'll try 10k thru 33k plate to see which sounds best for the AU7 side.

All of this is words until I simply try it out. So next update will be real-world testing.

[Phil_S]

I'm a mere amateur at this stuff. It seems to me, if your basic goal is to milk out of a 12AU7 voltage gain that approaches that of a 12AX7, it isn't going to happen. The 12AX7 is capable of somewhere between 3x and 6x voltage gain relative to the 12AU7. See the attached. In light of this, it is difficult for me to understand the persistence is pursuing this effort. As noted earlier in the thread, and perhaps stated a bit differently, the 12AX7 is all about voltage gain. The 12AU7 is all about current supply. They are two very different tubes. If you want the voltage gain, use a 12AX7. Of course, this is your amp and you should work out your thinking to whatever end you like.

On another point, it is quite difficult to obtain coherent advice without a proper schematic. If you aren't sure about some symbols or how to show something, just substitute a note -- make a box with a reference and explain the reference with a few words. I'd encourage you to get to it and draw out what you can to get maximum participation. Otherwise, I expect your thread will die a slow death due to lack of essential information.

Good luck with your project.

[GlideOn]

Yep, have the JJ spec sheet on the tube in question; both sides nicely contrasted to each other:

Screenshot_20241228-130249.png

I have been very curious about his tube type for awhile now and I can't help but try it out, but in fairness I am also considering a 12AT7 or reworking the 12AX7 if it doesn't pan out. At the very least I am going to try it in my V2 HotMod as a backup, so no big deal if so otherwise!

My primary motivation is to utilize the unused triode in this amp. It bothers me to not 100% take advantage of the possibilities present, so I opted for what I most desired - a switchable, loud, full-bodied clean tone as possible from a 20w amp. At least enough for practices with a drummer.

I think @cdemike and I came to the realization that in order for a 12AU7 to stand any chance at being heard near the level of a 12AX7, you have to goose the crap out of it with voltage, so the anode will have to be hooked directly to the B+ line (calculated to be within safe parameters).

I am aware of the somewhat long-shot nature of this. Maybe I'll get what I want from this, maybe I won't. It's a $20 experiment to find out and will be an invaluable lesson or at least a sign post to others. And I'm fine by that. It's more than most seem willing to try.

[TUBEDUDE]

I applaud your sense of adventure. Experimentation is key to discovery. That willingness to try something "just to see" will advance your knowledge. That said, the results may indicate why many were "unwilling to try". The ECC832, and it's brother, the ECC823, are great, saving an extra socket if you need one AX7 and one AU7 triode.

[GlideOn]

Looking at my layout I am noticing a potential flaw - in order to gain high DC voltage for headroom, my new triode's anode connection is bypassing the dropping resistor - that means it would share a filter node with the C2 Screen Filter. It's not hooked into the PI Filter.

Before I made the last round of updates to this amp, I had accidentally tied the V2 Cathode follower to before the V2 filter cap just right of the 10k dropper and everything was fine that way with no audible side effects.

Is there any potential stability issue to tap in a triode to the screen filter cap? If the value is sufficiently large enough, would it still just work?

Edit: I realized the V3 node is getting 360ish volts and that nets me 270ish volts with a 100k anode hooked to it. I'll just leave it hooked to V3. Close enough to max dissipation for a 12A type.

[B Ingram]

... my new triode's anode connection is bypassing the dropping resistor - that means it would share a filter node with the C2 Screen Filter. ... I had accidentally tied the V2 Cathode follower to before the V2 filter cap just right of the 10k dropper and everything was fine that way with no audible side effects.

Is there any potential stability issue to tap in a triode to the screen filter cap? ...

"Potential" issue, yes.

But if you're not getting oscillation or some other audible problem, then it's not a problem in your application.

Going back to your original ask:

I'm looking to utilize an unused side of a V3 triode in my single channel amp for a dedicated, switchable clean channel (see other thread on how to get the switching) ...

Previously I had thought of the merits of the JJ ECC823 tube which has the 12AX7 on pins 1, 2, 3 and the 12AU7 on pins 7, 8, 9.

I don't hear a lot of feedback regarding the 12AU7 as a dedicated clean stage ...

I realize it is a gain factor of 100 to 20, but could a purely clean channel be developed on the 12AU7 side ...

The best way to design an amp is to work on the output section (which is also focused on "How many Volts to send to the Speaker-Impedance") first. Or at least "the classic way to design audio amplifiers" was from the Speaker back towards the Input jack.

Bias of the output section tells the designer how much signal is required to distort the output section, because the power tubes will absolutely distort when the peak drive-signal equals or exceeds the Bias voltage (absent a special driver circuit rarely seen in guitar amps).

The Bias Voltage (and available Output Voltage Swing) of that "driver stage" then tells the designer whether a guitar pickup can fully-drive the power section, or if additional amplification is needed (and often it is).

Now we're at the Preamp stage (or stages), and here again we're looking at whether there is adequate amplification to make up for signal-reduction in "convenience features" like a Volume control, tone controls and/or onboard effects.


Kuehnel's Bassman Book will help you understand this approach by analyzing an already-designed amp.
I haven't read it, but I suspect Kuehnel's Fundamentals of Guitar Amplifier System Design will help you learn this approach (though perhaps the modern focus on master-volume amps leads to the preamp being covered first before the power section).

[soundmasterg]

Another thing you could try if the 12AU7 half isn't working out for you is to swap the tube for a 12AY7. A gain of 44, same pinout, and you could set up one side for higher gain and the other for lower gain. I happen to like the 'sound' of that tube more than the other common ones like the 12AX7, 12AU7, 12AT7, etc., The lower gain out of this tube will take the 12AX7 half a bit and clean things up in a way you might like.

Greg

[Phil_S]

I know I'm late to the party and more knowledgeable people have already replied. I simply wanted to note that a 12AX7, while rated for gain of 100, likely does not actually give you gain of 100 in a given circuit. The same is true for a 12AU7. So, while you might think that the 12AU7, rated for gain of 20, is going to give you 1/5 of the gain, that may be a faulty assumption. Do what RG says. Let us know what you find.

[B Ingram]

... I simply wanted to note that a 12AX7, while rated for gain of 100, likely does not actually give you gain of 100 in a given circuit. The same is true for a 12AU7. So, while you might think that the 12AU7, rated for gain of 20, is going to give you 1/5 of the gain, that may be a faulty assumption. ...

12AX7 in-circuit gain is something like "60" and 12AU7 in-circuit gain is something like "16."
16 / 60 = about 1/4th

The original question was (sorta), "Can a 12AU7 be 'as loud' as a 12AX7?"
With a given supply voltage, each tube type can achieve the same Output Voltage, so the 12AU7 will be "as loud as a 12AX7."
The only problem is the 12AU7 will need 4-5 times the driving signal to make that same output.

From there, a Designer can decide whether that fact is acceptable or not (or if they will add amplification some other way).