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12AX7 Question

JSimo

Member
2020-01-20 9:39 pm
I have been reading articles on the Valve Wizard and putting them into practice on my work bench. However, one aspect confuses me as I know I am missing something.


If I apply say 2 volts peak to peak to the grid of the 12AX7 and have it biased correctly, I end up with a nice sine wave without clipping. However, if I then apply that output to the other grid of the 12AX7 (same valve), which is now a much high peak to peak voltage the output clips.



If I look at a load line on the graph the bias negative voltage range (12AX7) from from 0V to -5V.

I understand that I need to keep within around 4 volts peak to peak on the first stage but how do you re amplify with the amplified signal within the same envelope?


Hope this makes sense


John
 
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how do you re amplify with the amplified signal within the same envelope?

The short answer is that you need to start with a small signal.

If we take a 4mV moving magnet phono cartridge and send that into the first grid (and we'll assume everything is ideal for easier computations), then you get 400mV coming off the plate of the first stage. That 400mV signal would normally go through a passive EQ and incur some loss, so you're still totally fine feeding that into the next grid even if the cathode is only biased up a volt or two.

If you are starting with a really hot signal, I would ask you why you need to send it through two stages of 12AX7 amplification. What's the end goal? In some circuits that do this, there is a feedback connection back at the first cathode, and that will work against what's being applied to that grid, which may be a chunk of the answer to the question you're asking.
 

JSimo

Member
2020-01-20 9:39 pm
Thanks for the explanation, what you say makes good sense and I had a feeling that a feedback connection is what would be required.



There is no reason for me to send a hot signal and re amplify within the same tube. I am just trying to get my head around the mechanics of how it all works.


I built a SE amp last year with a 6n1P driver going into two KT88's and it works fine. However, if I input a small signal (example Ipad) into this amp the output is very weak. So, I built a pre amp with a 12ax7 using only one half of the tube for each channel. This worked very well and I am happy with its performance. However, I thought the signal from the preamp (12ax7) would overdrive the 6N1P in the main amp but it does not distort.




I understand after your explanation that the signal is way to hot for re amplification.



I now need to study up on feedback connection to the first cathode.
 
At least 1 amp I read about here paralleled both sides of the 12ax7, to make a pleasant, tubey 2nd order (I assume) distortion generator. Since you have a free stage on each side of your stereo input... Might be something to try.

Another way to magnify the tubes distortion (assuming a feedback connection reduces it) is to use a voltage divider between stages - to knock some of the 1st stage amplification back down to where the second stage can handle it w/o being overdriven.

There isnt a lot of point to this outside of emphasizing the tubes sonic character, like the paralleled 12ax7. Dont want to overdo it in a stereo amp, guitar amp maybe -
 

6A3sUMMER

Member
2016-06-07 6:50 am
If you parallel a 12AX7, and use self bias, be sure to use individual self bias resistors (and individual bypass caps if any), for each triode.
It will always give better results, unless the two triode sections are Exactly matched at the exact quiescent voltages and currents you set them to.
 

JSimo

Member
2020-01-20 9:39 pm
Yes, I did use individual self bias resistors and individual caps.


Also, if using a preamp is the volume controlled from the main amp, preamp or both. I was of the opinion that the preamp is run at full volume and the main amp controls volume.
 
@JSimo - "I was going to add a 12AX7 to act as a pre amp" - I thought you had already installed it, using two tubes and one section of each for each stereo channel. Perhaps you were thinking of using that "empty" section for something, so you connected it up and it was way over the top. Hence your OP.

@^A#sUMMER - "be sure to use individual self bias resistors" Good point - I can imagine what your describing, but cant recall the previous post (I referred to) schematic showing the 12AX7 triodes paralleled at the input - I should have looked at it more closely.
 

JSimo

Member
2020-01-20 9:39 pm
jjasniew.


I am going to add a 12AX7 as a pre amp to my SE amp rather than use a separate pre-amp and power supply.



The earlier questions I put up in this thread was more of trying to understand using the second half of a 12ax7 to amplify the signal of the first half. Audiowize answered that question and I can fully understand the reasoning.


Sorry for the confusion.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
....... the output clips....

Sure. Count on your thumbs. The max output of a triode on 300V supply is like 60V peak. The gain of one 12AX7 stage is like 50. So the gain of two 12AX7 stages cascaded (withOUT any NFB or inter-stage loss) is like 2,500. 60V out divided by 2,500 gain is 0.024V peak input max.

So even an electric guitar can overload a 12AX7 cascade. It would not be enough for a phono pickup, except we always use heavy EQ between or around the two stages.

The 12AX7 comes very late in audio history because who the heck needs THAT much gain in one bottle?

It comes in for low-gain plans. Push-pull power amp drivers need one high gain stage and one unity-gain stage. Paraphase or cathodyne. Another is a line-amp followed by a Bax (unity gain) tone network.

In guitar amps the usual plan is one stage, a lossy tonestack AND the Volume pot, *then* another stage.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
Mu is not amplification. With resistance-coupled triodes the *working* amplification is 2/3 to 1/3rd (or less) of the Mu.

We can approach Mu with an infinite load, plus an infinite choke or current-limiter. But that's more costly parts. We can approach 1/4(Mu^2) by cascading two triodes, and that was always more practical than fancy loading of a single triode.
 

6A3sUMMER

Member
2016-06-07 6:50 am
nigelwright7557,

For a self biased and capacitor bypassed cathode, then . . .
Triode gain = u x (RL/(RL + rp))

Example, suppose a single stage of a 12AX7 triode has a plate resistance, rp, of 62.5k Ohms, and a load, RL of 125k Ohms.
The gain will be 66.67.
If that stage drives a next stage that has a grid resistor, RG, the parallel resistance of RG and RL will lower the gain to be less than 66.67.

u is not gain.
But you get the idea.
 
nigelwright7557,

For a self biased and capacitor bypassed cathode, then . . .
Triode gain = u x (RL/(RL + rp))

Example, suppose a single stage of a 12AX7 triode has a plate resistance, rp, of 62.5k Ohms, and a load, RL of 125k Ohms.
The gain will be 66.67.
If that stage drives a next stage that has a grid resistor, RG, the parallel resistance of RG and RL will lower the gain to be less than 66.67.

u is not gain.
But you get the idea.

Thanks for clarifying with that formula.

I have been there with 12AX7's and two consecutive gain stages and had problems myself. The triodes seemed to interfere with each other. It was if the 2nd stage radiated back into the first stage. A quick fix was a 12AU7.

Had a similar problem with a hybrid amp. The SS part oscillated and RFed it back into the valve ! causing mayhem.
 

Gnobuddy

Member
2016-03-01 4:10 pm
...12AX7's and two consecutive gain stages and had problems...
And yet, it is not at all unusual to run the two triodes in a 12AX7 in that manner, with the unattenuated output of the first feeding the second!

Leaping back in time about fifty-seven years, Leo Fender's "Blackface" guitar amps used lots of attenuation in between the two triodes in a 12AX7. The preamp will never overdrive with any Fender guitar with single coil pickups plugged in, nor with any normal humbucker-equipped guitar. That's what Leo wanted from his guitar amps - nothing but shimmery clean tones.

Fifty seven years is a long time. Not that long afterwards, tastes in popular music began to shift towards more and more distortion, starting with electric blues. The first tiny step towards a tube guitar preamp with enough gain to generate just a little bit of distortion is to drive one half of a 12AX7 directly from the other, with only a gain pot in between. Then move Leo's lossy tone-stack to the output of the second triode, where its output feeds the power amplifier stages.

I don't think Leo himself ever sold an amp like that, but the company he begat certainly did: the Blues Junior, for example, follows this topology.

Note that when the "Gain" knob in a Blues Junior is at maximum, the entire output of the first triode is indeed driving directly into the second triode's grid. This does not turn out to be a disaster by any means; with a typical guitar, there is just enough gain to get a wee bit of overdrive. Possibly enough to have shocked the conservative middle-class American parent in 1950, but certainly not enough to have stirred up the youth of 1965, who were starting to listen to bands like Cream.

Like PRR, I've usually seen a gain of about 50x (i.e. 34 dB) from a single triode in a 12AX7. Two cascaded would have an estimated gain of 2500. With a B+ of 300 volts-ish, max output is typically 200 volts peak-to-peak; divide that by 2500 and you need 80 mV peak-to-peak at the input to just start to overdrive.

The output of a guitar being played by a guitarist with a reasonable sense of touch (i.e. not a violently angry Pete Townshend) is not much bigger than this. The result is that with this topology, there is usually enough gain to get a little bit of preamp distortion, but not much. Not even enough for Cream-era Eric Clapton.

And if you turn down that gain knob to half-rotation, you've just attenuated the signal ten-fold (that's what the typical "10% taper" log audio pot does.) Now you won't even have a trace of overdrive from the preamp.

In short, driving one half-12AX7 into a second does not produce dramatically too much gain for electric guitar...not unless you have Les Paul's own taste in electric guitar tone. (He liked his guitar squeaky-clean - look up some of his songs on You Tube if you're not familiar with his tone of choice.)

Finally, IMO, please don't use negative feedback in an electric guitar preamp! NFB is the antithesis of everything you want in a tube electric guitar preamp.

(Small amounts of NFB in the power amp were not that uncommon, however, at least in amps designed mostly for clean guitar tones.)

Funny coincidence: earlier today I bread-boarded a common-source gain stage using a J112 JFET. Small-signal voltage gain was about 45 times - very close to what you usually get from one of the triodes in a 12AX7. But the JFET needed less than one-sixteenth of the B+ voltage, and only about one-quarter of the current, to produce the same gain.

Voltage gain was once an expensive luxury. Now you can buy a gain of 100,000 times (100 dB) for less than a buck. Amazing!


-Gnobuddy
 
And yet, it is not at all unusual to run the two triodes in a 12AX7 in that manner, with the unattenuated output of the first feeding the second!
-Gnobuddy

I suspect there was some positive feedback somewhere in my circuit.
It was certainly very unstable. But it was in my early days of valve amp design and I would be more careful with layout, decoupling etc now.

I haven't seen many circuits using two 12AX7 triodes in series.
I have seen them in parallel to drive tone stages or as an LTP.
I had a very old Simm-Watts amp that uses 12ax7's in parallel.

I had a look tonight through some of my valve amp designs and I did find one with 12ax7 triodes in series.
But that has separate decoupling between stages and grid stoppers and layout is very tight.
 
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