Everyone is telling you everything except a answer to your question. Change the plate resistor to 100k, cathode resistor to 1.5k. These are stock fender values. Try them then adjust to taste. Go here and type in different values and you will see the gain changes.
Cathode Bypass Capacitor Calculator
Cathode Bypass Capacitor Calculator
You could start by changing the 300k load resistor to 100k. Also, as previously suggested, snip off the cathode cap(s) in the preamp. That will bring your gain level down - but it won't sound exactly the _same_ as a 12AY7.
As I don't have a schematic, I'm working a little in the dark with this one.
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Thanks for the suggestion, AquaTarkus. In this particular amp, the bypass caps on both V1 and V2 are actually switched in and out with an FET sort of arrangement, so I don't even have to snip anything; it's part of the feature set.
It is so frustrating to have a seemingly simple experiment be so far out of my reach that I have started re-reading my Richard Kuehnel books, my Merlin Blencowe books and had a good read here: Tubes 201 - How Vacuum Tubes Really Work
As a result, I have drawn my first couple of load lines and seen visually what the differences are. Obviously, I have 1000 miles more to go but I am seeing a glimmer of light now.
I also see that the two tubes really will never respond the same regardless of operating conditions, the curves are very different. Compared to a 12AX7, a 12AY7 's curves are much closer to vertical and I can't change that with a resistor or capacitor.
It is so frustrating to have a seemingly simple experiment be so far out of my reach that I have started re-reading my Richard Kuehnel books, my Merlin Blencowe books and had a good read here: Tubes 201 - How Vacuum Tubes Really Work
As a result, I have drawn my first couple of load lines and seen visually what the differences are. Obviously, I have 1000 miles more to go but I am seeing a glimmer of light now.
I also see that the two tubes really will never respond the same regardless of operating conditions, the curves are very different. Compared to a 12AX7, a 12AY7 's curves are much closer to vertical and I can't change that with a resistor or capacitor.
That´s the point
You can change gain any day of the week, but since you said that´s not what you wanted ....
That's right!
Plus, as your issue seems to be mainly about reducing preamp gain, you absolutely can do that by changing a resistor or a capacitor. This white paper by Randall Aiken will help explain why - Designing Common-Cathode Triode Amplifiers
Actually, I recommend all of Aiken's white papers - White Papers
And I encourage you to keep on learning up about load lines.
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It is not how "vertical" they are. It is the -spacing- along your chosen load-line. Once you set a load-line, the circuit has no idea how the curves run above and below that line.
Do not forget to include the AC load!! With a 330K plate resistor, and typical 200K loading in many interstage networks, the AC loading is significant.
Do not forget to include the AC load!! With a 330K plate resistor, and typical 200K loading in many interstage networks, the AC loading is significant.
Why change the circuit when you can just buy a 12AY7? Because I don't feel like spending $50 for a tube when I have a tray full of 12AX7's from back in the day. I did have a 12AY7 but it went out with an amp for an opinion and both never came back, let it slid rather than cause family disharmony.
If I had the 12AY7 I would set up the two circuits with pots in the plate and cathode positions of the 12AX7 and twiddle away switching back and forth between the two input stages.
If I had the 12AY7 I would set up the two circuits with pots in the plate and cathode positions of the 12AX7 and twiddle away switching back and forth between the two input stages.
I set up a small amp circuit on an old chassis and had a pentode for the input tube. I had pots for the plate, cathode and screen along with a switch on the cathode cap. What would have taken forever to do if I had a board and subbed in components did not take too long before I found some sweet spots for the tube.
Funny thing, if all triodes really do follow the Child-Langmuir ("three halves") law, then it should be possible to make one triode (12AY7) sound exactly like another one (12AX7) - you just have to find the right operating conditions and signal amplitudes (twiddle pots like Printer2 says).
And yet we constantly hear people describing how swapping one 12AX7 for another changed the sound of their amp.
So we're left to wonder: do real triodes all depart substantially from the Child-Langmuir equations, each one in a different way?
Or, is the difference heard when "tube rolling" simply down to normal production variations between one triode and another? That is to say, you just happened to stuff a higher-mu one in second, got increased overdrive, and then mis-attributed the altered sound to the fact that the second 12AX7 was a NOS Telefunken, rather than simply to the fact that it happened to have a mu of 110, compared to the mu of 90 that the first triode happened to have?
There is good reason why true pentodes don't sound the same as beam tetrodes - they really do have differently shaped curves (beam tetrodes intrinsically have higher 2nd harmonic distortion, a problem discussed in the original white paper describing the creation of the 6L6). But why does one type of triode sound different than another?
-Gnobuddy
Sad truth is that there are **terrible** tube to tube variations, even within the same brand, let alone different ones.
Most inconsistent component in current use, only comparable to FETs , which are also terribly inconsistent.
That´s why my teeth screech when I see people attributing a certain sound to a certaint ube, a different one to another, and attribute that to **Brand**
Truth is:
* do different tubes sound different? ..... often YES.
Might expand that to "usually".
* so if I buy a Brand X tube will I get the same sound I got earlier with another Brand X one?
Yes/No/Maybe/Hardly/Possibly/Always/Never/No way! <------ pick one.
Suggest flipping a coin/rolling dice/dropping a ball on a turning red/black/green wheel
Most inconsistent component in current use, only comparable to FETs , which are also terribly inconsistent.
That´s why my teeth screech when I see people attributing a certain sound to a certaint ube, a different one to another, and attribute that to **Brand**
Truth is:
* do different tubes sound different? ..... often YES.
Might expand that to "usually".
* so if I buy a Brand X tube will I get the same sound I got earlier with another Brand X one?
Yes/No/Maybe/Hardly/Possibly/Always/Never/No way! <------ pick one.
Suggest flipping a coin/rolling dice/dropping a ball on a turning red/black/green wheel
I can verify that from personal experience! Yes, a lot of audible variation between manufacturers and a lot of variation in a batch from the same run.
I read that each "house" has it's characteristic fingerprint, too. That could be the choice of alloys, coatings, wire diameter, spacing of the components, how hard a vacuum they can maintain and so on. Those are all at the discretion of the manufacturer and in total they all sum together to create a recognizable sound.
I read that each "house" has it's characteristic fingerprint, too. That could be the choice of alloys, coatings, wire diameter, spacing of the components, how hard a vacuum they can maintain and so on. Those are all at the discretion of the manufacturer and in total they all sum together to create a recognizable sound.
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Child's Law is derived for an infinite plate device. There is a simple transformation for an infinite cylindrical device.
No real device is remotely infinite, or quite plane or cylindrical; and real grids are not sheets but wire fences with spacing comparable to distance.
12AX7 curves are very non-Child at low voltages due to the fine grid structure, "contact potentials", and electron velocity.
No real device is remotely infinite, or quite plane or cylindrical; and real grids are not sheets but wire fences with spacing comparable to distance.
12AX7 curves are very non-Child at low voltages due to the fine grid structure, "contact potentials", and electron velocity.
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