Yes, if you want the ideal three-halves response then first you have to find an ideal triode. Each electrode would be exactly the same shape as the others (e.g. all infinite planes, or concentric cylinders). No anode ribs etc. Infinitesimally thin grid wires, with no visible means of support. Then you get constant mu, and textbook variations in transconductance and anode impedance.
As a general rule, the closer the circuit is to 'typical' operating conditions for a valve then the smaller are the changes when 'tube rolling'. Use unusual bias conditions and you start exploring the edges of the characteristic curves, where sample variation is greatest. Use large grid stoppers and you start exploring the details of Miller effect, so gain and stray capacitance variations become audible. Many guitar amps seem designed to maximise the effects of sample variation; it is unclear to me whether this is deliberate or accidental, but I suspect the latter.
As a general rule, the closer the circuit is to 'typical' operating conditions for a valve then the smaller are the changes when 'tube rolling'. Use unusual bias conditions and you start exploring the edges of the characteristic curves, where sample variation is greatest. Use large grid stoppers and you start exploring the details of Miller effect, so gain and stray capacitance variations become audible. Many guitar amps seem designed to maximise the effects of sample variation; it is unclear to me whether this is deliberate or accidental, but I suspect the latter.
I'm new to this end of the business, but I've been up to my neck in guitar amps since the 1960s. I'd say the classic amps were accidental lucky combinations that made a wonderful sound when operated far outside what the designers anticipated. But these days, it's absolutely deliberate. I'm trying to learn enough to appreciate what they have done and perhaps tweak it more to my liking.
That´s the point.
Guitar amp design is myopically focused on maximum gain, period, and everything else be damned, and in power amps: maximum power, period, no matter what.
If such operating conditions make a tube give up the ghost within 6 months or a year, so be it, even if exact same tube can give 20 years or more if run "by the book".
Book written by designers and makers, who "should" know something about them, I might add.
A very experienced Tech friend (you would be surprised) moved from San Francisco to Saint Petersburg, Russia (after leaving his huge OEM repair business to former employees) and even without wanting to, started to dabble into Servicing again , this time as a hobby or to help friends.
He was amazed at finding perfectly working and measuring *good* 6L6 tubes (obviously Russian) in heavily used local Musician amplifiers, think "Uncle Vanya and his accordion: weddings, baptisms, any kind of party" used 5 or 6 nights a week fo the last 20 years or more.
He couldn´t believe his own eyes, until he noticed tubes were used "like the datasheet says", meaning some 360V plate (typical Guitar use: 430 to 520V) , 250V screen (instead of 400/450V or even 520V in Ultralinear Fender amps) and pulling 30W RMS from each pair (ample power in any Party setting) instead of coaxed out 50/60W RMS.
A real eye opening experience.
According to various reputable sources I've read on the 'Net over the years, the last generation or so of valves were actually designed using digital computers (which, of course, used valves as the active devices in them).
A series of powerful radio frequency transmitter triodes (some beam tetrodes too) eventually resulted, with almost perfectly cylindrical electrodes, and astonishing performance compared to the much older, and much more primitive, triodes that we hobbyists still tinker with.
This generation of beam tetrodes had no beam-shaping electrodes or other spit-and-string bodges: the electron beam was radially symmetrical, spreading perfectly uniformly out from a cylindrical cathode to a cylindrical anode.
My attempt to find some of my old sources online have failed for the last several minutes, but I did find some pictures of some of these weird and wonderful high-powered RF transmitter triodes. I'll attach a few.
For those readers who are not physicists, the mathematics that describes exactly how any vacuum tube will work (including every last tiny detail of electrode shapes, et cetera) was actually fully worked out in the mid 1800s, and came to be known as Maxwell's equations. This was well before vacuum triodes actually existed, so you would think it would have been easy to engineer perfect triodes from the start, since all the mathematics needed was already available.
The trouble was that, while the equations were known, only the simplest electrode shapes (infinite flat sheets, infinitely long perfect cylinders) could be solved purely with pen and paper; accurate solutions for more complex shapes (i.e., real-world electrode shapes, and finite sizes) had to wait until computers that were much faster than humans came along.
So early valves were designed using simplified math, and some clever contraptions that helped early researchers visualize the electric fields around the electrodes.
But then valves got good enough to build digital computers with, and those digital computers could crunch their way through numerical (approximate, but pretty accurate) solutions of Maxwells equations, and those solutions let engineers design better valves...
There's no doubt that much early guitar amp "design" was done by technicians rather than engineers, and that some of them had, shall we say, a fairly limited understanding of what they were doing.
But the fact of the matter is that using zero negative feedback produces the most gradual transition from linear, small-signal operation, to less-linear, large signal operation. And many electric guitar players like this sort of fairly progressive "touch sensitive" distortion, so most guitar amps use little or no negative feedback, compared to Hi-Fi amps of the same era.
And what happens when you use little to no negative feedback? Yup, the amp becomes very sensitive to sample variation from one valve to another.
So, in this case, I agree with you about many guitar amp designs being highly sensitive to normal production parameter variations in valves. I also agree with you that this was probably an accidental byproduct - but a byproduct that occurred accidentally only because a higher priority took precedence, that of producing controlled, progressive, "touch-sensitive" distortion.
I have a foot in both worlds: if I'm trying to quickly put together a budget dynamic mic preamp for a friend with no money, I use an NE5532 or two, and a ton of negative feedback, virtually eliminating the effects of parameter variations between one 5532 chip and another. But if I'm building a triode input stage for a guitar amp, I use no negative feedback at all, because I want the gentlest possible transition from clean to overdrive. And, as a result, my guitar input stage is, unfortunately, wildly susceptible to parameter spreads in the valves used.
-Gnobuddy
Attachments
Leonidas was an accountant first, and an electronics tech second. So of course if he could squeeze out the gain he needed from one less valve (or from a cheaper valve), he would immediately do it, to improve profits.
Apparently Bogner didn't get the same memo - at one time they made the "Bogner Fish" preamp, which, I think, used eight 12AX7s...sixteen triodes!
I grew up building solid-state electronics, and one rule you learn to always take very seriously with transistors is: never, ever, exceed the maximum voltage rating!
So I was completely shocked when I compared the voltages in vintage Fender guitar amps with the datasheets for the corresponding valves. I have a '65 Princeton Reverb reissue, and the factory schematic shows 440V DC on the 6V6 anodes, while vintage Tung Sol datasheets show 315V maximum for a 6V6 or 6V6GT.
I know, I know, AC voltage at the wall outlet has crept up since 1965, sure. But that's not enough to explain having 140% of the absolute maximum voltage rating on the anodes. The mains AC voltage didn't go up that much! This absurdity probably has everything to do with Leo trying to squeeze an extra watt or two out of the cheapest power valves he could buy.
-Gnobuddy
Agree but add: overvoltage failure mode is very different in tubes and transistors, with the advantage being on the first.
Tubes basically need an internal arc to be destroyed and that can easily need 2/3/4X the design voltage, so 40/50% excess can very often be tolerated.
If anything, it will cause overdissipation but the killing mechanism will be turning a metal part first red, then orange, and only burn a hole with incredibly high dissipation.
Yes, abuse will also cause gassing, loss of emission, etc. , but those are relatively slow killers.
Now in SS, to begin with, only a few Volts above normal rating they "Zener" ... or to be more precise, avalanche.
This is very destructive, to worsen things up that creates hot spots so destructive force concentrates on a small area and works even faster.
And Transistors are semi-conductors only because they have a very precise atomic structure, held in a precise shape by careful doping in a tightly controlled way; any disruption in that structure, even in a small part of it, turns it back into a stupid piece of metal: a short.
That´s why tubes can be abused without great pain: they will deliver and worst case will have to be replaced more often; transistors die on the spot.
Oh well.
Tubes basically need an internal arc to be destroyed and that can easily need 2/3/4X the design voltage, so 40/50% excess can very often be tolerated.
If anything, it will cause overdissipation but the killing mechanism will be turning a metal part first red, then orange, and only burn a hole with incredibly high dissipation.
Yes, abuse will also cause gassing, loss of emission, etc. , but those are relatively slow killers.
Now in SS, to begin with, only a few Volts above normal rating they "Zener" ... or to be more precise, avalanche.
This is very destructive, to worsen things up that creates hot spots so destructive force concentrates on a small area and works even faster.
And Transistors are semi-conductors only because they have a very precise atomic structure, held in a precise shape by careful doping in a tightly controlled way; any disruption in that structure, even in a small part of it, turns it back into a stupid piece of metal: a short.
That´s why tubes can be abused without great pain: they will deliver and worst case will have to be replaced more often; transistors die on the spot.
Oh well.
Bear with me...I have actually been thinking about the original problem, just trying to assemble enough jigsaw pieces inside my head to maybe come up with something useful.
So, having thought about it a bit, and slept on it last night, I think we can put several of the posts in this thread together, and come up with some things that I think have to happen for one triode to mimic another:
(a) changing the anode resistance (to mimic some of the effects of the lower rp of the 12AY7, more on this later), and
(b) Changing the input voltage slightly (12AY7 has higher transconductance than 12AX7) to provide the same excursion along the curves, and
(c) Using the gain control to send the same signal strength on to the subsequent stages of the amp
I have a feeling that the first of these (adjusting the anode load) will be the biggest factor. My hunch is that ratio {Ra/ra} (i.e., the ratio of external anode resistor to the valve's internal anode resistance) needs to stay the same for one triode to sound like another. I haven't fully worked this out mathematically, yet, it's just a hunch at this point.
Looking at the datasheets, the 12AY7 is specified at approximately 25 kilo ohms ra; the 12AX7, at approximately 62.5 kilo ohms. (These numbers will surely vary with operating point, so they are only rough starting points.)
Using those datasheet values, that means the 12AX7's internal anode resistance is 2.5 times higher than the 12AX7s.
I would suggest, therefore, that a good starting point would be that, if you use a 12AX7 instead of a 12AY7, you have to *raise* the anode load by a factor of around 2.5, to have a chance of getting them to sound alike. In other words, if the 12AY7 had a 47k anode load, I think the 12AX7 will need around 120k at the anode to have a chance of sounding similar.
This is counter-intuitive...but we know that a bigger anode resistance makes a triode sound "cleaner". Bigger compared to what, you ask? I do, too, and the obvious contender is the internal anode resistance of the triode itself.
In other words, I think the dimensionless scaling parameter that sets this is the ratio of (Ra/ra), which is why you'd need to raise the anode load for the 12AX7 to get it to sound like a 12AY7.
After this Ra scaling is done, I think the input signal strength (from the guitar!) will have to be scaled to produce the same amount of harmonic distortion from both valves. I haven't quite got my brain around this - not sure if that means reducing the input signal to the 12AX7, or increasing it, though I suspect it's reducing; let's hope that's correct, because that is much easier to do! (Turn down the guitar volume!)
Finally the output signal has to be scaled so that the same voltage is sent on to subsequent stages. This should be doable with the existing gain control knob...
Whether all this will actually make a 12AX7 sound like a 12AY7, I don't know. Even if it does for clean tones, as soon as any overdrive enters the picture, I have a hunch that different triodes may not sound alike (DF96 suggested this too, when you get closer to the edges of the operating area, there is likely more departure from "three-halves" ideal behaviour.)
-Gnobuddy
A split load rather than a pot to scale the output. The transfer characteristics of the AY is -6v while the AX is -3v. So the AY has twice as much input headroom than the AX. So a signal cut in half for the AX.
Should look at the bias the AY would be under with the components of a 5E3 or 5F6 and offset the bias for the AX the same amount from center bias. I may look up the datasheets yet but for now I am a little under the weather and I need something warm in me.
Should look at the bias the AY would be under with the components of a 5E3 or 5F6 and offset the bias for the AX the same amount from center bias. I may look up the datasheets yet but for now I am a little under the weather and I need something warm in me.
Tried making apples to apples with this simulator and tried to past the two images (12AX7 and 12AY7) in one picture but the online editor was not cooperating.
Interactive Valve Data Sheets
Interactive Valve Data Sheets
Good idea! That will lower the output resistance of the stage too, which will also bring it closer to 12AY-ish behaviour.
That certainly sounds like a great starting point!
I like the general idea, but didn't Fretts say:
So it seems Fretts liked a 12AY7 with a 330k anode load...multiply by 2.5, and that would equate to about an 820 k load for a 12AX7!Fretts said:
We may be nearly in constant-current-source load territory - 820 is more than ten times the internal anode resistance of a 12AX7, so it probably acts pretty much like an infinite load impedance, i.e. a current source. Which is exactly what the valve Hi-Fi guys use when they want the lowest possible distortion from a triode gain stage.
Sorry to hear it, hope you feel better soon. My office-mate has been sneezing and sniffling and wheezing for a few days, I wonder how long before my immune system gives in to the onslaught and I get sick too.
I wanted to try something similar to what you did with 12AX7 and 12AY7 curves, but haven't found the free time yet. Maybe tomorrow, with a little luck. (We are recovering from a building renovation project gone awry at work, every day is an adventure in playing whack-a-mole with a stream of unexpected problems that keep popping up.)
-Gnobuddy
Making the wild guess (for convenience) that B+ was 333 V, I plotted a load-line for a 333.3k resistor (330 k anode, 3.3k cathode) on a set of 12AY7 characteristics.
I also plotted the cathode load line, so we can make a rough estimate of the operating point when Fretts plugs a 12AY7 into his Fender.
Keeping in mind that 330V B+ is entirely a guess (though likely to be in the ballpark), I come up with approximately 0.7 mA anode current, and Vgk around -2.2 volts.
The attached image shows what the 12AY7 is expected to be doing in this circuit.
I also estimated the second harmonic distortion from the graph, for an input signal of 2V peak to peak. The amount of distortion is very low, and the graphical estimate will therefore be inaccurate. But, for what it's worth, my estimate is that second harmonic distortion is somewhere between 1% and 2.3%.
Now, even 2% is very low (probably too low to detect by ear with a guitar as the source signal). And this is for a fairly large 2 Vpp input signal - an actual guitar signal is virtually certain to be much less than this, unless Fretts has the most overwound humbuckers in history, cables for guitar strings, and a heavy strumming hand.
Realistically, then, the input signal straight from the guitar is probably at least ten times smaller, call it 200 mV pp. And the distortion will be vanishingly small for such a small signal. Too small to hear.
So: it's probably a safe estimate to say that the 12AY7, in this particular circuit, is almost certainly behaving as a perfectly clean amplifier, with essentially zero distortion.
The question now is, can we make a 12AX7 do more or less the same thing?
It's immediately obvious that my earlier idea of scaling up Ra to match ra is not going to fly in this case. Ra is already 330k for the 12AY7, which scales up to 820k for the 12AX7, and that is simply not going to work with a 330V B+ (or anything anywhere near that).
In fact, it's now obvious that one thing I completely missed earlier, is that B+ needs to be scaled as well!
However, in this particular case, we don't need to make a 12AX7 reproduce what a 12AY7 does; instead, what we really need to accomplish is to get a 12AX7 to produce inaudibly low levels of distortion.
And the obvious thing to try is simply to remove the cathode bypass capacitor. (I think this was suggested earlier in this thread by other people, too.)
Fretts, if you're still here, why not lift one end of the input stage cathode bypass cap, and see if that lets your 12AX7 behave close enough to the (virtually) zero-distortion 12AY7 that you liked?
-Gnobuddy
I also plotted the cathode load line, so we can make a rough estimate of the operating point when Fretts plugs a 12AY7 into his Fender.
Keeping in mind that 330V B+ is entirely a guess (though likely to be in the ballpark), I come up with approximately 0.7 mA anode current, and Vgk around -2.2 volts.
The attached image shows what the 12AY7 is expected to be doing in this circuit.
I also estimated the second harmonic distortion from the graph, for an input signal of 2V peak to peak. The amount of distortion is very low, and the graphical estimate will therefore be inaccurate. But, for what it's worth, my estimate is that second harmonic distortion is somewhere between 1% and 2.3%.
Now, even 2% is very low (probably too low to detect by ear with a guitar as the source signal). And this is for a fairly large 2 Vpp input signal - an actual guitar signal is virtually certain to be much less than this, unless Fretts has the most overwound humbuckers in history, cables for guitar strings, and a heavy strumming hand.
Realistically, then, the input signal straight from the guitar is probably at least ten times smaller, call it 200 mV pp. And the distortion will be vanishingly small for such a small signal. Too small to hear.
So: it's probably a safe estimate to say that the 12AY7, in this particular circuit, is almost certainly behaving as a perfectly clean amplifier, with essentially zero distortion.
The question now is, can we make a 12AX7 do more or less the same thing?
It's immediately obvious that my earlier idea of scaling up Ra to match ra is not going to fly in this case. Ra is already 330k for the 12AY7, which scales up to 820k for the 12AX7, and that is simply not going to work with a 330V B+ (or anything anywhere near that).
In fact, it's now obvious that one thing I completely missed earlier, is that B+ needs to be scaled as well!
However, in this particular case, we don't need to make a 12AX7 reproduce what a 12AY7 does; instead, what we really need to accomplish is to get a 12AX7 to produce inaudibly low levels of distortion.
And the obvious thing to try is simply to remove the cathode bypass capacitor. (I think this was suggested earlier in this thread by other people, too.)
Fretts, if you're still here, why not lift one end of the input stage cathode bypass cap, and see if that lets your 12AX7 behave close enough to the (virtually) zero-distortion 12AY7 that you liked?
-Gnobuddy
Attachments
Gnobuddy, I really appreciate you sinking your mental teeth into this puzzle! Now, the amp as built has an option to lift the bypass cap already, it goes to a pushbutton labelled "treble boost" which is kind of inside reference to the supposed practice of using an external treble booster box with a Marshall Bluesbreaker, back around 1965 or so. The Bassbreaker line of amps is Fender kind of laying claim to the fact that the entire Marshall amp line was borne out of the Fender 5F6A Bassman, so why can't Fender do it too.
Anyway, as it happens, I don't like what the bypass cap adds, so I leave it disconnected. The sound I normally hear is without it regardless of tube type.
I think you may be right, that the first triode - the way *I* like it, may be running nearly clean as a whistle, and is likely overdriving the second triode. If so, that is the same thing that happens in a classic fuzztone - a clean stage extremely overdrives a subsequent stage, a practice I am familiar with. The other factor of tonality that attracts me to this amp doesn't show up until the whole amp is really cranking, i.e. the EL84 power tube is distorting as well - that's when all the pieces fit together, the new harmonics and "bite" that happens when the whole amp is turned up are the final sound I'm after. It's far better with the milder tube installed in the first position.
I haven't been able to set up my bench or scope since I moved (no room) so I can't see for myself on a scope what is happening, but I can still solder and listen. Maybe all it will take is to put the 12AX7 into the clean zone with load and bias, followed with an attenuator to bring the level down to the 12AY7 level, and make the whole amp work harder. it's only a 7 watt amp, so it spends a lot of its time working hard.
Anyway, as it happens, I don't like what the bypass cap adds, so I leave it disconnected. The sound I normally hear is without it regardless of tube type.
I think you may be right, that the first triode - the way *I* like it, may be running nearly clean as a whistle, and is likely overdriving the second triode. If so, that is the same thing that happens in a classic fuzztone - a clean stage extremely overdrives a subsequent stage, a practice I am familiar with. The other factor of tonality that attracts me to this amp doesn't show up until the whole amp is really cranking, i.e. the EL84 power tube is distorting as well - that's when all the pieces fit together, the new harmonics and "bite" that happens when the whole amp is turned up are the final sound I'm after. It's far better with the milder tube installed in the first position.
I haven't been able to set up my bench or scope since I moved (no room) so I can't see for myself on a scope what is happening, but I can still solder and listen. Maybe all it will take is to put the 12AX7 into the clean zone with load and bias, followed with an attenuator to bring the level down to the 12AY7 level, and make the whole amp work harder. it's only a 7 watt amp, so it spends a lot of its time working hard.
Last edited:
The B+ supply at V1 is 285v, anode resistor 330K, cathode resistor is 3.3K, there's a "gain" control at the output from the plate that's around 450K consisting of a 220K resistor in series with a 250K pot to ground, the wiper feeds the next stage. The next stage is set up very much like standard Fender - 100K on the anode, 1.2K on the cathode, same B+ supply, AND since this is a dual triode, the 12AY7 provides both sections, with all the non-12AX7 differences.
Ah, thank you, I misunderstood you earlier - I thought you said the voltage at the anode of the first triode was 285 V (not the B+ voltage feeding that stage). So I assumed the B+ would be higher (to account for the voltage drop across the anode load resistor).
B+ being 285V will alter that load line's position a bit, and the 450k gain control will rotate it (AC load line) clockwise a little. But not too much when a 12AY7 is installed, because of the low "ra" of around 25k. 450k is a lot bigger than 25k, so only minimal loading will occur.
Not to lose sight of the forest for the trees, though - the general conclusion that the input stage is providing only squeaky-clean gain should still be true.
Speaking of gain, there isn't very much of it. If you look at where the load-line crosses the Vgk=-1 and Vgk=-3 curves, you can see that a 2V signal swing at the grid causes about a 78 V swing at the anode. So voltage gain of the first stage (with a 12AY7 installed) is only about 40 times.
Thank you for that, I completely missed the obvious - that the second stage will also be a 12AY7!
Here's the load line for the conditions you described (12AY7, 100k Ra, 1.2k Rk, 285V B+). I didn't have time to do more than draw the lines, mebbe tomorrow.
Meantime - I realize this is a PCB amp, so how easy would it be to disconnect one end of the cathode bypass cap in the 2nd stage?
'Cos using a 12AY7 is going to clean up that stage compared to a 12AX7, and lower the voltage gain. Removing the cathode bypass cap will move a 12AX7 in the right direction - lower distortion, less voltage gain.
-Gnobuddy
Attachments
Sorry about the delay replying, it's been a busy few days.
So: we seem to now be at the point where a 12AX7 with a fully *unbypassed* cathode resistor still has too much gain, and too much nonlinearity, to suit you personal preferences. The easy solutions have been tried (remove cathode bypass caps), and we haven't reached the holy land yet. Now what?
Well, the actual question we need to answer is, "How do we get the distortion and gain of a 12AX7 even lower than you can get with a fully unbypassed cathode resistor?"
I can actually think of three different ways of doing this (and a couple more ways, which are practical in a scratch build, but not in modding a commercial product made on a PCB.) I will list them in what I think is increasing order of difficulty:
1) Add some local negative feedback from anode to grid of the second triode. This would involve tack-soldering one resistor and one capacitor onto your Bassbreaker PCB. Some experimentation would be needed to find the proper resistance value. If it works, and you find the results satisfactory, it is the simplest solution I can think of (short of just plugging in a 12AY7!)
2a) Remove the first 12AX7 from its socket, build a solid-state preamp to replace it, using a small high-voltage MOSFET or two. This will give you a squeaky-clean first two gain stages. With little TO-92 MOSFETs (like the LND150), it may be possible to make the preamp small enough to fit in the space formerly occcupied by the first 12AX7.
This promises to be challenging, but if you have the skills, it can be done. I once shoehorned an additional 12AX7 into a Superchamp XD to replace the (dead) solid-state preamp, so I know a modification like this is possible. But it will take patience, skill, and time, and will definitely void your Bassbreaker's warranty. If things go wrong (wiring errors, etc), it might also cause damage to the Bassbreaker.
2b) Build a solid-state preamp just like option 2a, but make it external, and self-powered. Basically, modify the Bassbreaker only enough to allow you to feed an audio signal into it at a point immediately after the first two stages (i.e. add one more 1/4" jack). Run your external preamp to that point.
3) Replace the 1.2k cathode resistor of the second gain stage with a current source, made from a transistor and a couple of other parts. Partially AC bypass the current source with a resistor that's bigger than 1.2k, in series with a capacitor.
The idea here is to allow that second stage to still bias up to the proper idle current, but at the same time, increase the AC impedance at its cathode higher than a 1.2k resistor. Current sources have very high AC impedance, so substituting one for the cathode resistor will effectively behave like an extremely high cathode resistor, dropping the gain to nothing, and the distortion very low. We don't want a gain of nothing, so you then bypass the current source with a series R-C combination, and tune the "R" to get you the gain you need, but no more.
The trouble is, to do this mod, you will have to cut traces on your amps PCB, come up with low-voltage DC power for the transistor-based current source, and build the current source itself. Not only is this complex, it is also risky on several levels - and you won't know if you'll like the results or not, until after you've done it. So this should absolutely be tried outside your amp first - build a test rig, as Printer2 suggested, make sure that works and you're happy with it, only then even begin to consider modding your Bassbreaker.
Truthfully, if it were my new Bassbreaker amp, I wouldn't even consider options 2a, 2b, and 3. Option 1 is minimally intrusive, so if I were in your shoes, I might try it out to see if it made me happy.
Or you could solve all your headaches by simply plugging a 12AY7 in - definitely the simplest solution by far, if you can come to terms with it.
Whaddya think?
-Gnobuddy
So: we seem to now be at the point where a 12AX7 with a fully *unbypassed* cathode resistor still has too much gain, and too much nonlinearity, to suit you personal preferences. The easy solutions have been tried (remove cathode bypass caps), and we haven't reached the holy land yet. Now what?
Well, the actual question we need to answer is, "How do we get the distortion and gain of a 12AX7 even lower than you can get with a fully unbypassed cathode resistor?"
I can actually think of three different ways of doing this (and a couple more ways, which are practical in a scratch build, but not in modding a commercial product made on a PCB.) I will list them in what I think is increasing order of difficulty:
1) Add some local negative feedback from anode to grid of the second triode. This would involve tack-soldering one resistor and one capacitor onto your Bassbreaker PCB. Some experimentation would be needed to find the proper resistance value. If it works, and you find the results satisfactory, it is the simplest solution I can think of (short of just plugging in a 12AY7!)
2a) Remove the first 12AX7 from its socket, build a solid-state preamp to replace it, using a small high-voltage MOSFET or two. This will give you a squeaky-clean first two gain stages. With little TO-92 MOSFETs (like the LND150), it may be possible to make the preamp small enough to fit in the space formerly occcupied by the first 12AX7.
This promises to be challenging, but if you have the skills, it can be done. I once shoehorned an additional 12AX7 into a Superchamp XD to replace the (dead) solid-state preamp, so I know a modification like this is possible. But it will take patience, skill, and time, and will definitely void your Bassbreaker's warranty. If things go wrong (wiring errors, etc), it might also cause damage to the Bassbreaker.
2b) Build a solid-state preamp just like option 2a, but make it external, and self-powered. Basically, modify the Bassbreaker only enough to allow you to feed an audio signal into it at a point immediately after the first two stages (i.e. add one more 1/4" jack). Run your external preamp to that point.
3) Replace the 1.2k cathode resistor of the second gain stage with a current source, made from a transistor and a couple of other parts. Partially AC bypass the current source with a resistor that's bigger than 1.2k, in series with a capacitor.
The idea here is to allow that second stage to still bias up to the proper idle current, but at the same time, increase the AC impedance at its cathode higher than a 1.2k resistor. Current sources have very high AC impedance, so substituting one for the cathode resistor will effectively behave like an extremely high cathode resistor, dropping the gain to nothing, and the distortion very low. We don't want a gain of nothing, so you then bypass the current source with a series R-C combination, and tune the "R" to get you the gain you need, but no more.
The trouble is, to do this mod, you will have to cut traces on your amps PCB, come up with low-voltage DC power for the transistor-based current source, and build the current source itself. Not only is this complex, it is also risky on several levels - and you won't know if you'll like the results or not, until after you've done it. So this should absolutely be tried outside your amp first - build a test rig, as Printer2 suggested, make sure that works and you're happy with it, only then even begin to consider modding your Bassbreaker.
Truthfully, if it were my new Bassbreaker amp, I wouldn't even consider options 2a, 2b, and 3. Option 1 is minimally intrusive, so if I were in your shoes, I might try it out to see if it made me happy.
Or you could solve all your headaches by simply plugging a 12AY7 in - definitely the simplest solution by far, if you can come to terms with it.
Whaddya think?
-Gnobuddy
Meaning even cleaner than a real 12AY7, possibly to the point of sounding sterile.
Gnobuddy - wow, what an answer - thank you! Of the three, I agree that option #1 is the most practical for modding the amp itself. Sure I can just continue with the 12AY7 but I hate to say it, I have 18 amps of various sizes and after a while, it really is hard to remember that ONE of them requires a special tube to sound right. For my situation, it would be so much nicer to get the sound I like without having to remember special tube needs. Once I am sure I have what I'm looking for, I can start digging into a hardware mod so I can (hopefully) set it and forget it.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.