Hello every one . due to the lack of valves tubes and in general all this situation that prevailed with the war in Ukraine, I open this topic .. generally looking for a way to replace a valve tube with a transistor… in general I have made several emulators pedals such as: Mesa boogie / soldano / fender / Marshall ,, and all I did was copy exactly like the schematic only I changed the bulbs with jfet (j201) and put a trimpot in the drain and adjusted it 4.5-5.00 volts depending on the occasion, .. now this What I am looking for is to make a replacement of a valve, the only problem is that I do not know what jfet to use for 300+ volts ,,, I have found similar products such as jetcity retro valve and something called fetron…. I know very well that in sound quality it will differ, I just want to have a planB because there are no more valves and the ones that do exist are really very high prices! ..
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Don´t remember the name at the moment but there is a high voltage depletion MosFet (hint: "it biases like a tube") , NOT a "normal" enhancement MosFet ,(99.999% of those out there) which is being used by everybody, including Marshall.
That one should do the trick.
EDIT: here
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
More of a Pentode operation than a Triode, but hey, you eat what´s served in your plate hehe.
VERY tube sounding anyway.
That one should do the trick.
EDIT: here
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
More of a Pentode operation than a Triode, but hey, you eat what´s served in your plate hehe.
VERY tube sounding anyway.
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Wait what ?!?! This transistor replace the triode of valve with out any modification ?! Interesting 🤔. …Don´t remember the name at the moment but there is a high voltage depletion MosFet (hint: "it biases like a tube") , NOT a "normal" enhancement MosFet ,(99.999% of those out there) which is being used by everybody, including Marshall.
That one should do the trick.
EDIT: here
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
More of a Pentode operation than a Triode, but hey, you eat what´s served in your plate hehe.
VERY tube sounding anyway.
confusing now ,, I do not want to make pre-amplification…. I want to make an emulator of 12ax7. to replace it… ie instead of the 12ax7 I will put (this thing that will have the mains transistor and logically some circuit) which should go into a 9 pin socket… the photo of JMFahey shows a preamp circuit. but I understand more or less what is happening. I have found schematics with other valves that convert them to solid state, but none with 12ax7 .. that's why I need your help, who knows more things than I do.
Also I found that schem of fetron . He say that use 2x silicon N- channel J-FET …but which??? Maybe the LND150 do the work ? And I found some othe solid state replacements with IRF730 …
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That's not going to be easy.
You can find Fetrons and even generic MOSFET devices that can replace a triode as a generic amplifier but as is they won't emulate the characteristics of a 12AX7. Fetron never even tried to; its pentode-like characteristics were much superior for higher gains and common switching applications. MOSFET as is will have very different plate characteristics (again more similar to pentode tubes), teansconductance, impedance levels and bias points but yes, it is "voltage driven" and some of them can operate at high voltages. VFETs have triode-like characteristic curves but I suppose there weren't many high voltage ones and they have been obsolete for about 40 years. None of them, I think, tried to specifically emulate 12AX7 so again different transconductance figures, impedance levels and bias points.
You may find some more realistic 12AX7 emulations from likes of AMT ("Warmstone" triode) or Robert's "Retrovalve" but these are not trivial designs. Particularly the later is a very complex circuit. Retrovalve is patented technology so the patent documents will provide some insight of its inner working. Few minutes of google search should find AMT schematics and the Warmstone whitepaper. It's basically A high voltage MOSFET circuit that mimicks grid characteristics of a 12AX7.
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very nice and informative post. Thank you very much for your help, the point is that I do not find any schematic on retrovalve or warmstone .. whenever I look for other pre amp vacuum tubes that have made them solid state and somehow I try to find an edge in all this mess .. I believe that I will succeed with your help.
They are proprietary designs so you probably will not find schematics. IIRC, Retrovalve is largely a digital signal processor assisted by high voltage MOSFET, and Warmstone is loosely described in their whitepaper and similar circuitry employed in the AMT pedals. Warmstone is fully analog circuit that exploits specific plate cathode feedback and a clipping diode at gate circuit in order to emulate the 12AX7 triode.
¿¿¿¿???? IF you want to emulate/substitute a 12AX7 of course you will use it in a preamp circuit, what´s your point?
This was just an example.
In principle you take any 12AX7 based Guitar preamp, leave it as is, put LND150 replacing each triode, only difference is you use 330 ohm cathode resistor instead of 1k5 and add protection diodes across Gate ("grid")
You will be quite close to what original 12AX7 did, only higher gain, with a cheap simple substitution.
Nlow if you want to make an "SS 12AX7" which plugs in without any other mod and works/sounds exactly the same, then the circuit becomes quite complex, as those mentioned above.
Forget Fetrons and high voltage JFets, not made for 40 or 50 years by now, and they are not any closer to 12AX7.
my goal is to make as he said a replacement of 12ax7 so that it enters the socket without any absolute change in the circuit of the amplifier .. I know it will not be the same quality, I know it will not be the same sound .. just I want to start somewhere, so far I understand that it is complicated but I want to try ..
Thank you but must find to built it ,, it’s easy to buy everything but the pursuit of knowledge is quite difficult
hanks for that interesting circuit.
That said, it is NOT a universal 12AX7 substitute, but only in that very particular organ oscillator circuit, nowhere else.
* WAY too low input impedance.
* clips like a transistor
* might stand , say, 60/80V +V, definitely not +250/300V
* MPSA42/92 can stand up to 300V, but gain is very poor, lowering input impedance even more.
That said, IF you are committed, fine with me 🙂 , keep searching and start experimenting 😉
Suggest you build a standard 12AX7 gain stage circuit, by it another socket (which does not even need to be noval, simply a modern 0.1" or 0.156" pitch) for ease of connection/disconnection, build as many prototypes as you wish and plug them there for comparison.
A toggle switch or even better a foot operated relay for hands free operation will let you play your guitar and go back and forth through either of channels (tube an SS emulator) at will.
Then let your ears decide.
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I really hope to find an edge .. let me experiment and let nothing come out… tomorrow I am waiting for a friend to bring me a transformer from a damaged old tube amp .. and I will make a simple pre amp circuit for 12ax7 like fender champ 5F1 and experiment there ..
The enhancement fet is an interesting idea.
However the 12ax7 may be as cheap and low current 100v supplies are easy to make.
However the 12ax7 may be as cheap and low current 100v supplies are easy to make.