freestompboxes.org

Sometimes things don't go as you planned. You try and try again but the issues persist. I've started working on this project 11 months ago, but it's finally in a state I'm happy enough with to share it.
Whether I want to record easily or practice without disturbing others, I'm fond of Guitar Rig. One amplifier model it features is the Hot Solo, a Soldano SLO (100). Since it's the foundation of my “no-frills metal” preset, I thought it would be fun to turn the preamplifier circuit into a DC-powered box I could plug into my interface or the power amp input/fx return of an amplifier, or even as distortion pedal in front, for no other reason that I could, and while I had already made a DC-powered tube amplifier, a preamplifier was an idea I wanted to try for a while. It's well known that the SLO-100, being an amplifier provided of master volume, at most times concentrates its distortion timbre in the preamplifier, making this a worthy endeavor.

I started from the schematic available on el34world. As many others did, I focused on the Overdrive channel, which is the most popular one and the one I was interested in, to keep things compact and use few tubes. The first thing I wanted to address is the B+. This, as many other amplifiers, uses very high voltages in the preamplifier, in this case quoted at 359V, 350V, 378V, probably because they're derived from the even higher voltages of the power amplifier. These voltages are above the maximum ratings of most 12AX7 tubes, and also near the limit of the boost converter I was using. Do they need to be? Of course too low voltages can cause issues too, but I chose to use the safe and standard 250V supply. This doesn't seem enough “fiery heavy metal high voltage” to you? Well, I'm sorry .

This doesn't mean I just carelessly changed the voltage and called it a day hoping for everything to fall into place. The most interesting part of this adaptation for me was to rescale everything to work the same at this voltage. The two things to address are output swing and bias, since I've checked with a load line tool that the voltage gain stays roughly the same between these voltages.

• About the first point, I've realized two important factors just in time: for all stages, the peak-to-peak input headroom (from grid conduction to cutoff) gets scaled down by the same factor of 1.5, save some small variations due to the different B+ voltages. The output swing also is scaled down by the same amount, because the anode cannot swing as high with the lower supply voltage (this was also measured on load lines. So while I was tempted to scale the output of each stage down, proportionally to the reduced input headroom, this was already done by the same reduction of supply voltage and I didn't need to do anything more, no attenuation between the stages or changing anode resistors. In hindsight this was to be expected if gain had stayed the same.
  This doesn't apply completely to the first stage, since the input headroom is reduced but the input is determined by the external source. Thankfully though, as it's usually the case for a good, strong, input stage, this stage doesn't clip by itself under normal circumstances. You need at least a 1.1V positive peak (which is the one clipped by soft grid conduction) or 3.4V negative (for the harder cutoff) to make it clip, and if you boost into it that much it means that's what you wanted after all!
  In conclusion, after all these measurements I just had to account for the 1.5x larger signal coming from the first stage, but not coincidentally there's the gain control right there. Instead of tweaking the series resistor while keeping the shelving filter the same, I left the extra amplitude in the hands of the gain pot: think of this as having a little “extra” 3dB at the end of the travel.
• The second point is bias. Changing the B+ moved the bias point sometimes considerably. Since the peak-to-peak swing was taken care of in point 1, my goal was to keep the same bias “temperature” of the original, the relative balance between cold and hot bias, the proportion of positive to negative swing from the bias point. I achieved this with a load line tool and by changing the values of the cathode resistors and capacitors accordingly.

Another change from the original is the smaller grid stopper resistor on the input stage, according to the suggestions in Designing Tube Preamps for Guitar and Bass, 2nd edition for lower noise in the crucial input stage of such an high gain circuit. I did calculate the equivalent grid capacitance (130pF) but I went from a smaller capacitor to ground than the required 750pF to keep the same cutoff to not affect the guitar pickup resonant peak too much, an effect not considered by the author.

After this it was time for a quick count. I had four common cathode amplifiers, which means a pair of tubes already. After checking that the fx send and return had complementary amounts of attenuation and amplification, and given that there's nothing else past that to clip before the master volume and the phase inverter, I just had to account for the cathode follower and another buffer after the tone stack and volume to give me a nice low output impedance fit for a preamplifier.
The cathode follower is where I had to make a judgment: being fashioned after Marshall (as most of the circuit is), it has the same bias that results in it softly clipping. While this effect is present, I don't know its importance among the vast amounts of distortion this circuit can provide, so my choice was to replace it with a MOSFET follower which doesn't clip, neither nicely or badly, and just faithfully drives the tone stack, which allowed me to keep the tube count to two. You aren't forced to make the same choice.
The IRF710 I've used is a cheap and unimpressive high voltage MOSFET, with a generous 400V maximum Vds, completely at ease in the undemanding application. The 710 has smaller gate capacitances and higher Rds(on) than the others in its line, which seems suitable for the purpose.
There's not much to say about this stage, other than the additional zener diode to clamp Vgs and the resistors in series with gate and source to prevent oscillation.

The tone stack is the same as in the original. Instead of the master volume, since I needed to bring down the levels to line sooner or later, I put a fixed attenuator. I decided for this position instead of leaving the attenuation at the output because this way I could adjust it without worrying about the output impedance or the volume pot value. The amount of attenuation was an educated guess based on the peak-to-peak output voltage of the last stage which I later refined. It's easy to change it with R24. The load on the tone stack is roughly 500k, just as in the original with the two parallel 1M master volume pots.

The output buffer is another source follower, this one AC coupled and with a simple bias that works well in this position. The fixed bias and the source make me quite confident that I don't need another zener diode here. The 1k resistor isolates the output and limits current to the 12V zener clamps which are the last line of defense against output spikes. I've measured the maximum output voltage to be about 3V peak, so lower voltage zeners would work just as well, but they're just added safety since the signal is already attenuated.

While I had calculated the gain and cutoff of the presence control according to the transformer ratio and gain of the stages, which agrees with other sources online which have done the same, I couldn't find a place to add a shelving filter after the clipping without another active stage. The input to the tone stack needs to stay low impedance; the output of the tone stack would almost work if it weren't that its output impedance is highly variable, throwing off the filter completely; having it at the output would ruin my output impedance. I decided that while it is possible to have this control, it is not of crucial importance to the core timbre of the circuit, and I can achieve the same effect with an equalizer if the tone stack isn't enough. Another fun challenge while beginning this project was to make my own high voltage boost converter. A preamplifier would be the perfect candidate, since the voltage is high, but the current is just a few mA. There are a few such designs, many of them featuring a simple 555 oscillator driving a MOSFET, with voltage feedback. Here's one example, while my friend Ethan has come up with his own variation.
Since I had all the necessary components, I tried this kind of circuit, and it kind of worked but with poor performance, with the output never reaching too far above 100V even when tweaking the feedback. I've tried this both with some commercial inductors I had in that value range, both with homemade ones with wire around a ferrite core, which again measured a suitable inductance. The DIY ones weren't probably very good, but neither the commercial ones reached the desired performance before arcing over. Turns out small inductors very rarely have a voltage rating, not being intended for this application, so while mine clearly wasn't up to the task, I couldn't find any I was sure would, so I put this idea aside for now.

Instead, this build features a return of the “yellowboi” 8-32V to ±45V-390V, UC3843-based boost converter, with all its merits and flaws, even if it's honestly overkill for this power. The heater supply is again the MP1584 buck converter, which had proven itself ideal already.
Since I knew the output from the 'boi wasn't very clean, I was sure to provide plenty of filtering before hitting the sensitive, high gain stages. The supply for the whole circuit is first passed through a MPSA42 capacitor multiplier, with the high voltage low power BJT being fit for the purpose and then through a simple RC filter to the tube amplifiers. Since this drops a certain amount of voltage, the output of the boost converter was turned up to compensate, giving the tubes the 250V by design and a bit more to the relatively more current thirsty MOSFET followers that don't need as much filtering. The result is satisfactory, but I suggest trying a different, more suitable boost converter if possible. I've built the circuit in a tight 1590BB enclosure with a mix of point-to-point wiring, perfboard and the PCB of the boost converter boards. You can see it as the natural history of electronic circuits . The point-to-point includes the tube pins, the central socket pin used as ground point and two terminal strips as additional support and was done for space concerns more than anything. The perfboard includes the gyrator and the MOSFETs. Both perfboard and boost converter are secured in place with brass standoffs that screw to the bottom lid.
The enclosure design is made of a polished aluminum enclosure on which I tried caustic soda etching for my second time. The etch came cleanly but not very deep, so I had to forgo the planned filling of the etched areas with black paint because it wouldn't come out as I wished. I like the subtle look of it and it's sill readable. If this had been it, the build would have been completed months ago. But things don't always go as planned. Usually I don't give up easily, and if I set on soldering together something it's because I like it enough to see it to the end. I don't take it apart just because it doesn't work. That's why I was both ashamed and disheartened that it spent a long time in a cupboard when I decided I needed a break from struggling with it.
The first small struggle was power supply noise, but I was expecting that from the 'boi, so I added more filtering until it was decent. The real issue was an impossible to ignore, loud oscillation at a couple of kHz with the gain past half. I wasn't surprised at first, this circuit has tons of gain, even more than a similar sounding distortion pedal because of the relatively larger voltages required to clip tubes. Those same tubes are in high impedance stages and their output swing is proportionally gigantic, so it takes little to start a fire.
My optimism went down and down as I checked off all my possible fixes: shielding the input wire, increasing the input capacitance, the grid stopper, rethinking grounding more carefully, mixing up tubes, redoing the perfboard, adding series resistors to the MOSFET and some other things I don't remember. Then I started breaking the circuit in various points to see how extensive was the problem, finding that I didn't have oscillation before the MOSFETs but I did after the tone stack. I tried taking apart the tone stack then to see if it was involved. I must have repeated this step a few times, forgetting the exact results after some days of break between the attempts. I also noticed that, other than being affected by the gain control, grounding the input or using a low impedance source, instead of terminating it with a resistor or pickup, prevented the oscillation. So the feedback loop enveloped almost the whole circuit, from the input to the DC follower.
This is where I left it for a long time. After finishing my last project, I pulled this out of the cupboard and set on finishing it once and for all. After a while, moving things around and powering it up, to avoid memorable shorts, I noticed that having the perfboard and the pots out of the box made the oscillation go away. I thought it was the wire going to the first gate but it turned out the oscillation was caused by the perfboard and pots coupling signal into the input jack. The wire going from it was shielded, but the jack itself was not. To solve this, or at least make the problem much less severe, I made a shielding bracket from an aluminum sheet that mounts to the input jack, generously padded with electrical tape. I still have a bit of oscillation in the last bit of the gain pot, which I don't use, unless I use a low impedance source like a boost pedal. I hope a different internal layout might solve the issue completely for someone else, but I'm happy to have a working, completely usable circuit anyway. Here's a quick demo of what you should expect this to sound like.
I'm happy to have reached a state with this project that I can call done. It's a fun and beautiful box if I can say so, but does this mean I will never use the Hot Solo again? Probably not, since the convenience of having not only the amplifier but also any necessary tube screamer-like pre-emphasis in software is hard to beat, and while I always thought VST models gave their best with high-gain amplifiers, I can only confirm this for how close the two sound. The SLO preamplifier might be the choice in winter since it also works as space heater though .
I'm interested as always in your thoughts and valuable comments.

OK so first of all this is fantastic

However, I also thought about using only two tubes and having a hybrid cathode follower using a mosfet but I was dissuaded from it cos supposedly they dont distort the same way as tube cathode followers do

I mean is the demo you recorded on full gain? Cos to me it sounds like it lacks some gain and I know SLOs have TONS of gain and this could be your problem

I had no issue running 3 tube preamps off of a simple 555 based SMPS so you could use 3 tubes in your build with no issue, however a larger enclosure would be a must

You could use the last half of the 3rd tube as either a clean channel 2nd stage and retain the mosfet output buffer or you could use the last half as the output buffer itself

Reachahighernoon wrote: ↑03 Sep 2022, 10:02 OK so first of all this is fantastic

However, I also thought about using only two tubes and having a hybrid cathode follower using a mosfet but I was dissuaded from it cos supposedly they dont distort the same way as tube cathode followers do

I mean is the demo you recorded on full gain? Cos to me it sounds like it lacks some gain and I know SLOs have TONS of gain and this could be your problem

I had no issue running 3 tube preamps off of a simple 555 based SMPS so you could use 3 tubes in your build with no issue, however a larger enclosure would be a must

You could use the last half of the 3rd tube as either a clean channel 2nd stage and retain the mosfet output buffer or you could use the last half as the output buffer itself

Oh no no, in the demos the gain was around half, with or without the boost! I like it this way, it's a bit to fuzzy at max. If anything this should have a bit more than the original, I'll refer to the main post, speaking about gain. Speaking about clipping, where that gain ends up, as I said the dccf clips while the mosfet is clean. I'm happy with this tradeoff instead of having a third tube and having to use the other half as the other follower or something else. Whether the very soft cathode follower clipping is important or not I leave to others to investigate and test. I'd think it's more like the cherry on top of a distortion cake than anything.

For the clean channel, a more economical solution would be to use one switch pole to disconnect the cathode bypass capacitor on the second stage (optionally a resistor in parallel or series can be switched by the same pole to change the bias but that's not really necessary), the other pole to change the output to the different attenuation/shelf network. I'll just leave this here as an idea.

I know current isn't an issue for using a 555 smps here, but I didn't get good results with the inductors I tried. I'm glad you did, which kind did you use?

Thanks anyway for your words.

dylan159 wrote: ↑03 Sep 2022, 12:55

Reachahighernoon wrote: ↑03 Sep 2022, 10:02 OK so first of all this is fantastic

However, I also thought about using only two tubes and having a hybrid cathode follower using a mosfet but I was dissuaded from it cos supposedly they dont distort the same way as tube cathode followers do

I mean is the demo you recorded on full gain? Cos to me it sounds like it lacks some gain and I know SLOs have TONS of gain and this could be your problem

I had no issue running 3 tube preamps off of a simple 555 based SMPS so you could use 3 tubes in your build with no issue, however a larger enclosure would be a must

You could use the last half of the 3rd tube as either a clean channel 2nd stage and retain the mosfet output buffer or you could use the last half as the output buffer itself

Oh no no, in the demos the gain was around half, with or without the boost! I like it this way, it's a bit to fuzzy at max. If anything this should have a bit more than the original, I'll refer to the main post, speaking about gain. Speaking about clipping, where that gain ends up, as I said the dccf clips while the mosfet is clean. I'm happy with this tradeoff instead of having a third tube and having to use the other half as the other follower or something else. Whether the very soft cathode follower clipping is important or not I leave to others to investigate and test. I'd think it's more like the cherry on top of a distortion cake than anything.

For the clean channel, a more economical solution would be to use one switch pole to disconnect the cathode bypass capacitor on the second stage (optionally a resistor in parallel or series can be switched by the same pole to change the bias but that's not really necessary), the other pole to change the output to the different attenuation/shelf network. I'll just leave this here as an idea.

I know current isn't an issue for using a 555 smps here, but I didn't get good results with the inductors I tried. I'm glad you did, which kind did you use?

Thanks anyway for your words.

Oh! Then if the gain is around half forget I said anything cos it does sound like an SLO, not really sure how much the CF clipping is really important to the overall sound

The idea with my take on the channel switching is that you can tune it to sound more Fender like but your idea is good too

As for inductors I had no issue with using the ones from Tayda, 100UH 2.1A power inductors. 180V with no load, 160-ish with load and the schematic is the same 555 based that's floating around the internet for ages now

And you are very welcome, do you mind if I make a PCB of this?

Reachahighernoon wrote: ↑03 Sep 2022, 15:29 Oh! Then if the gain is around half forget I said anything cos it does sound like an SLO, not really sure how much the CF clipping is really important to the overall sound

The idea with my take on the channel switching is that you can tune it to sound more Fender like but your idea is good too

As for inductors I had no issue with using the ones from Tayda, 100UH 2.1A power inductors. 180V with no load, 160-ish with load and the schematic is the same 555 based that's floating around the internet for ages now

And you are very welcome, do you mind if I make a PCB of this?

Yeah, I was thinking about the normal channel in the SLO but you're free to do as you like to. The first stage is far from being fully bypassed, so I think for a Fender-like sound what needs to change is the tone stack in some way.

Those are the same voltage I was able to reach, inductors arcing over or not, and my inductance was also in the 47-100u range. Glad to know it works reliably for you, and those are very usable voltages, but I'd like to get at least above 200.

You're welcome to do a pcb for this, this is a freely shared schematic. Just keep in mind that, according to the share-alike clause you should use the same CC license as I did. More info here. The "use the license" at the bottom gives options to add it.
Other than that I can just wish you good luck, and be careful with coupling between in-phase traces with those pesky high amplitude, high gain, high impedance signals!

dylan159 wrote: ↑04 Sep 2022, 12:04

Yeah, I was thinking about the normal channel in the SLO but you're free to do as you like to. The first stage is far from being fully bypassed, so I think for a Fender-like sound what needs to change is the tone stack in some way.

Those are the same voltage I was able to reach, inductors arcing over or not, and my inductance was also in the 47-100u range. Glad to know it works reliably for you, and those are very usable voltages, but I'd like to get at least above 200.

You're welcome to do a pcb for this, this is a freely shared schematic. Just keep in mind that, according to the share-alike clause you should use the same CC license as I did. More info here. The "use the license" at the bottom gives options to add it.
Other than that I can just wish you good luck, and be careful with coupling between in-phase traces with those pesky high amplitude, high gain, high impedance signals!

Incase you are wondering here's how I usually implement a second channel
Oh yeah as far as I know the 555 timer ones arent stable above 190ish volts though I've seen 555 based SMPS that actually handle 250 volts I think, either way 160 volts under load is good enough for me. I read that you couldnt get more than 100 volts though...I have no idea why, I heard the amperage has a lot of play here

And what do you mean by "be careful with coupling between in-phase traces with those pesky high amplitude, high gain, high impedance signals!"

And of course, thank you

Reachahighernoon wrote: ↑04 Sep 2022, 14:44
Incase you are wondering here's how I usually implement a second channel

Oh yeah as far as I know the 555 timer ones arent stable above 190ish volts though I've seen 555 based SMPS that actually handle 250 volts I think, either way 160 volts under load is good enough for me. I read that you couldnt get more than 100 volts though...I have no idea why, I heard the amperage has a lot of play here

And what do you mean by "be careful with coupling between in-phase traces with those pesky high amplitude, high gain, high impedance signals!"

And of course, thank you

I see you're implementing some personal tweaks in the "overdrive" path, but I'll just comment on some things that catch my attention:

• The second tone stack hanging off the input of the first stage is going to load it down. That's not very gain of your SLO. you probably want to disconnect it when you switch channels
• What's the purpose of R4?
• R23 and the 470k are going to taper the volume controls. The resulting attenuation is beneficial, the tapering, although small, might not. On a related note, is the total attenuation before the output sufficient? I suggest something close to mine, which is x0.045. That results in a useful output for both instrument and line inputs.
• About the bias on the last cathode follower. You don't have to bias it that cold but it's beneficial to keep current draw lower and that's still plenty of headroom. You might want to make the bias resistors larger or use cathode self-bias to cut down on that ~1mA of current in the divider at 250V (a bit less if the voltage is lower of course). Fixed bias also requires a diode between grid and cathode.
• Since this isn't a pedal and they're equally common, I'd use a 12V power supply, so the heaters aren't so undervolted.

I was able to get 140-160V at most, that's what I meant with "not that far above 100V", less than 200V anyway.

I meant that if you're not careful with the pcb layout, you might have the same oscillation issues I had. I think this kind of circuit has all that it needs to cause that kind of issue.

dylan159 wrote: ↑04 Sep 2022, 20:12

I see you're implementing some personal tweaks in the "overdrive" path, but I'll just comment on some things that catch my attention:

• The second tone stack hanging off the input of the first stage is going to load it down. That's not very gain of your SLO. you probably want to disconnect it when you switch channels
• What's the purpose of R4?
• R23 and the 470k are going to taper the volume controls. The resulting attenuation is beneficial, the tapering, although small, might not. On a related note, is the total attenuation before the output sufficient? I suggest something close to mine, which is x0.045. That results in a useful output for both instrument and line inputs.
• About the bias on the last cathode follower. You don't have to bias it that cold but it's beneficial to keep current draw lower and that's still plenty of headroom. You might want to make the bias resistors larger or use cathode self-bias to cut down on that ~1mA of current in the divider at 250V (a bit less if the voltage is lower of course). Fixed bias also requires a diode between grid and cathode.
• Since this isn't a pedal and they're equally common, I'd use a 12V power supply, so the heaters aren't so undervolted.

I was able to get 140-160V at most, that's what I meant with "not that far above 100V", less than 200V anyway.

I meant that if you're not careful with the pcb layout, you might have the same oscillation issues I had. I think this kind of circuit has all that it needs to cause that kind of issue.

Oh, no that's basically the high gain channel of a Rectifier not an SLO, I just posted the schematic to show how I usually add a clean channel along with the distortion channel rather than to show tweaks to the high gain channel

Oh sorry, I misread you then, I did see 555 SMPS that can deliver more than 200 volts, I think the Trex Squeezer compressor has one

Any recommendations then for proper PCB design?

Reachahighernoon wrote: ↑05 Sep 2022, 03:36 Oh, no that's basically the high gain channel of a Rectifier not an SLO, I just posted the schematic to show how I usually add a clean channel along with the distortion channel rather than to show tweaks to the high gain channel

Oh sorry, I misread you then, I did see 555 SMPS that can deliver more than 200 volts, I think the Trex Squeezer compressor has one

Any recommendations then for proper PCB design?

You have to admit, they're pretty similar schematic-wise hehe, never realized how much until now.

Thanks for telling me about the t-rex squeezer, The two main differences are the use of a transformer and that there's no feedback. I might have some SMPS transformers that could work there. On a side note, what stupid use of a tube there.

I really don't have much experience with pcb design, but whenever two signals are in phase especially, you want to keep one away from the gridb of the other. if there's a ground pour in between it should help.

dylan159 wrote: ↑05 Sep 2022, 07:50

Reachahighernoon wrote: ↑05 Sep 2022, 03:36 Oh, no that's basically the high gain channel of a Rectifier not an SLO, I just posted the schematic to show how I usually add a clean channel along with the distortion channel rather than to show tweaks to the high gain channel

Oh sorry, I misread you then, I did see 555 SMPS that can deliver more than 200 volts, I think the Trex Squeezer compressor has one

Any recommendations then for proper PCB design?

You have to admit, they're pretty similar schematic-wise hehe, never realized how much until now.

Thanks for telling me about the t-rex squeezer, The two main differences are the use of a transformer and that there's no feedback. I might have some SMPS transformers that could work there. On a side note, what stupid use of a tube there.

I really don't have much experience with pcb design, but whenever two signals are in phase especially, you want to keep one away from the gridb of the other. if there's a ground pour in between it should help.

Eh, not really but the basic 5 stage preamp design is quite similar, the Recto has that overcomplicated network array at the 2nd stage which is typical Mesa over engineering

As far I know the Trex doesnt use a transformer it is very similar to the typical 555 with feedback network

Well I will try and keep it mind, though I think I need an example as to what lines to keep in mind

Reachahighernoon wrote: ↑05 Sep 2022, 08:55
Eh, not really but the basic 5 stage preamp design is quite similar, the Recto has that overcomplicated network array at the 2nd stage which is typical Mesa over engineering

As far I know the Trex doesnt use a transformer it is very similar to the typical 555 with feedback network

Well I will try and keep it mind, though I think I need an example as to what lines to keep in mind

You mean after the first stage? It's more complicated than it needs to be, but it boils down to a bright cap/high shelf substantially the same (with the due differences) to the one in the slo. The 22p is dominated by the miller capacitance of the next stage anyway.

This is what I saw according to a schematic here on the forum

dylan159 wrote: ↑05 Sep 2022, 11:05

Reachahighernoon wrote: ↑05 Sep 2022, 08:55
Eh, not really but the basic 5 stage preamp design is quite similar, the Recto has that overcomplicated network array at the 2nd stage which is typical Mesa over engineering

As far I know the Trex doesnt use a transformer it is very similar to the typical 555 with feedback network

Well I will try and keep it mind, though I think I need an example as to what lines to keep in mind

You mean after the first stage? It's more complicated than it needs to be, but it boils down to a bright cap/high shelf substantially the same (with the due differences) to the one in the slo. The 22p is dominated by the miller capacitance of the next stage anyway.

This is what I saw according to a schematic here on the forum

Yeah, it is basically a remnant of the channel switching from the real amp? I think I adopted it from the Dr. Boogie schematic

And oh, my bad, it did have a transformer

Hi,

great project! i have done a similar job years ago. It sounds great. Knowing that the first stage has less influence on the sound than the cathode follower, why not place the mosfet stage as input and the triode as cathode follower.
A first stage based on LND150 could be awesome, right?

Lemontheo wrote: ↑09 Nov 2022, 08:35 Hi,

great project! i have done a similar job years ago. It sounds great.

Knowing that the first stage has less influence on the sound than the cathode follower, why not place the mosfet stage as input and the triode as cathode follower.
A first stage based on LND150 could be awesome, right?

Aww, I thought for a moment you had built this one, but very cool anyway. What choices did you make differently from me?
About the MOSFET, I actually considered it, depletion or not. The main issue is input capacitance, even worse than normal because of the miller effect. Also they're noisy, probably even more than the unimpressive tubes. JFET might not have enough gain, and good luck finding high voltage ones. The best choice is probably BJT, which should have enough gain even with degeneration to keep the input impedance at 1M, and can always be bootstrapped. Worst come to worst, a two transistor feedback amplifier.

dylan159 wrote: ↑09 Nov 2022, 09:40

Lemontheo wrote: ↑09 Nov 2022, 08:35 Hi,

great project! i have done a similar job years ago. It sounds great.

Knowing that the first stage has less influence on the sound than the cathode follower, why not place the mosfet stage as input and the triode as cathode follower.
A first stage based on LND150 could be awesome, right?

Aww, I thought for a moment you had built this one, but very cool anyway. What choices did you make differently from me?
About the MOSFET, I actually considered it, depletion or not. The main issue is input capacitance, even worse than normal because of the miller effect. Also they're noisy, probably even more than the unimpressive tubes. JFET might not have enough gain, and good luck finding high voltage ones. The best choice is probably BJT, which should have enough gain even with degeneration to keep the input impedance at 1M, and can always be bootstrapped. Worst come to worst, a two transistor feedback amplifier.

....Really? The capacitance/miller effect of FETs is that big that it is not a viable solution to place them in the beginning?

Also, I am making a PCB, I will get back to you on this

Reachahighernoon wrote: ↑09 Nov 2022, 13:35 ....Really? The capacitance/miller effect of FETs is that big that it is not a viable solution to place them in the beginning?

Also, I am making a PCB, I will get back to you on this

Pretty sure it does, especially with common source (miller) and with large high voltage FETs.

Isnt there a way to mitigate this? I mean the cathode follower is reported to be more tonally critical

Hi folks,

In the very first amplifier stage of Dumble amps was an option to employ a fet rather than a half of 12AX7:



I think that the combination of 1Meg and 100pF in parallel are to compensate the mentioned Miller effect...

Cheers,
Jose

LND150 has surprisingly low gate capacitance for a depletion mode mosfet. There is some company that has a first stage boost built with a LND150. The first stage might be best as a triode anyway as players want to drive it into oblivion with boosts and overdrives.

bmxguitarsbmx wrote: ↑09 Nov 2022, 17:37 LND150 has surprisingly low gate capacitance for a depletion mode mosfet. There is some company that has a first stage boost built with a LND150. The first stage might be best as a triode anyway as players want to drive it into oblivion with boosts and overdrives.

I've calculated just 37pF for a common source with a gain of 60. That's very good! But nothing can be done about the fact that mosfets are noisy, especially in the input stage of a very high gain circuit.