Is a solid state input stage the way to go?

[loopfinding]

Hey all, pondering this question. I haven't seen an SS input on too many tube amps - offhand I can think of some of the egnater stuff, or the modern vox ac10, and then the music man RD 50 (but that's a little bit different than what I'm pondering here). Not talking about the dumble FET where it's an additional stage, but rather just replacing the input triode entirely. I'm having doubts as to whether an input triode is really necessary on any tube amp.

My thinking is basically:

- Most of the distortion in the preamp is going to be coming from the second stage and PI, unless you are slamming the input with a boost, in which case the input stage is contributing to asymmetric clipping. But you could very well just set up a solid state device to not even flinch at an 8-16Vpp signal (like from a boost), pass an even hotter unclipped signal to the second stage, and essentially get the same transfer curve out of that second stage as you would from stage 1 + stage 2.

- Normally with my 2 stage (input + driver/cathodyne) amp I run either a compressor with the output volume jacked or just a 100% clean NE5532 boost in front of the amp to goose it. In a 3 stage design, I'm not really sure when you are driving a clipping stage, the PI, and the power amp hard how much any of that additional asymmetric clipping on stage one really contributes much to the character or feel of the amp.

- The input is the part of the amp most sensitive to noise. Paralleling triodes on the input is one way to get a better s/n ratio. Running DC heaters is a more involved way of getting better filament hum rejection. But a better s/n ratio or hum rejection using a solid state device is much more effective and trivial to implement. It would make for an extremely quiet amp.

- Using a high voltage JFET like an LND150, you wouldn't even need to worry about separate rails for it, and a device like that would pass the signal clean and hot.

- It's considerably cheaper (both short term and long term) than using a tube, allows for a more compact build, and is much less prone to failure.

Idk, what do you guys think, what are your experiences?

 

[gabasa]

I don't know if I can offer much here, but I recall that vintage Music Man amps had solid state preamps and tube output sections. Arcade Fire has used these amps extensively and they get some great sounds. I'd say go for it.

 

[cyclopean]

I really like how stiff musicman amps are.

 

[loopfinding]

I really like how stiff musicman amps are.

i think some of that might have to do with the SS rectifier and high amount of filtering on the power supply. some of them it might also be because of the weird cascode power tube setup.

i'm actually really gassing for either an rd-50 (tube clipper, solid state PI) or 65 (solid state pre but tube driver/cathodyne PI) at the moment and i want to take a crack at modding either to see if i can make them squishier but still get all the low noise/reliability benefits.

 

[Wildeman]

I was just playing my Marshall 4203, Artist 30, it's exactly that, SS pre, EL34 pwr and 12ax7 PI, it sounds like a killer Marshall should, I'm not the only one who thinks so either.

 

[loopfinding]

I was just playing my Marshall 4203, Artist 30, it's exactly that, SS pre, EL34 pwr and 12ax7 PI, it sounds like a killer Marshall should, I'm not the only one who thinks so either.

interesting, i'll check it out. in my case i'm more interested in still getting some pre clipping (before the PI) from tube, just swapping out the boring/utility stuff like the input, or input + stack recovery (if there's a later third tube stage, like a BF fender) to solid state and seeing what happens.

i recently saw this vid on the valvestate (with the 12ax7 clipper in the preamp) and it piqued my interest. at around 10 min he takes the pre and runs it into the power section of the diezel. it sounds more "correct" to me (hard to tell with high gain though). wondering how it actually feels or sounds with lower gain applications.

 

[Willie Johnson]

I had a stupid KMD with a solid state preamp and a 2x6l6 power amp and it sound like a spittoon full of angry bees, but good amps sound good and bad ones don't.

 

[Wildeman]

 

[edvard]

Your reasoning is sound, and is worthy of a try. The LND150 is a good, and maybe best option. The folks at SSGuitar and DIYStompboxes have discovered it to pretty good results. User KMG at SSGuitar has designed a part-for-part (mostly) JCM800 simply replacing the tubes with LND150s (of course with a few tweaks to accommodate them, as they are not a 1-for-1 replacement option). Close enough to be a relatively simple job though. If I were a bit richer, I'd spring for a cheap tube amp kit and give it a shot myself.

I'd personally like to see op-amps in the 300-ish volt range, which is probably easier done as a discrete stage rather than monolithic, as the highest voltage one I could find was the ADHV4702; ±110V supply, but the max input voltage is ±2V. That stops it from being suitable to be slammed with a boost. Heck, even the lowly 741 has a max input of ±15V...

 

[SerpentRuss]

I've built two amps with LND150 first stages, mainly so I could use a really small PT that didn't have to support one more tube's heater current requirements.

It's hard to say since I don't own a scope, but the LND150 if properly biased seems to be a pure boost stage that doesn't color anything. I think the first stage of any amp, if it's a center-biased tube, could easily be replaced with that DMOSFET. You do have to properly protect the gate with Zener diodes. I think reducing the heater noise in the first stage would be the biggest benefit. Losing the all-tube mojo would be the biggest con.

 

[tlsmack]

With no technical knowledge to support my opinions, I will just keep them to myself and follow this thread in the hopes of learning a few things.

 

[jrblue]

Personally, I think you're overthinking it and should get out there and just try some stuff. Personally, I think that relying on power stage saturation for anything is no longer viable or desirable, and there are now all kinds of SS preamps that are simply fabulous and many of them offer loads of control and adjustment to get what you like. I still use tube amps at the end of my signal chain, but I use neither preamp or power stage OD or distortion. I currently run a few excellent preamp and OD pedals that yield beautiful tones, but expect to gravitate to a Custom Tones Ethos Overdrive -- the amp version that has its own 30W SS amp, which will allow me to skip my tube amps in many/most venues. I think the OP is on the right track and would simply say, "Go out there and find what works for you." Options range from cheap/easy stuff to Millionaire Rabbit Hole" rigs that become a hobby all their own -- to be avoided, IMO.

 

[Peegoo]

Absolutely it's worth a shot. Done right it can sound good.

The last amp like this that I worked on was an 80s Peavey Classic 2x12 combo, 50 watts. The amp was a beast, weighing somewhere around 75 lbs. Unlike the Classic 50, the Classic was a hybrid with a SS front end and a 2x6L6GC power stage.

Super loud and super clean. A lot of country and steel players liked this amp. It was also the amp of choice for Andy Gill of Gang of Four.

 

[FenderLover]

I've been through the experiment in the past and found value in having a triode fist stage. True, it doesn't impart a great deal of character by itself until you hit it with a greater amplitude than a guitar can offer. Enter you favorite boost X. It also depend how you tune your amp, regarding gain staging. A scope is handy. If your PI falls apart while the pre stages have more to give, trim the input to the PI. With two triode stages and low loss EQ between them, try leaving the second stage un-bypassed and hit the first stage harder. There are many ways to squeeze the juice out.

 

[TequilaCaster]

Howard Alexander Dumble put a slectable JFET input stage on his OverDriveSpecial (ODS) amps. David Lindley (Jackson Browne) is supposed to have preferred using that input.

 

[loopfinding]

Personally, I think you're overthinking it and should get out there and just try some stuff. Personally, I think that relying on power stage saturation for anything is no longer viable or desirable, and there are now all kinds of SS preamps that are simply fabulous and many of them offer loads of control and adjustment to get what you like. I still use tube amps at the end of my signal chain, but I use neither preamp or power stage OD or distortion. I currently run a few excellent preamp and OD pedals that yield beautiful tones, but expect to gravitate to a Custom Tones Ethos Overdrive -- the amp version that has its own 30W SS amp, which will allow me to skip my tube amps in many/most venues. I think the OP is on the right track and would simply say, "Go out there and find what works for you." Options range from cheap/easy stuff to Millionaire Rabbit Hole" rigs that become a hobby all their own -- to be avoided, IMO.

oh i mean, i've tried a good amount of stuff, just not this configuration (as seen in some amps).

i have a small chassis right now with 2 noval and 2 octal sockets, and no transformers. i actually need a 50W amp that will be pushed to its limits for an upcoming project (unmic'ed punk stuff). may as well do it with the real thing. just trying to get a sense of the leanest thing i can get away with, if there are any good approaches worth considering that i’m unaware of, if there are any problems i can avoid, etc.

 

[Scottcurry13]

Try it and we will follow your journey. I’ve found that we can speculate all we want about circuits and mods, but until we try it for our own ears, we will never know.

 

[FenderLover]

Howard Alexander Dumble put a slectable JFET input stage on his OverDriveSpecial (ODS) amps. David Lindley (Jackson Browne) is supposed to have preferred using that input.

I've tried this approach too and abandoned it if favor of a boost out front. It goes with a build philosophy of keeping amplifier circuitry baggage to a minimum. Example: Why build in a spring reverb if one can add an FX loop and plug in anything we want?

There is an advantage of using a boost out front, aside from choosing the flavor of the day. The first amplification stage should ideally be as close to the source as possible. If there is a long cable run to an amplifier, the boost belongs at your feet, not at the end of the cable.

 

[sds1]

Howard Alexander Dumble put a slectable JFET input stage on his OverDriveSpecial (ODS) amps. David Lindley (Jackson Browne) is supposed to have preferred using that input.

Indeed, as the D-folklore goes this input was intended to accommodate Lindley's guitars with piezo pickups.

 

[loopfinding]

I've tried this approach too and abandoned it if favor of a boost out front. It goes with a build philosophy of keeping amplifier circuitry baggage to a minimum. Example: Why build in a spring reverb if one can add an FX loop and plug in anything we want?

There is an advantage of using a boost out front, aside from choosing the flavor of the day. The first amplification stage should ideally be as close to the source as possible. If there is a long cable run to an amplifier, the boost belongs at your feet, not at the end of the cable.

yeah i've thought about it in this way too. like if you're using a solid state first stage anyway, then maybe don't even include a gain knob on the amp, just have a master volume (after the phase inverter)...or no volume control. put the gain stage on the floor (e.g. a boost) then just use a volume pedal to control the level of dirt. if a 9V boost is enough to goose a tweed deluxe or an 18 watt, then an 18V boost might be fine for something with a tonestack. essentially just treat the amp as a power amp.