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Solid state high gain amp design f.hard rock/heavy metal using depletion MOSFETs

Started by stratitis, March 23, 2020, 04:21:02 PM

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stratitis

Hi,
I like the sound of 5150/6505, SLO100, hot rodded marshalls and similar tube amps, but I don't like all the tube typical disadvantages (heat, wearing, microphony, weight, costs, high voltage...).
So I am looking for solid state alternatives to tube amps and found some interesting circuits using depletion MOSFETs (LND150) by KMG and others.
I'm not quite sure if I need a tube or tube emulating FET output stage using a big output transformer to get that overdriven sound by the output stage, or if same or similar sound could be generated in the pre amp section. If possible I'd like to use a "normal" transformerless transistor final stage to get rid of the weighty and expensive output transformer. I'd also want to get rid of higher voltages >>60-100V.
Does anybody know if there's a chance?
I'm electronic technician, but I'm not an electronic developer, so I need a little help by the amp experts here...  ;)

Loudthud

If you ask 50 people your question, you will likely get 50 different answers. Now if you set up a test where you ask 50 people to play an amp and ask them if it's tube or solid state, it's one of those situations when you can fool some of the people most of the time, but you can't fool all of the people even once.

In my experience, you can come pretty close, but you can fool more people if you think outside the box (non-conventionally).  A tube power amp has a high output impedance (low damping factor), it has some gain compression and produces a non-symmetrical square wave when you overdrive it. If you can reproduce those characteristics,  you've come a long way.

joecool85

I'd be thinking along the lines of a nutube type amp with class-d power amp.  Vox has it's MV series and they sound pretty amazing.  Digital amps also have come a long way, though I've yet to play one that fully "clicked" with me.

Vox Nutube gear: https://voxamps.com/type/nutube/
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

stratitis

I forgot to mention I used some modelling amps before, a few of them I'm still using (Peavey Vypyr, Line 6), but I think it's not the real thing.
The nutube approach I think, will not be not the same as real tubes because lower gain per stage and lower anode voltage. You could also use one normal tube with lets say 50-60V anode voltage. Gain will be lower then automatically. So I do not see much advantage for the nutube beside power dissipation and dimensions (volume). 

joecool85

Quote from: stratitis on March 24, 2020, 11:50:28 AM
I forgot to mention I used some modelling amps before, a few of them I'm still using (Peavey Vypyr, Line 6), but I think it's not the real thing.
The nutube approach I think, will not be not the same as real tubes because lower gain per stage and lower anode voltage. You could also use one normal tube with lets say 50-60V anode voltage. Gain will be lower then automatically. So I do not see much advantage for the nutube beside power dissipation and dimensions (volume).

I see the biggest advantages of nutube vs low voltage regular tubes being: less weight, less space, less cost (sometimes anyway).  Also longer life span @ 30,000 hours of use!  Most regular tubes are rated at 5,000 - 10,000 hours from what I've read.  Mechanically nutubes should take a beating better as well (being dropped, banged around etc).

Sound wise, verdict is out, but many folks seem to feel that it cuts the mustard.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

stratitis

Quote from: joecool85 on March 24, 2020, 12:56:47 PM
I see the biggest advantages of nutube vs low voltage regular tubes being: less weight, less space, less cost (sometimes anyway).  Also longer life span @ 30,000 hours of use!  Most regular tubes are rated at 5,000 - 10,000 hours from what I've read.  Mechanically nutubes should take a beating better as well (being dropped, banged around etc).
Sound wise, verdict is out, but many folks seem to feel that it cuts the mustard.
Maybe I should test one of the nutube amps or pedals.
But I really want to build an amp or preamp in the style KMG did.
Is KMG still active at this forum?

joecool85

Quote from: stratitis on March 24, 2020, 02:37:24 PM
Quote from: joecool85 on March 24, 2020, 12:56:47 PM
I see the biggest advantages of nutube vs low voltage regular tubes being: less weight, less space, less cost (sometimes anyway).  Also longer life span @ 30,000 hours of use!  Most regular tubes are rated at 5,000 - 10,000 hours from what I've read.  Mechanically nutubes should take a beating better as well (being dropped, banged around etc).
Sound wise, verdict is out, but many folks seem to feel that it cuts the mustard.
Maybe I should test one of the nutube amps or pedals.
But I really want to build an amp or preamp in the style KMG did.
Is KMG still active at this forum?

It's been a few years since his last login. Send him a PM or email, you might get a response.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Katoda

Hello to all!

I am not KMG and I too haven't been active for quite some time here, but I have built a few amps with this topology, so I can help in any way, should the need arise.

I built the EVH 5150 preamp inspired by KMGs excellent articles and it did in fact sound good with the LND150. So good that I later modded my main (real tube) amp into a 5153, because I found I really like the sound and the feel of those EVH amps. I would have absolutely no problem running my LND150 preamp through it instead of real tubes, but since I already had an amp, I found it more practical to have a single unit to carry around.

I haven't tested LND150 circuits extensively at lower voltages, but I did have some problems at ~120V. If the voltage went any lower, I could not bias the MOSFETs to sound good, they were clipping in a weird way. If you do end up going the KMG-style route, I recommend at least using a charge pump, like the one used for nixie tubes, so you can at least get into that 300-ish volt range. That way you don't need to buy a high voltage transformer with a low voltage winding for the solid-state power amp. The transformer is usually the most expensive part of any build (especially for tubes), so if you can just get one to power the power amp and derive the high voltage from there with a charge pump, your wallet will thank you.

As for the real tubes at low voltages - I wouldn't bother with them, if you want a high gain sound. Van Halen did use to run the tubes on his Marshall Plexi (it was a Plexi, right?) pretty cold (it was still above 100V), but that was meant to get more distortion from the power tubes, not so much the preamp. The tubes will distort differently on low voltages, and you probably won't like the result.