A schematic for a pure 50w FET / MOSFET guitar amp?

[marcoharder]

Does anyone know of any amp schematic that purely uses FETs and MOSFETS in the signal path? I'd like to create a 50 watter that can power a 12 inch speaker for gigging purposes.

The reason I asked is that I've been doing some reading and apparently, FETs and MOSFETS behave similarly to valves when you pump a signal into them. I think it's an idea worth trying out.

Schems anyone?

[dfx]

try this site:

leamps.blogspot.com

look for amp designs that uses MOSFET as output devices

hope this helps....

[Gus]

IMO it has more to do with the output transformer

[bancika]

Don't expect too much from FET, they don't clip the same way as tubes which is very important.

[theundeadelvis]

Try here: http://www.ssguitar.com/

[dfx]

BTW, the average power output of your amp is dictated by the power supply you will use, that is:

1. the VA rating of the transformer (in terms of watts)

2. the secondary voltage of the transformer

example: a 36-0-36 volts  center-tapped transformer with a 300VA rating will give you 100 watts RMS of average power.


cheers!

[R.G.]

I've been doing some reading and apparently, FETs and MOSFETS behave similarly to valves when you pump a signal into them. I think it's an idea worth trying out.

It's an idea that's been tried and found not to be so. You cannot expect a MOSFET-output amplifier to sound like a tube amp. Think about it - if that were true, all musical instrument amps would by now be MOSFET output, right? The big companies don't just ignore economies like that.

The other responses you got were right on.

[Minion]

I suggest you simply look for a Mosfet power amp schematic and put a fet preamp in front of it, maybe something like a DR.Boogie.....

But even mosfet amps with have some BJT"s in the design for CCS and VAS so there probably aren"t any mosfet amps that use stricly just mosfets, usually they are BJT amps with mosfet outputs......

Cheers

[Ben N]

Pay attntion--a lot of attention--to heat. You absolutely have to sink those power devices to death.

[soulsonic]

IMO it has more to do with the output transformer

Absolutely! I'm totally with you on that. Everyone seems to ignore the transformer....

[squidsquad]

Here's a 10 watt that has always caught my eye:

http://www.redcircuits.com/Page52.htm

[Minion]

Here's a 10 watt that has always caught my eye:

http://www.redcircuits.com/Page52.htm

that isn"t a mosfet amp it is a darlington amp (uses darlington output transistors)

[marcoharder]

Hey thanks for the replies guys!

[marcoharder]

By the way, are there any schems available for transformer coupled solid state amps?

[d95err]

By the way, are there any schems available for transformer coupled solid state amps?

The reason output transformers are used in tube amps is that tubes have very high output impedance (too high to drive a speaker directly). Instead, they work with high voltages and thus high signal amplitues. The output transformer converts the high impedance high amplitude signal to a low impedance low amplitude signal that can drive a speaker.

Solid state devices typically have very low output impedances, so a transformer is not necessary. Even if an output transformer would be used, it would not be the same and it would not bring any tube magic.

No, the only hope of making a solid state amp sound like a tube amp is to simulate the tube output stage somehow and then amplify it. (Note that the solid state amp would need prerhaps 5-10 times as much power as the tube amp to avoid clipping in the output stage).

[blanik]

My first amp was a clean solid state Yamaha power amp that my dad modded into a guitar amp (clean-only preamp, cabinet, etc...) but i couldn't get a metal sound (it was the early 80's after all...lol) he then bought me my first commercial amp... the "Marshall MOSFET" it was called.... like a baby JCM800 with 4X10" cabs, i haven't seen one of those around in a long time.. but there was no tube in there, just mosfets (i later traded the whole stack to a music store and some cash for a JCM800)

R.

link to the schem
www.drtube.com/schematics/marshall/3210.gif

[Gus]

I have an old PRS head. Huge output transformer solid state IC and discrete parts.  I think E.P. designed it.  My amp is a mess bought it at a guitar show years ago holes drilled in it work done to the boards etc.  I have been trying to find parts to bring it back to stock.  I contacted PRS years ago but they offered no help.  I think 190 made if what I read is true

Problem it is about 80watts I believe, and to get it to sound good it is to loud on a marshall 4x12.

musicman amps had output tubes to transformer with solid state pre and PI.
I believe I saw a Fender patent for a BJT to transformer amp from the 70's  I think I saw this in a book been looking for it even had the output as a collector out for voltage and power gain IIRC.
Macitosh solid state amps had output transformers IIRC.

  There is the alloy and winding and BH and signal levels vs core alloy/size and heat treatment and lam shape and......... from transformers

I build condenser microphones as a hobby solid state and tube and the transformer is a big part of the "magic" when you get it right.

Good stuff in the big red book and other older tube books

Part of the sound is the pentode or pentode type tubes workings at overload levels, screen currents ...........................................

[Joe Kramer]

I've got an old 45W Peavey Pacer (    yes, I know) that has a large coupling xfrmr between the preamp/tone stack stage and the power output stage.  This amp has better body and tonality in the all-important low-mids than other SS amps I've heard, and I have to think that the xfrmr is partly responsible.

Regards,
Joe

[soulsonic]

I remember the adverts for that PRS amp. I didn't know they only made so few.... I guess when you're a kid you assume that if it's in an ad, then they must be selling thousands.

I've seen quite a few designs for early solid state amps that use output transformers.
The Sunn Concert series amps use an interstage coupling transformer... they can sound pretty warm too, as long as you take it easy with the Treble control. The have a JFET preamp and simple BJT poweramp with the transformer splitting up the signal to four 2N3055s that are set up in a bridged arrangement. I've repaired several lately; they don't sound "just like" a tube amp, but they do sound good.

You could use some high voltage MOSFETs with a standard tube output transformer and it should work, you just have the use the correct topology and get the biasing right. Maybe try it small first - like with a little 1 watt thing just to see how well the concept works.