5W fet power amplifier with "tube" sound

Tube or solid-state, this section goes to eleven!

Re: 5W fet power amplifier with "tube" sound

Postby KMG » 23 Aug 2010, 10:26

Another one test with different knob settings
http://www.youtube.com/watch?v=rlkIXhEtxqQ
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Re: 5W fet power amplifier with "tube" sound

Postby KMG » 17 Sep 2010, 11:07

Reamping samples using Fet version of ENGL530 MOD2 as preamp.
Tone controls at preamp in middle position, "Presence" and "Depth" at PA also in middle position.
Digits after channel name represents "Gain" setting.
http://milas.spb.ru/~kmg/files/projects ... mp/5W4x12/
Recording path: E-MU Tracker Pre->Fet Engl->Fet PA 5W->Bull 412V+L(4x12)->Sennheiser e906->E-MU Tracker Pre
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Re: 5W fet power amplifier with "tube" sound

Postby THChrist » 29 Dec 2010, 22:03

Well, this project looks cool. I've been checkin' your website (KMG) and these Fet/micro poweramps may be what i've been searchin' for.

I posted these projects (micro PA + fet PA) at the SSGuitar Forum, and I had this answer about your PA:

I don't know... to me at least the MicroPA output looks pretty much like one coming from a generic tube amp: somewhat rounding up on the clipping onset and it even features the same type of crossover/blocking distortion mechanism due to shifting bias point. But, I think those scoped waveforms are either from running to a resistive load or the amplifier's response stays fairly linear regardless of load impedance. So the mechanism where amp's gain interacts with a reactive load is not showing up in there. Otherwise, it pretty much nails the "PP tube power amp thing".


Can you tell me anything about the underlined part?

Silly questions: what are the -sonic- differences between this amp and the microfet pa? I saw Micro's schematic and the structure looks slightly different than this one. Also, the values on the Micro are pure Marshall ("Phase inverter", bias splitter, etc).
Can I built this 5w with Marshall values?
How effective are the presence and depth controls?
how the "cut" works? is it effective?
is the simmetry knob something i can mess around? or is it like a "set and don't move" kinda knob?

Thank you. Looks like a cool project and I'm dyin' to know more about it.
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Re: 5W fet power amplifier with "tube" sound

Postby RnFR » 29 Dec 2010, 22:38

sounds great!
"You've converted me to Cubic thinking. Where do I sign up for the newsletter? I need to learn more about how I can break free from ONEism Death Math." - Soulsonic

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Re: 5W fet power amplifier with "tube" sound

Postby KMG » 30 Dec 2010, 12:51

Well, this project looks cool. I've been checkin' your website (KMG) and these Fet/micro poweramps may be what i've been searchin' for.

I posted these projects (micro PA + fet PA) at the SSGuitar Forum, and I had this answer about your PA:

I don't know... to me at least the MicroPA output looks pretty much like one coming from a generic tube amp: somewhat rounding up on the clipping onset and it even features the same type of crossover/blocking distortion mechanism due to shifting bias point. But, I think those scoped waveforms are either from running to a resistive load or the amplifier's response stays fairly linear regardless of load impedance. So the mechanism where amp's gain interacts with a reactive load is not showing up in there. Otherwise, it pretty much nails the "PP tube power amp thing".


Can you tell me anything about the underlined part?

It`s really true, but modelling speaker complex load is not simple problem.
I think you can try to use magnetic system of speaker from small transistor radio (without cone) for load.

Silly questions: what are the -sonic- differences between this amp and the microfet pa?

Any speaker simulation sounds like a "rubber woman" - similar, but not those feelings.
But micro PA gives you opportunity to have a sound of PA overdrive at low sound volume when you connect its output to PA loaded on guitar cabinet.
I saw Micro's schematic and the structure looks slightly different than this one. Also, the values on the Micro are pure Marshall ("Phase inverter", bias splitter, etc).
Can I built this 5w with Marshall values?

Micro PA has "split load phase inverter", 5W PA has "Long tail phase inverter" like in most tube PA (marshall for example).
I tried to use long tail phase inverter in micro PA, but it gives much more gain.
With it sensistivity of micro PA was about 5mV. So i returned to split load phase inverter for micro PA.
How effective are the presence and depth controls?

They works like in tube poweramps.
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how the "cut" works? is it effective?

Cut is an optional control. It allows to suppress "sand" in sound
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is the simmetry knob something i can mess around? or is it like a "set and don't move" kinda knob?

Yes, simmetry is a trimmer potentiometer for matching aplitude at phase inverter outputs.
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Re: 5W fet power amplifier with "tube" sound

Postby THChrist » 31 Dec 2010, 03:29

RnFR wrote:sounds great!


Have you tried it? Can you give us some samples?

KMG wrote:Micro PA has "split load phase inverter", 5W PA has "Long tail phase inverter" like in most tube PA (marshall for example).
I tried to use long tail phase inverter in micro PA, but it gives much more gain.
With it sensistivity of micro PA was about 5mV. So i returned to split load phase inverter for micro PA.


Well, I think I'll go with the 5w or maybe the 15w. I some minor differences between those 2:

R4&8; C15; D1, 3&4

Now, is it ok if I change the values of the long tail resistors to Marshall specs.? Will I have more gain, noise, Marshallesque sound?

R4&8 to 470 ohm; R5 to 10k; R6 to 4k7? (and maybe C4 to 47p?)

I'm asking b'cos I'm some kind of OCD person and I prefer typical values. This project is what I want (or need) and I want to make sure it will be perfect (and that I won't damge my speaker). Also, if I build the 15w and apply less voltage (say 24v) and change the bias, will it be the same as the 5w version?

Thank you for your patience.
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Re: 5W fet power amplifier with "tube" sound

Postby KMG » 31 Dec 2010, 10:43

Well, I think I'll go with the 5w or maybe the 15w. I some minor differences between those 2:

R4&8; C15; D1, 3&4

The gain of PI stage depends on R4&8 value, because 5w requires less current of output stage it requires less PI gain in order to have same sensitivity.
C4&15 - frequency compensation for stability
Voltage of zener D1 is selected according to supply voltage in order to have correct PI stage biasing.
Maximum reverse voltage of D3&4 must be greater than twice power supply voltage

Now, is it ok if I change the values of the long tail resistors to Marshall specs.? Will I have more gain, noise, Marshallesque sound?

R4&8 to 470 ohm; R5 to 10k; R6 to 4k7? (and maybe C4 to 47p?)
I'm asking b'cos I'm some kind of OCD person and I prefer typical values.

If you`ll increase R5&6 you`ll need to increase R3 to 36k, R7 to 47k RV1 to 30k.

Also, if I build the 15w and apply less voltage (say 24v) and change the bias, will it be the same as the 5w version?

One of my friends on GTLab made interesting version of this PA.
As a base he took 40W version.
He powered PI stage from separate supply (separate winding of power transformer), but output stage from switchable winding (variable supply voltages).
So he have PA wtih 5W/15W/40W of output power in one device.
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Re: 5W fet power amplifier with "tube" sound

Postby THChrist » 08 Feb 2011, 04:32

KMG wrote:
Now, is it ok if I change the values of the long tail resistors to Marshall specs.? Will I have more gain, noise, Marshallesque sound?

R4&8 to 470 ohm; R5 to 10k; R6 to 4k7? (and maybe C4 to 47p?)
I'm asking b'cos I'm some kind of OCD person and I prefer typical values.

If you`ll increase R5&6 you`ll need to increase R3 to 36k, R7 to 47k RV1 to 30k.


Same things goes for the 40w version, right? 3 times the value.


KMG wrote:
Also, if I build the 15w and apply less voltage (say 24v) and change the bias, will it be the same as the 5w version?

One of my friends on GTLab made interesting version of this PA.
As a base he took 40W version.
He powered PI stage from separate supply (separate winding of power transformer), but output stage from switchable winding (variable supply voltages).
So he have PA wtih 5W/15W/40W of output power in one device.


Ok. Now i want this. How can I do it? where do I have to put a switch? R10?

Thank you!
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Re: 5W fet power amplifier with "tube" sound

Postby KMG » 08 Feb 2011, 11:46

Ok. Now i want this. How can I do it? where do I have to put a switch? R10?

Scalable down 40W version
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Re: 5W fet power amplifier with "tube" sound

Postby THChrist » 08 Feb 2011, 15:50

KMG wrote:
Ok. Now i want this. How can I do it? where do I have to put a switch? R10?

Scalable down 40W version
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Oh I see. Just change the voltage of the OT. Nice. Thank you KMG!
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Re: 5W fet power amplifier with "tube" sound

Postby KMG » 14 Jun 2012, 20:05

5 W PA assembled in the new enclosure.
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Re: 5W fet power amplifier with "tube" sound

Postby file1 » 22 Nov 2017, 21:45

Dear All,

I want to build 90W PA version for my brother , but I have a huge problem with output transformer. Plase could somebody provide me some more information about it? Link on internet where it could be bought also can be usefull. Does it someone tried to build it with EI core and could provide parameters for 90W version? Which type of tpk-190, exactly could be used for rewinding ? Thank you in advance, Igor
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Re: 5W fet power amplifier with "tube" sound

Postby bmxguitarsbmx » 25 Nov 2017, 00:06

file1 wrote:Dear All,

I want to build 90W PA version for my brother , but I have a huge problem with output transformer. Plase could somebody provide me some more information about it? Link on internet where it could be bought also can be usefull. Does it someone tried to build it with EI core and could provide parameters for 90W version? Which type of tpk-190, exactly could be used for rewinding ? Thank you in advance, Igor


Why not just build a 100watt tube power amp?
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