Good question. The tranni in the pic is only 30VA. However given that the amp is 90+% efficient and is (obviously) not running at continuous full power 30VA seems to work well. I have run music (not tone) through it running at full power and the tranni (and the PA) are barely warm.

Hi people. I have been designing, developing and building a few different Guitar Amps over the past few (many!) years.
This is my latest effort.
I would much appreciate any thoughts and/or critiques that you may have.
I have actually sold a few of these to mates and acquaintances... but am now at the point where I would like to retire from my retirement... if you know what I mean!?
I'm just getting to old for this stuff!
So if you, or anyone you know might be interested in picking up the chalice I would be glad to hear. Depending on their context there may need to be some "nominal costs" attached.

Anyways... enough of that... the technical stuff follows...

In its simplest from the unit mimics a "regular" guitar amp...
* Outputs up to 50 watts or so
* Has a standard FMV tone control stack
* Both gain and volume can be separately adjusted
* Standard HI / LO high-impedance Inputs
* Etc... etc.

Now for the good bits!...
* Is built in a much smaller and lighter case than a "regular" Amp - actually in a standard Hammond case... roughly 6 x 8 x 6 inches... and only a few kilos in weight! (No more lugging a huge Marshall Amp up the stairs!)
* IMPORTANTLY... uses a (discrete) FET based preamp that mimics the characteristics of a triode valve preamp... e.g. the ubiquitous 12AX7. How this is done is somewhat technical. Google "Dimitri-Danyuk FET Triode Emulator" for a full description. His Paper on this was originally presented at an AES conference some years back.
* (In this version) Uses a high efficiency Class D amplifier (pre-built) module... but
* Can also be built with a TRANSFORMER coupled output stage that I have developed driven by power MOSFETS. While a little larger now also gives a nice power amp overdrive tone... and 100+ watts of output power... just like the "standard" EL34 / 6CA7 config.
* Using the preamp and power amp in combination can produce anything from clean "jazz" tones to all-out crunch!
* Has the standard HI / LO (unbalanced) input connections - on 1/4" TS connections... but...
* Also has a low-impedance BALANCED input connector - on a 1/4" TRS connection which provides really nice tones on guitars using an acoustic pickup. This input has a separate Gain control (as shown)
* (Looking along the front panel) Has a "BOOST" switch that (in a simplified view) alters the characteristic of the preamp that produces a more "edgy" tone. This can also be controlled by a simple (pull to ground) Footswitch through the adjacent TS connector. Great for switching clean to edgy tone quickly and cleanly.
* (Next along the row) A simple Line Output Jack that can be easily connected to a Mixer, Bigger Amp, Foldback Amp, etc...
* Also has an "INSERT" TRS Jack that can be selected as PRE or POST EQ... if you prefer to use an (outboard) EQ in lieu of the standard FMV EQ... or any other devices... Foot Pedals even!

OK... enough words... will await your thoughts.
Some pics attached. The one of the inside was from a prior version and does not include the front panel I/O's or the purpose built PCB's that I have subsequently done.

Cheers,
Graham

I need help!

Since my (formal) retirement from the Electronics business back 10 years or so ago I have been designing, building, and selling a series of CUSTOM GUITAR AMPS and related items.

Up until now this has all be done by yours truly... BUT... I am now at the point of needing to (somewhat) back away from all of this as there are other priorities... and I am struggling with the time, stamina, and application required to take on any new builds.

SO... I am now looking to engage with someone that might be interested in picking up my pursuits... either for their own purposes or a "third-party" builder.

They would need to have suitable skills and the equipment to do the complete build of the items - PCBs, Enclosures, Labelling, etc... and then final testing and QA.

These units are relatively simple... but, nonetheless... use some unusual designs to achieve excellent results whether used in "Clean or Crunch" modes... and are comparable to many larger, heavier, unreliable and expensive valve-based equivalents.

I have an extensive inventory of schematics, mechanical designs, PCB's, and parts... albeit that they may need to be "upgraded" to suit quantity construction rather than the (largely) one-offs that I have been doing.

I can also offer support and mentoring to get the process started... and then, potentially, on an as required, on-going basis.

If this is of interest to you please message me and I am happy to give you my mobile number and/or email address for further discussions.

A few (of many) random pics attached as a general guide to the sort of stuff I have been building.

FYI... I am based in Perth. Someone local would be preferred, but, with the technology that we have available nowadays more distant options may be feasible.

I need help!

Since my (formal) retirement from the Electronics business back 10 years or so ago I have been designing, building, and selling a series of CUSTOM PRO GUITAR AMPLIFIERS... and related items.

Up until now this has all be done on a fairly informal basis... BUT... I am now at the point of needing to (somewhat) back away from all of this as there are other priorities... and I am struggling with the time, stamina, and application required to take on any new builds.

SO... I am now looking to engage with someone that might be interested in picking up my pursuits... either privately or commercially.
I am very much open as to how this might happen... I have an extensive inventory of schematics, designs, PCB's, parts, and the like.

If this is of interest to you please message me and I am happy to give you my mobile number and/or email address for further discussions.

A few (of many) random pics attached as a general guide to the sort of stuff I have been building.

FYI... I am based in Perth, Western Australia. Someone local would be preferred, but, with the technology that we have available nowadays more distant options would likely be feasible.

I have been fortunate to have got a supply of original 2SK and 2SJ FETs... like the 2SK30, 2SK369, 2SK170... etc... which has given me a wider choice of options to vary and optimise designs... still using (primarily) the Danuk process.
I have even developed a couple of XL spreadsheets which integrate the key formulas... and then expand the calculations to other areas e.g. Voltages, Currents, Gain, etc...  If you would like the xls file send me an email address(s) and I will send it that.
Graham

Hi. Edvard is correct about FETs having a wide range of specs... that is why they are not favoured in mass-produced equipment. However it is possible to allow / design for this.
There are simple circuits that will allow you to measure Idss and Vgs cut-off of FETs using a few components and a multi-meter. Once measured a precise design can then be done... and component values can be calculated. The Drain resistance value you have mentioned seems way high.
There are multiple articles on the net re biasing calculations. There is one in particular by Dimitri Danyuk to set up bias so that an FET works like a triode vacuum tube... Google it. I have built many circuits using this principle. They work very well.
For your (and others) info I have attached something that I wrote for a magazine a couple of years back. It is copyright... but I would welcome your feedback and comments.
Cheers,
GB

Hi Joe... here's a further suggestion following (broadly) your Option 2... but using FETs rather than TL072's.
It follows the VOX AC-30 structure including Normal / Boost selection and the VOX Bass / Treble tone stack.
Notably... it follows the "Danyuk" FET biasing method to get them to perform like (valve) triodes. The schematic shows 2N series FETs... but I have also built versions using Toshiba 2SK series... which sound better to me... but are harder to get hold of.
Let me know what you think. I have a circuit description... and even a (FET and Transformer) based power amp to go with it... but that's a whole other story! 

A schematic might be revealing just a little too much at this point. How about a brief sound clip instead? 

Yeah... agree with JM. Not worth it.
The TDA will inherently reject power supply ripple in any event.
The TL072 should be de-coupled from the PS with a couple of (say) 100 ohm resistors in series and 100uF caps behind them on each of the + and - supply rails... as OP Amps normally are.

Hi people... I'm aware that this a very old thread but I just wanted to acknowledge the very many good thoughts in it... particularly from Phatt and Teemuk (whose publication I have read from to cover!) Many thanks for that. 
I have also read Danyuk's paper on FET triode emulation... along with a lot other related stuff.
I could now probably write a book of my own!  ... but I built an Amp instead... also emabracing many of the thoughts in related posts.

(Very) Breif Summary:-
* The signal chain is very closely modelled on VOX AC-30
* It uses the Danyuk FET biasing method - done correctly it works
* It uses Power MOSFETs AND an Output Transformer.

I am NOT a musician... but several who are... have used it - and like it! I have sound clips if anyone is interested?

Below is a pic... I'll wait to see if any interest / questions???

Cheers... GB

Hi Guys... notwithstanding the (very valid) point about them failing... Cooling Fans can be very effective... potentially reducing the size of a given heatsink by 50% - or more.
Attached is a pic of a 100 watt with a relatively small heatsink - but with two 40mm fans directed down the cooling fins. The amp runs of a nominal 50 volt supply so these are two 24 volt DC units - with a little bit of additional circuitry to ensure that the voltage is equally distributed between them.
Bottom line it works very well.
PS. I have contemplated adding some sort of thermal protection (fuse or breaker) as a last line of protection - but haven't got that far yet.

GB

Hi HS... thanks for that! Just working on the updated version now. Will keep you posted.
Cheers,
GB

Hi guys... been of the grid for a bit... but there have been some developments.
Most notably I have "sold" (albeit only covering component costs) a couple of units and the feedback (forgive the pun) has been very encouraging.
One customer is a pro muso and also a guitar teacher - so quite critical of sound.
To address a couple of the point raised by others:-
1) The gate clipping of FETs will be different - and hard to accurately duplicate. It is what it is.
2) Nonetheless the FETs (PROPERLY set up) can do a good job of emulating a triode (12AX7). I have tested this using a DSO with FFT analysis.
3) The schematic of the output stage which is posted in this thread is mine - but a simplified view. Actual ccts need a bit more added.
4) Re the 1+1:1 transformer. That is one option. They can be salvaged from old PA amps but difficult (impossible) to buy new ones off the shelf.
5) I have used PA Line transformers. These have been applied to valve guitar amps - mainly using a (high) 100 volt version of same. I have used a (low) 25 volt version which, approximately, has a 3+3:1 primary / secondary ratio. A long and detailed story for another day - but with correct MOSFETs, and setting up currents and voltages correctly they work! - including the saturation effects mentioned by Katopan and Bulldozed in this thread.

Bottom line... I am now thinking about rolling all of this accumulated knowledge in to a kit and/or publishing details.
Would be most interested in any comments / interest on this possibility.

Cheers,
GJB

PS - there seems to be two threads going on here. Search "GB" for the other.

Hi... I have looked at the schematic you referenced. Although it is using JFET (presumably) to emulate tube sound it is not ideal. If you are interested I have a better circuit that you could use?
GJB

Hi... the Schematic posted above is not the latest design... gotta keep some secrets! :-)
But shows the general layout... and is (obviously) the preamp only.
GB