Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: GB on January 08, 2017, 03:53:10 AM

Title: Solid State version of a VOX AC-30 Guitar Amp
Post by: GB on January 08, 2017, 03:53:10 AM
Hi people... I would like to get a few comments please...
Some time ago I started building custom Guitar Amplifiers... not because I am a Muso... but rather because my (then teenage) son was.

Fast forward 15 years...

I have most recently completed a few different versions of a Solid State Guitar Amplifier modeled on the (classic) VOX AC-30. Its key attributes are:-
1.   Does not use ANY Integrated Circuits (in the signal path)
2.   Does not use ANY bi-polar transistors
3.   Uses FET semiconductors throughout ... JFET’s (small signal) and specialist MOSFET’s (power)
4.   Amp architecture / signal path / tone controls based on the (classic) VOX AC-30 amplifier
5.   Uses an Output Transformer... like ALL Valve Amps... which provides significantly tonal differences to direct-coupled circuits
6.   Output Damping factor of transformer is much “looser” than solid state amps
7.   Lower damping factor substantially changes the dynamic characteristics of the speakers
8.   Is switchable between Class A(?) (like the VOX AC-30) and Class B for more “conventional” operation. This provides some tonal differences liked by musicians (Clean / Crunch)

While I could write lots more I will perhaps wait for any specific questions and/or general interest first.

Thanks
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: phatt on January 08, 2017, 05:40:55 AM
Hi GB, and welcome,
So when do we get to hear the sound samples? 8|
Oh yeah,,, where did you get the idea that AC 30 was class A?
Unless I've read all wrong they where All AB1 ;)
no matter,, as long as it sounds good. :tu:
Phil.
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: aquin43 on January 08, 2017, 08:26:22 AM
Sounds interesting. I think that we will need to see the schematics so that there is something to discuss.

Arthur
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: J M Fahey on January 08, 2017, 06:21:46 PM
Same here   :)
Please at least share some pictures but specially sound samples or maybe a YT video  :tu:

The strawbery on the cake would be some kind of schematic  :dbtu:
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: joecool85 on January 10, 2017, 11:35:17 AM
I'd love to see some pictures and sound clips along with schematics!
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: galaxiex on January 10, 2017, 07:34:21 PM
+1
I'd be interested to see more details and schematic.  :)
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: bradosaurus on January 12, 2017, 04:41:36 AM
Yes, please! I'd love to see what you've done on a schematic and hear how it sounds! How well do you feel that you've replicated the sound of the AC30? What led you to entirely avoid IC's and BJT's?
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: J M Fahey on January 12, 2017, 11:30:51 PM
We are talking to the sea waves, the OP dropped his bomb and left for good. Has not even checked the Forum to see whether somebody read his post or not.  ::)
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: phatt on January 13, 2017, 06:19:25 AM
Oh the Argentine Falcon does not miss much,  8|

Well who knows?
Anyone's guess,,Maybe he stepped outside right after posting and got struck by lightning and now the world will never know if he cracked the code or not.
Phil.
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: Loudthud on January 13, 2017, 04:58:57 PM
Looks like GB started a similar thread on DiyAudio and posted a couple of reply's.

http://www.diyaudio.com/forums/instruments-amps/301909-solid-state-version-vox-ac-30-guitar-amp.html

Post Edit: Schematic posted in the above link
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: smackoj on January 23, 2017, 04:03:56 PM
I guess he meant some other kind of "interest" other than people responding to his post?
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: phatt on January 24, 2017, 07:03:51 AM
Looks like GB started a similar thread on DiyAudio and posted a couple of reply's.

http://www.diyaudio.com/forums/instruments-amps/301909-solid-state-version-vox-ac-30-guitar-amp.html

Post Edit: Schematic posted in the above link
Yes thanks for the link and the schematic is there,,,,, but you have to be a member to see the circuit.
Phil.
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: J M Fahey on January 26, 2017, 12:56:41 AM
No problem, I´ll copy there and paste it here :)
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: phatt on January 26, 2017, 02:04:08 AM
No problem, I´ll copy there and paste it here :)
Yes I thought that also but was not sure ,, thanks,, Phil.
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: J M Fahey on January 26, 2017, 04:02:30 PM
(http://www.diyaudio.com/forums/attachments/instruments-amps/592212d1484461992-solid-state-version-vox-ac-30-guitar-amp-ac-30-guitar-amp-pcb.jpg)


(http://www.diyaudio.com/forums/attachments/instruments-amps/592213d1484462054-solid-state-version-vox-ac-30-guitar-amp-ac-30-guitar-amp-chasis.jpg)

(http://www.diyaudio.com/forums/attachments/instruments-amps/592903d1484724042-solid-state-version-vox-ac-30-guitar-amp-dscn1727.jpg)

(http://www.diyaudio.com/forums/attachments/instruments-amps/592904d1484724042-solid-state-version-vox-ac-30-guitar-amp-dscn1733.jpg)

(http://www.diyaudio.com/forums/attachments/instruments-amps/592905d1484724042-solid-state-version-vox-ac-30-guitar-amp-dscn1741.jpg)

hope you can see them; if not I´ll try something else :)
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: joecool85 on January 26, 2017, 04:21:00 PM
Sorry, not loading here.

Sent from my XT1055 using Tapatalk

Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: blackcorvo on April 21, 2017, 01:57:57 PM
Sorry, not loading here.

Sent from my XT1055 using Tapatalk

Here, try this: http://imgur.com/a/1dfRh
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: GB on March 17, 2018, 10:48:07 AM
Hi people... been absent for a while... won't bore you with the details.
I have (mostly) read the comments since my last post... some good thoughts in there. Thanks.
I have got a little more active I the last month or so and being doing some more development... aided by a supply of Toshiba FET's that I have been able to secure... 2SK117, 170, 369, 30's... etc.
This has been very helpful as they are:-
a) Graded at manufacture (BL, V, G, etc. versions) albeit that I still INDIVIDUALLY test and select them, and,
b) Especially made for audio applications
Importantly they have helped me refine the design process of biasing in "triode" mode as per the article I referred to some time back
(Google "Dimitri-Danyuk FET Triode Emulator")
I have developed an XL sheet making it easy to get all the components values I need by simply specifying Vgs, Idss, and the supply voltage.
BTW - There are also some "wider" considerations (not mentioned in the above) that need to be considered.
While I could go in to detail... this process seems to work... I have looked at numerous FFT spectrums on my scope... and, much more importantly, had the critical (positive) evaluation of several (good) musicians... unlike myself! :-(
I have also refined the output stage further with (again) a good supply of 2SK1058 and 2SK2221 (audio) power MOSFETs...
I could continue... but perhaps I'll wait for any feedback / questions about specific items before continuing...

Cheers
GJB
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: GB on March 17, 2018, 10:53:22 AM
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
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: GB on April 27, 2018, 12:40:32 AM
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.
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: HamSandwich on April 28, 2018, 02:32:16 AM
I’d buy a kit/pcb, but id also be interested to see and learn from the schematic!
Title: Re: Solid State version of a VOX AC-30 Guitar Amp
Post by: GB on April 28, 2018, 06:29:55 AM
Hi HS... thanks for that! Just working on the updated version now. Will keep you posted.
Cheers,
GB