Small SMPS Guitar Amp - Build Progress from Poland - A guide

[M23Bomber]

Hello All,

I like a small amp that can give me the pleasure of playing and the sense of acheivement of knowing I built it. 

I decided to build it and keep posting the build progress here through pictures and personal notes, maybe it will be usefull for someone.

I will start here and update it as I go. Right now only the idea, Later on some pictures and concepts.

Regards,
M.

[M23Bomber]

Hello reader,

First step is to get the basic ideas together, and not try to reinvent the wheel.

Let decide to use a push pull power amplifier configuration, that can give arround 0.8watts of power, with the right setup, selecting for that task a good old 12AU7 tube , which widely available and for which we can find reliable datasheets. This means that the Firefly amp can be usefull guide in a way, and that the Output transformer it uses ( the Hammond 125), or one of similar characteristics ( in Poland the TELTO TG-28 easy to find ) ( No publicity intended, just part numbers) can be usefull. What does it mean in numbers? The primary winding should be arround 25k and the secondary 8 ohms, frequency range from 50Hz - 100Hz to 10.000Hz - 16.000Hz. And a power dissipation of 3Watts - 5 Watts.
Here is the firefly schematic just in case ( mine will be here later on) :



Also how many preamp tubes and which?
I choose the 2 x 12AX7 seven because they will provide enough triodes to make some "good sound".

The best part about 12A#7 tubes is that at 12V they eat up only ~150mA of current for those heaters. This comes in handy later on. So you guesses it, 3x 150mA = ~450mA for heaters.  Keep the number in mind because it will be usefull to choose the power supply for amperage for the power transformer. This part also means that the Power supply unit ( the mains tranformer / PSU /u get what I mean ) will need to be 12VDC.

And now how much power and at which "high voltgage"? Lets say that I would like it to run at 200VDC. I know from experience that each triode from the 12AU7 will need around 8mA at this voltage , this means 2 x 8mA = 16mA. The 12AX7 each triode will need 1mA in the right setting, so 4 triodes in the amp , 4 x 1mA = 4mA, this means that the SMPS will nees to deliver 200VDC and 20mA total. This leads me to believe that the 555 HV SMPS for nixie tubes will come in handy. Thanks to Rick Holt and Frequency Central here is the basic layout for it, From my testing i know it can provide 200v and 20 ma under the right setup. And it will eat up arround 800mA from the 12VDC PSU. I will metion it later on.



Then we need a box , i will choose a Gainta BS-11 because I have it arround. Also this amp will have 1 input, 1 output, and 2 effects loop jacks, a fan with a MCU to make sure it runs cool.On off  and select switch, status led and MCU led for the heat. I also want it to have a Gain, Tone and Volume knobs.

This is all for now. Best Regards,
M.

[M23Bomber]

Hello all again

Here is step one of the conceptual production stage, grab a piece of A4 papper and a pen during lunch, and more or less have a general idea of part size by reading their datasheets, how to layout comes naturally after that.

But here is the initial concept obviously it may change :



I decided to add 2 U-Bolts in the edges to protect the tubes in case of fall and to overlay some parts because its good for space economy.

Best Regards,
M.

[frequencycentral]

Well I'm interested! The image above is too low res to see however.

[TejfolvonDanone]

Quick question if you don't mind: Why cascode boost?

[M23Bomber]

Hey TejfolvonDanone,


No problem, good question, I would like to know too why exactly, but I have a guess, the Firefly designers might have used a cascade ( mu amp configuration i think its called) right in the beginning so that they could add a boost switch with a and not waist a triode.

The Firefly schematic is just a guide line, I will not clone it and make it work with a SMPS.

Hey Frequency Central I will try to post it again using another hosting service.

Regards,
M.

[gtudoran]

@M23Bomber, here you have my version of full tube 1W amplifier (using 6N17B and 6N16B russians submini tubes).

Project's page (with some pictures): http://www.dextersworkshop.com/portfolio/the-sfa-1w-full-tube-amplifier/

Build document: https://dl.dropboxusercontent.com/u/15275178/OSEQ_UPLOADS/THE_SFA/THE%20SFA.pdf

How it sounds:

Regards,
DeX

[TejfolvonDanone]

Hey TejfolvonDanone,

No problem, good question, I would like to know too why exactly, but I have a guess, the Firefly designers might have used a cascade ( mu amp configuration i think its called) right in the beginning so that they could add a boost switch with a and not waist a triode.

The Firefly schematic is just a guide line, I will not clone it and make it work with a SMPS.

If you look closer it isn't really a cascode amp because the second triode's grid isn't connected to ground. It isn't really a SRPP neither because the output isn't taken from the cathode of the second triode. I don't really have much experience with tubes so i'm maybe wrong.

[gtudoran]

A nice article to read

http://www.valvewizard.co.uk/cascode.html

Hey TejfolvonDanone,

No problem, good question, I would like to know too why exactly, but I have a guess, the Firefly designers might have used a cascade ( mu amp configuration i think its called) right in the beginning so that they could add a boost switch with a and not waist a triode.

The Firefly schematic is just a guide line, I will not clone it and make it work with a SMPS.

If you look closer it isn't really a cascode amp because the second triode's grid isn't connected to ground. It isn't really a SRPP neither because the output isn't taken from the cathode of the second triode. I don't really have much experience with tubes so i'm maybe wrong.

[M23Bomber]

Hey all,

I had time to build the amp, I called it "Art is...Skulptor©",  between work ,home and family I didn't have a lot of time to keep posting the progress or write some kind of build log build properly. I am thinking about creating a little free blog and posting the log  there for whoever wants it. I think 1GB free blog will be enough.

@GTudoran Thanks for sharing your documents I read around your webpage  I liked what I saw, especially in the comment section, keep up that good work I also like the way you create your documents.

@FrequencyCentral - I finished the build its not as small as yours but also It has more features. I used photobucket this time

@TejfolvonDanone - The link for Valvewizard.com is a cool place to understand tubes, you will like. Remember, Curiosity is the first step of a great journey . Me on the other hand its more try and fail


So here is a picture of the finished "Art is...Skulptor" >



I will try to post a sound demo soon.

Regards,
M.