12AX7, EL34 or 6L6 design wanted

[Fiat1]

Hi
I am wanting to start building my first valve amp. As a first project I would like something around the 20watt mark, and do it cheap! I have found this website that uses a 100volt impedance matching transformer as the output tran. http://www.alphalink.com.au/~cambie/...ls_6CM5amp.htm
I would like to do something similar but I want to use valves that I have avaliable from local electronics shops. As stated in the website the transformer is 8k:8ohm.The valves I have avaliable are 12AX7, EL34 and 6L6. I assume the design would be PP. So if you have a design for such an amp please post it!

[aletheian]

Hey.
8k seems a :little: high for 6l6...that is more in the el84/6V6 territory...BUT it can be done.
What topology are you looking at, and what will it be used for? Phono? Home Stereo? Will you need a preamp section?
It sounds like a push pull tranny that you will be using. And what is your price-to-tone ratio? If you just want to experiment with tubes, you could try a sinple output stage, like a self-split and eliminate the need for a phase splitter tube, thus keeping the current/voltage demands on the power supply lower, but they are not super popular...one of those lost designs I guess.

[aletheian]

I just whipped this up as an example of a super simple circuit. It uses a concertina (split-load) driver with 1 gain stage before it, and runs the 6L6's in cathode bias/triode connection...simplest design I could thing of in the push-pull realm. You could use a 12ax7 as the driver/splitter, but the output impedance would be a bit high, a 12au7 would be a bit better, but either will function.

[Fiat1]

Hi aletheian

I will be using it for home stereo. A pre amp is not 100% nessary, but if there is a simple all in one solution then great. I can buy a vavle preamp as a kit from a local electronics store so I could go that route for the preamp.

Thanks for making that schematic! Since I am new to valves and their design do you have component values?

One question about power supplies, say I need 2 voltages ie 250volts and 6.3volts for the heaters, can I run the heaters off a seperate transformer?

[aletheian]

Hey man,
There are TONS of psu schematics floating around this website. The simplest type is the 'ol "brute force" unregulated supply...that should work fine for a first amp attempt. You could probably do it with as little as 2 diodes, 2 resistors and 2 capacitors in a simple RC filter circuit...like an old Dynaco.
I drew up an example diagram, but you can up the value of the capacitors to lower the ripple/noise if you want, AND if you really want to get spunky, replace the 50 ohm resistor with a choke (I would recommend placing another capacitor before the choke if you take that route.
For the heaters, find any 12v or 6v power transofrmer with a sufficient current rating and it should be cool...somewhere around 2.5 Amps for 6L6's, or 3.5+ for EL34's. I did a project with a little transformer that I bought from Radio Shack once...dunno if they still sell them though. Hammond makes some nice cheapish ones. You could also get a main power transformer with a 6v heater tap too, that would be easier. Just take the voltage rating (175-0-175 for emample) add it up (175+175=350v) and multiply by .7 (350x.7=245v) to estimate ABOUT what plate voltage you will end up with. You'll lose a bit in the psu resistors, and in the rectifier too. As for the current, add the plate current at your bias point + grid current. The 12ax7 will draw a few milliamps, so don't worry about that. A good guess is around 125mA, but more available current is always better.

I'll work on putting in some general component values tonight, but you might want to try it yourself if you like math. The Aikenamps.com website has some killer info and all the formulas that you need. Since you only have one gain stage to play with, use that to set your EQ cutoffs from 20Hz to 20kHz.

[Fiat1]

Thanks for that website, very helpful. I will have a good read through it. I am looking foward to seeing how this amp turns out!

[aletheian]

Hey man,
I ran a little math and here is what I came up with. It uses no gobal feedback, otherwise the specs would chance. There is some local feedback in the form of the first stage cathode resistor...and I suppose you could add some high value resistors from the plates to the grids of the output tubes for some local feedback there is you want, but I wouldn't worry about that right now.
The potentiometer at the input WILL alter your input freq response as it is turned down, so either take it out and replace it with a fixed resistor if youe source has an output control, OR experiment with the input DC blocking cap value until you like the sound...or take out the cap and lower the value of the input pot...but that will change the gain of that stage...just stuff you can try.
You will probably want to put some grid resistors on he output tubes...I forgot to draw those in...just to be safe if the tubes start drawing crazy current. 1K should be cool. Just run them from the 470K resistors to the input grids instead of connecting them directly.
ALSO, I just did the load lines real quick for the output tube cathode bias resistor...i think it is cool, but you might want to check around or do the math yourself. You can err higher if you want to play it safe, take the mesurements when it is built and adjust accordingly.

[Fiat1]

Thanks alot for your help! With the volume control, why is it that varying the pot changes the freq responce?? What if I replaced that with a fixed value and added a 50k pot before the input? I might try for a bit more of a refined power supply. What sort of wattage would you expect for this thing to put out?? Hopefully once I get my 35mm projector back together and off my desk I can start on this amp!

[Fiat1]

Aletheian or anyone else know the answer to my above question???

[SY]

You could probably get about 10-12 watts out of that.