12AU7 SE practise amp project - diyAudio
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Old 21st December 2010, 11:49 PM   #1
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Default 12AU7 SE practise amp project

Hey people that are interested, ive recently been working on a relatively low voltage project (for valves/tubes(just over 100v)) using spare parts i had laying around, the initial idea came from the AX84 firefly, i wondered if i could build something similar at 100v instead of 260v, the 12AU7's in push pull didnt seem to have any power at all, and the second tube was doing practically nothing.

So my main changes are (other than plate/cathode resistors etc.) , a tone control which needs a fair bit of tweaking, theres alot of treble loss, even when its on full, a higher pot would be useful i think. i also added an input cut with a switch to add to the grid stopping resistance, as it is, the change when the switch is flicked is quite noticable, though i think it could be improved. The power stage is the two 12AU7 triodes in parallel single ended configuration, it gave me alot more volume and a bit more clean headroom. no matter how you play with the pre amp controls, there is no distortion, im thinking of changing a plate resistor in one stage but im not sure yet. the power amp overdrives very easily, for a clean sound its probably twice as loud as playing unplugged, with a bit of push it hasnt got a bad rock tone, but anything past about 40% on the final volume and it gets mushy. the power transformer is about 37.5v RMS, which is being doubled in the rectifier stage. the heater winding is 12.6v with no CT, the output transformer is a 100v line transformer i found, which is probably what sparked the whole project, it has a primary impedance of about 40k to a 4 ohm load. (im only using half of it from the 50v tap since i changed it from PP to SE) if anyones interested any input will be greatly appreciated the only problems ive got is at some settings when the input cut is switched to a variable grid stopper, there is quite alot of hiss in the amp, like a loud whistle, theres a bit of hum, but most likely due to heater wiring and a ground loop somewhere.

Thanks for reading this little essay and thanks in advance for any input and im still fairly new to this forum so if theres anything i should know about long posts like this then im all ears
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Old 22nd December 2010, 12:43 AM   #2
taj is offline taj
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Why not use real power tubes for the output section?
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Old 22nd December 2010, 12:54 AM   #3
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You need a high impedance tone control so the load doesnt affect it much.

Get an EL84 or two banged in there for a bit more power.
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Old 22nd December 2010, 07:16 AM   #4
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Use the 1M input pot for your tone control. Your capacitor should not be going to the cathode but to ground. You will not get much gain on the preamp tubes as you do not have bypass capacitors on the cathode resistors. If the power transformer can handle the load I would use a voltage quadrupler, the voltage is way too low for both the pre and power tubes. Push-pull would make a lot more sense with the line transformer as the dc through the core would cancel out. And you should also bypass the cathode resistor on the 12AU7.
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Old 22nd December 2010, 09:42 AM   #5
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as for normal power tubes, i said this was inspired by the AX84 firefly, and i had a spare 12au7 laying around,

i will try connecting the tone cap to ground, and il add some bypass capacitors. il think more on the push pull bit, i quite like the sounds of it in SE the hum isnt what bothers me, its the occasional squeal it lets out. but a voltage quadrupler? ive never heard of that...
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Old 22nd December 2010, 10:26 AM   #6
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Nice to see you are not afraind of trying tubes laying about, and not always building like veryone else
Here is a few mods that'll make it more guitar friendly. Notice how the tone output goes to the gain knob. The tone knob should be 500k-1meg ohms. The coupling caps are way too large in value for guitar. Try 1nF to 10nF and I promise it'll cut much sweeter.
You can bypass the first cathode resistor with 0.5-10uF, but with such low voltage I doubt you'll like that any better.
The impedance of the OPT shouldn't be so high if you want to swing any power out of it with such low b+.
But for practice work, a few hundred milliwatts can do nicely.
Good luck.
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Old 22nd December 2010, 10:27 AM   #7
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btw...the 12AX7's plate resistors should be a value that gives about1/2 b+ on the plates.
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Old 22nd December 2010, 01:42 PM   #8
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thank you very much , il try those ideas, and im also going to try the voltage quadrupler aswell. as for plate resistors, they were scrounged out of an old TV when i first started to learn about tubes, so i dont have alot of parts. so i biased them the best i could.
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Old 22nd December 2010, 04:14 PM   #9
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The bypass on the cathode resistor will work the just as well at low voltages, you just do not have the headroom before that stage clips, 1uF or about their if you have too much bass, up to 20uF or more if you do not need to roll it off.
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Old 22nd December 2010, 04:22 PM   #10
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il keep that in mind i tried the voltage tripler first, but it fried the first capacitor, i think it may have been wired the wrong way, the quadrupler is almost built now, hopefully everythings working, and then il play with the rest of the amp if it works.
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