12AU7 SE practise amp project

[razorrick1293]

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

 

[taj]

Why not use real power tubes for the output section?

 

[nigelwright7557]

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.

 

[Printer2]

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.

 

[razorrick1293]

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...

 

[SemperFi]

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.

 

[SemperFi]

btw...the 12AX7's plate resistors should be a value that gives about1/2 b+ on the plates.

 

[razorrick1293]

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.

 

[Printer2]

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.

 

[razorrick1293]

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.

 

[SemperFi]

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...

Excactly, which is why I said he might not like it any better

 

[razorrick1293]

Excactly, which is why I said he might not like it any better

i tried bypass caps on both but still no preamp distortion, just power amp distortion occurred even quicker, though just before the volume cuts out at its lowest point a clean sound can be heard.

 

[SemperFi]

My iphone doesnt seem to want to show the schematic, and I cant remember from yesterday, but is the last level ('master') control before or after the last 12AX7 triode? For full on preamp crunch without output crunch, the master must be after. You should be able to crunch the preamp at least to a rounchy blues tone.
Unless you get higher b+, your project is more like an effects pedal than amp. Nothing wrong with a pedal that also can drive a cab, or headphones.
Btw, the hum you described is also likely from the SE output. Not much ripple is required when using triodes and the OPT has such high Z.

 

[razorrick1293]

My iphone doesnt seem to want to show the schematic, and I cant remember from yesterday, but is the last level ('master') control before or after the last 12AX7 triode? For full on preamp crunch without output crunch, the master must be after. You should be able to crunch the preamp at least to a rounchy blues tone.
Unless you get higher b+, your project is more like an effects pedal than amp. Nothing wrong with a pedal that also can drive a cab, or headphones.
Btw, the hum you described is also likely from the SE output. Not much ripple is required when using triodes and the OPT has such high Z.

the master volume is after the last gain stage, and yes i can get that blues tone, at the moment im happy driving it more with a clean boost pedal.
The hum isnt much of an issue, its more the occasional high pitch squeal.

 

[jerluwoo]

Here's how I used 12au7's as outputs.

 

[M Gregg]

Just for interest have you looked here?

http://www.diyaudio.com/forums/tubes-valves/177997-low-wattage-valve-amps.html

Regards
M. Gregg

 

[razorrick1293]

if you mean the 12v stuff, i first learnt about tubes with 12-30v designs. now im trying to get more familiar with higher voltages and the precautions. not only that but just amps with better tone, the low voltage stuff didnt exactly sound the best...

ive now doubled the voltage again so i have 220v HT now, but im still getting alot of distortion, nothing big, quite rocky and not too bassy either, i just have no cleans, the tone is also now connected to ground, and the input pot has been removed as i found the wires around that were attracting s0me radio frequencies which just added noise.

 

[M Gregg]

12V? I was refering to low Wattage.

I would have thought 200V is about right.

Using Push-Pull EF86 Pentodes in a Low-Power Guitar Amp

Link from the above thread.

Regards
M. Gregg

 

[razorrick1293]

well i saw something about spacecharged tubes, but no i havnt really looked at other low wattage designs, though i have 2 EL84's that i plan to do something with now ive discovered the voltage quadrupler. but il give the EF86's a look, and it seems i forgot id have to rebias the tubes after the voltage change...