Help with tube project

[blackcorvo]

Hello everyone

i have here a EH 12AU7 and a U.S.A. 6AU6 (also a Sovtek 12AX7, but seems it is not working right. i tried making a lil tube booster and it sounded really bad), and i was thinking of making something nice with them, but i know just a little about tube theory, nothing about formulas and that math stuff, and wanted some help.

Thoug of the following ideas:

- pre-amp with overdrive, using the 6AU6 pushing the 12AU7;
- pre-amp with overdrive, using the 12AU7 pusing the 6AU6;
- low-power SE amp using the 12AU7 as power and the 6AU6 as pre-amp (more power & cleaner, i think) ;
- low-power SE amp using the 6AU6 as power and the 12AU7as preamp (less power and more distortion, i think) ;

Since i didn't find any info about the plate impedance on the 6AU6 and i've seen the 2W SE power-amp on AX84, though of maybe building the lil tube amp with the 6AU6 as pre. But i've heard that it sounds bad as a pre-amp for guitars, so i'm really stuck here with the tubes and no idea of what to do, and also have no idea of how to control the gain on it to get some overdriven or clean sound in a pre-amp so i'd appreciate any kind of help!

[Ripthorn]

Here is what I would do, I would make a very nice little preamp out of the 12au7.  Perhaps do a valvecaster with no tone control and let that push the 6au6.  This could sound quite nice and give a nice range of tones from clean to some overdrive, and a booster in front of it all would get you easily into solid overdrive territory.  If the 12ax7 can be gotten to work, you could really have fun.  My question about that tube regards the voltage you put on the plates.  12ax7's do not work on a starved plate (i.e., low plate voltage) nearly as well as a 12au7 does.  I would stick with relatively low voltages for a first tube project.

[blackcorvo]

i tried building a 12v booster from the brazillian DIY site's forum www.handmades.com.br , where they used a 12AX7 and got some nice sound from it. i dunno wich brand they used but everyone that tried got nice results in the end.

i'll try that idea and build something here. i really want to try using the 6AU6 as a power tube in a 1/2W amp or something like that. but i cant spend much money in this, because i have like 23 bucks right now...

i'll be talking to one of my teachers form my Tech School where i did Electronics and ask him if he could help me. Its hard to find tube stuff here in Brazil, and when you find its kinda expansive, so i wanted to try this idea because i got the tube as a gift from a friend, and i know he'd love to see this tube in use for something nice.

[andrew_k]

12AU7 gain stage as per input stage of any fender amp (except with a 12AU7 triode rather than 12AX7 triode) AC coupled from plate to 6AU6 grid, running plate load of ~100K (high enough for good gain, hopefully low enough to avoid excessive microphonics) into second 12AU7 triode as cathode follower into 1M volume pot and out. Worth experimenting with a tone stack before the volume pot perhaps.
6AU6 screen bypassed (eg: 50-100n from screen to cathode), Cathode bypassed with 22uF cap, try a pot between cathode bypass cap and ground as adjustable bypass; could give an interesting gain/breakup control.
Why?
The first 12AU7 triode will provide a decently high input impedance, and hopefully drive the 6AU6 into compression at the "knee" of its gain curve without having to wring stupid amounts of gain from the 6AU6 alone. The second 12AU7 triode wired as a cathode follower will provide a good low-impedance output to drive cable or any effect following it.
Voltages? Ummm... not pedal voltages. A 12VAC 1.5A plug pack outside of the pedal feeding the two valves' heaters in series (12au7 heater wired for 6v operation) and then the 12V feeding a reversed small 240V:12V transformer (suitable transformer available from hoffmanamps), rectified, filtered as the B+ supply.
Hmm.. sounds kinda usable. Might have to try it myself  

[Salvatore]

Check out this link here, and the last 2 posts + schematics there.
I suppose this could be a starting point for the 12au7 driving a 6AU6.

http://www.el34world.com/Forum/index.php?topic=6749.0

And this one..
http://www.el34world.com/Forum/index.php?topic=6759.msg63633#msg63633
(Original schematic and some explanation)

[blackcorvo]

thanks a lot Salvatore!!! The links really helped me.

Sorry andrew but i didn't understand what you said... my poor english didn't help me either ^^''

but after some time wondering of what should i do, i though of doing something i've seen some people doing when they have no money or can't find the output transformer: use a common power transformer, modifying the core plates' position. You just gotta remove the E and I plates, and mount the E's all in the same direction, put a piece of cardboard or folded paper to make a "air gap", then put the I plates over this and the OT transformer is done! you can get some wax and dip it to make everything get a bit more firm, but its not really needed since the structure that comes with the transformer can be used to hold everything up with no problems.

It would look like this: E|airgap|I

I also found a formula for using these comon PT's as OT's : Zo * (Vi/Vo) ² = Zl , where:
Zo is the Output Impedance (speaker impedance)
Vi is the Input Voltage for the transformer (110, 220)
Vo is the Output Voltage for the transformer (6, 9, 12, etc)
Zl is the Load Inpedance (usually specified in your Output Tube's Datasheet)

so i though of using a 6v/500mA transformer, that would give me an input impedance of ~21k, and be able to tolerate 3w, wich is really close to the max plate dissipation for the 6AU6 - that's 3,5w - , and i'm not prettending to go too far from the 2,75w of plate dissipation. i want something as close to the 1W output as possible, but 1/2w would be nice enought to me. i'm thinking of using the AX84's system with the 1R-1kPOT-100R on the cathode for getting the right spot for the 6AU6. now i just need help to know if i need to put a resistor in series with the grid that is conected to the B+ or if i can connect it directly to the B+...  

[blackcorvo]

okay, i have here a little preview schematic of what i though for this lil amp:


the resistor marked with a "?" is the one i dunno wich value should be, if i should use the 1K / 5W as in the AX84 project or if should be directly connected to the B+...

And i marked 850mA for the first PT because 10,2W is more than enough power for the filaments + power... just do the maths:

1st PT:
12v * 0,85 = 10,2W.

2nd PT:
12v * 0,3 = 3,6W

Filaments:
(6 * 0,3) * 2 = 3,6W

Fil. + 2nd PT = 7,2W

10,2W - 7,2W = 3W

So i can easly power this amp that way and i have more unused 3W. Lets see if i can find a 850mA Transformer... lol

[petemoore]

  the resistor marked with a "?" is the one i dunno wich value should be, if i should use the 1K / 5W as in the AX84 project or if should be directly connected to the B+...
 IIRC has to do with repelling some of the electrons from plate, should be be a small % voltage less than the plate, looking for those references to re-read again now, ''Screen Grid", "6AU6 Data Sheet".
  http://en.wikipedia.org/wiki/Screen_grid

[blackcorvo]

thanks for the help pete.

i did some math here and found that with a 1K resistor i'd get a voltage drop of around 3v on the 2nd grid, so i think it's okay for using the AX84 project as reference yay!

And for anyone that's asking theirself, the 1N4007 in series with the reticified B+ is there for reducing any chances of hum on the power supply, just in case.

[petemoore]

And for anyone that's asking theirself, the 1N4007 in series with the reticified B+ is there for reducing any chances of hum on the power supply, just in case.
  I don't understand that a diode would do anything there except drop voltage one diode drop.
  Usually a smallish resistor between filtercaps is shown right there, helps the caps fill smoothly as I understand it.

[blackcorvo]

just make some tests in a noisy power supply next time with that lil diode and two caps like in the pic (but use larger values, at least 1000uf), and you'll notice the diference.

also, its not needed, that was just a sugestion.
Myself, i'll try without the diode first, and if i notice any "hum", then i'll add it.
and since i have no 16 ohms speakers here, i'll use 2x 8" ones i have here connected in series and try to make a nice cabinet to them (we have some pieces of wood here that may work nicely for that). i'll try to go tomorrow buy some of the components i don't have here and start building it.

oh, and today i tried the 12AX7 and it seems to be working. just sounding kinda "muddy" when i get some distortion on it but seems to be because of the low voltage i'm using... i'll try it when i finnish building the whole thing.

[blackcorvo]

okay, i was making some tests here with my tubes but this morning i looked to my 12AU7 and the glass was broken... i think it couldn't take the recent weather changes around here...

anyway, my 12AX7 works and it's still all together, so does my 6AU6... but i was thinking a bit and had another idea based on something i saw on http://handmades.com.br/

they created a project of a one tube amp using an ECL82, that they called the IALL 3.5, or "I Am Little Lucifer", that is a little tube 'demon' amp of around 3,5 Watts for having fun in home, with friends, travelling, or just recording.

So, i found this site that sells tube stuff here, the http://altanatubes.com/, but buying the original ECL82 would be taking all the monet i have ATM, and since i wanna make some other things i though of buying it's lil bro, the ECL84.

the greatest difference between them is that the ECL82 can take 7W on it's plate, while the ECL84 can take only 4W. But, for my idea of an amp, and since i alreadly have a transformer to use as the OT, i would have around 2W of output power, thats more than enought for me. that way i could make the whole amp using only one tube.

what do you guys think? oh, and since it was based on the IALL, i though of calling it IAML, or "I Am Micro Lucifer".

[blackcorvo]

giving this post a lil update: unfortunately, my mother needed the money i had for this last idea of buying an ECL84, so i'll try to build something with the 12ax7+6au6.

i though of maybe getting the high voltage using a lil 15+15, 300mA transformer i have here, using a simple voltage muiltiplier (8x to get around 280v, that may drop to 250v with a load), using the diodes and caps technique. it must have enought power for this (since 30V * 0.3A = 9W).
The filaments will be fed by a small dc supply, using one 6V/1A regulator.

yeah, i think this must work nicely. but i'll first try a lil SS amp for the travell i'll be doing this week.

[blackcorvo]

okay, heres a new update:

i tried the circuit out, but with mods:



i didn't put the filaments because i used a sepparate power supply for them in my tests.

it works just like a tube amp should, has a lot of gain and distortion. if you want it to be cleaner, try making another pre-amp config, or with another tube, or whatever you want. i only put a one knob tone because i don't have much components here ATM for a tone stack...

and if it sounds too bright, try adding a 22nF cap (or higher/lower value depending of your taste) in paralel with the OT primmary.

hope you people like it! i LOVED it! no i just need to make it in a definitive version ( its actually all in my 2 protoboards XD )

EDIT: the non-marked resistor in the input is 470k. i forgot to note that, sorry!!! i was SO excite because it worked that i didn't notice i forgot to put that value there! XD

[Salvatore]

Hey, you tested the design, looks cool ! 

I see that you used a 12 V transformer for the output, I'm wondering, does it manage to retain the lows well ?
I would love to hear a sound clip, I bet a tiny wattage, few/cheep component, nice sounding tube amp would be welcomed by a bunch of diy musicians.

[Thomeeque]

For more effective filtering I'd advice to add some filtering caps yet (without them those 5k6/1W resistors do not make much sence, they could even cause some nasty behavior this way IMO), on the other hand you can significantly lower capacity of original ones:



T.

[blackcorvo]

Hey, you tested the design, looks cool ! 

I see that you used a 12 V transformer for the output, I'm wondering, does it manage to retain the lows well ?
I would love to hear a sound clip, I bet a tiny wattage, few/cheep component, nice sounding tube amp would be welcomed by a bunch of diy musicians.

http://www.youtube.com/watch?v=bqj3IFxM7og here is a video sample of this lil devil rocking. i know, my playing sucks but the tone os nice =P

For more effective filtering I'd advice to add some filtering caps yet (without them those 5k6/1W resistors do not make much sence, they could even cause some nasty behavior this way IMO), on the other hand you can significantly lower capacity of original ones:



T.

thanks for the help, man. i just used what i had in hands, really. and i know like NOTHING about tube theory, so i just used my knowledge about electronics and tried it out.

and anyone that have sugestions to make it better, be free to add your sugestions here!!!

[PRR]

>> you can significantly lower capacity

Agree. 940uFd??

> i know like NOTHING about tube theory, so i just used my knowledge about electronics

So use your knowledge as a lever.

A transistor power amplifier at 24V probably pulls 0.5 Amps. The ratio is about 50 ohms. (In general, about 6 times the speaker impedance.)

We don't know the tube amp's voltages and currents, but 220V AC gives about 300V DC. The lovely diagram does not say what tube you used, but you mentioned 6AU6 which can stand 3 or 4 Watts, 10 or 13 mA, no more than 20mA for the whole amplifier. 300V/20mA is a ratio of 15,000 ohms

15,000/50 = 300:1. It appears that you can use a cap 300 times smaller uFd than you would use in a transistor amplifier.

What would the transistor amp use? 470uFd is cheap. 2,000 is fine. 10,000uFd is generous.

So for the tube amp, using only the knowledge you have, 1.5uFd to 33uFd is your first guess.

The transistor amp has huge negative feedback and probably good ripple rejection. The pentode has some rejection but not as much. I would make the cap 2 or 3 times bigger. 3uFd to 100uFd.

When you do not need the MAXimum power available (and using 6AU6 as a "power" stage means that you are not power-hungry), you will get even better ripple reduction by using a C-R-C filter, and letting the R drop about 10% of the voltage. From 300V to 270V. The 20% drop in power is hardly audible. But the ripple may be reduced 10:1 or even 100:1. If you want to drop 30V at 20mA, you use a 1.5K (or 1K) resistor. It cruises at 0.6 Watts, so must be a 1W part. At turn-on the energy is very high, and 2W or 5W metal-film will last longer.

Design of a low-performance C-R-C filter is not real critical. You can just copy values from an old Champ: 8uFd, 500, 8uFd. Or since caps are cheap today, 16uFd, 470, 16uFd. The smaller R gives less filtering, but also less heat, allowing use of low-cost 1/2 Watt resistor. 16uFd is an old-number value, 15 or 22uFd in modern parts is all the same.

Notice that "16u+16u" is very nearly the same as that first-guess "33u". The pentode's need for more filtering is solved by cutting the 33u cap in half (16u+16u) and adding a resistor.

> i looked to my 12AU7 and the glass was broken... i think it couldn't take the recent weather changes around here...

Tubes just like that were used from the arctic cold to burning deserts and 5 miles high in the sky. Even Brazil's weather won't break a 12AU7. Somebody cracked it. Tubes are tough, but people who don't know tubes can be careless.

[blackcorvo]

Thanks for all the tips and information, PRR.

In the circuit i used in my tests and the video demo, i used a 80uF - 1K/5w - 100uF C-R-C Filter. the 470 resistor on the 6AU6 Cathode was adjusted to give 10mA Plate Current, and the 4700uF bypass capacitor was to give a better bass response, but it can also be corrected adding a capacitor in paralel with the OT primary, like i've seen in the Firefly cricuit.

the power section, as i mentioned before, was based on yout project Third Watt, but i wanted a little more power and only had one 6AU6. thats why i used 220V. In the end it turned to be 277V DC under load, and i did the P = U * I to get the dissipation in the 6AU6' plate, that was around 2,7w at 10mA. Mine's a USA production, no brand 6AU6, so i think it can easly take 4w plate dissipation, but i didn't want to damage it, so i just kept my maths in the safe area.
I really think my 12AX7 is somehow damaged, because i tried using the Fender Pre-amps configuration with 100k/1k5+22uF resistors and capacitor, but the sound became too low and the notes were muted after 1 second or so... so i adjusted everything till it sounded good. I should buy other twin triode pre-amp tubes... but i really have no money for that ATM. Maybe after Christmas i can get something... we'll see.

[Cliff Schecht]

First off, I've used 2 6AU6's in the preamp of the tube amp I posted in the November Effects Submission thread and I must completely disagree that they sound bad in a preamp. Gibson and a few other manufacturers also used these tubes in a few of their weirder small amplifiers to much success. Good sounding little tube IMO and gives a very pleasing distortion when pushed.