I have one 12AX7. What simple tube drive should I make?

[Taylor]

I am building a multi-effects box which will have a headphone output. I'd like a nice tubey drive to sound a little more like my amp when playing through headphones. I have a single preamp tube sitting around not doing anything, and I'd like to put it to use. It should be something low voltage, which, I know isn't going to sound great, but I don't want to mess with high voltage for this.

Any simple projects that sound a bit like a tube amp (I'm not aiming too high, just a bit like a tube amp)? Would I be better off forgetting tubes altogether?

[frequencycentral]

For those of you who want to try (or have tried with poor results) a 12AX7 in your Valvecaster, I have some ideas.........

12AU7 is a medium mu dual triode
6111 (Subcaster) is also a medium mu dual triode

12AX7 is a high mu dual triode
6112 is also a high mu dual triode

I just breadboarded a Subcaster using a 6112 in place of a 6111 - blarty due to too much gain, so I experimented with a couple of resistor values and would recommend:

For a 12AX7 Valvecaster: change R2 to 100k, change R4 to 100k.

For a 6112 Subcaster: change R5 to 100k, change R6 to 100k.

I haven't got any 12AX7 to try this with, but I'm assuming that a 12AU7 is roughly equal to a 6111, and a 12AX7 is roughly equal to a 6112, based on mu.

I've got an idea for a new very dirty high gain tube pedal based on my little experiments with 6112 (should work well with 12AX7 too), it's working title is "Promiscuous Girlfriend" - watch out for the thread!

http://www.diystompboxes.com/smfforum/index.php?topic=63479.1020

[frank_p]

If I remember well, the "real McTube" could be an option:
http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

It goes in high voltage but with "ordinary transformer low voltave derivative -deviation-".
If you feel a bit adventurous, it might be an option.

Maybee for a second project.  Or a project with help from other forumites.
Still a high voltage project anyhow...  But some report: to be considerated option.
If you feel unsure go on with the other project in the other topic.

Yours.
FHP

[brett]

Hi
in the 12A_7 series, only the 12AU7 will have enough current capacity to operate as a power tube and drive your headphones.  You'll probably want to operate it in self-biasing, push-pull mode (like the Firefly amp).
The output transformer will cost $$$.  For a headphone amp with nice tone, I'd either go simple (an LM386 with bells and whistles) or do something that will need some effort but sound great (like a Deacy amp or similar discrete push-pull that uses an output transformer).

cheers

[Salvatore]

Any simple projects that sound a bit like a tube amp (I'm not aiming too high, just a bit like a tube amp)? Would I be better off forgetting tubes altogether?

No way, tubes are so important in music history that they at least need some attention, besides that they can do stuff with sound that you can't get any other way.
And as an electric guitar player, you need some tube experience, cause tubes where essential in the evolution of guitar sound.
Rock guitar players who never played through a great tube guitar amp, will never fully understand the history of rock music.

Now a starved plate tube preamp will never sound like a full blown guitar amp, but it is interesting and great fun.

At this moment the Valvecaster is being build by a bunch of people on this board, so if you try that you can count on some immediate support from the members here, but it needs some special attention cause the schematic is based on some other tubes.

An other very popular design is the shaka tube, it is based on the 12ax7, you can find countless mods for it, and many people on this board has build it, so you can count on some help there as well.
http://www.diystompboxes.com/smfforum/index.php?topic=46137.0

There are many other starved plate 12ax7 preamp designs around, but I have no idea if those are any good.

Enjoy. 

[Paul Marossy]

Build a Shaka Tube. http://www.diyguitarist.com/DIYStompboxes/ST1590BB.htm

[Taylor]

Hi
in the 12A_7 series, only the 12AU7 will have enough current capacity to operate as a power tube and drive your headphones.  You'll probably want to operate it in self-biasing, push-pull mode (like the Firefly amp).
The output transformer will cost $$$.  For a headphone amp with nice tone, I'd either go simple (an LM386 with bells and whistles) or do something that will need some effort but sound great (like a Deacy amp or similar discrete push-pull that uses an output transformer).

cheers

To clarify, the tube is not going to be used as the headphone amp. I am building a solid state HP amp, I just want something a little tubey for the headphones, since I normally play through a 100 watt 1968 Univox. I just want a little of my real amp's sound when I can't play loudly.

Any simple projects that sound a bit like a tube amp (I'm not aiming too high, just a bit like a tube amp)? Would I be better off forgetting tubes altogether?

No way, tubes are so important in music history that they at least need some attention, besides that they can do stuff with sound that you can't get any other way.
And as an electric guitar player, you need some tube experience, cause tubes where essential in the evolution of guitar sound.
Rock guitar players who never played through a great tube guitar amp, will never fully understand the history of rock music.

Er, I didn't mean forget tubes completely as in not ever using them in anything I play through. Note my comment above about my Univox - tube preamp, power amp, reverb amp, and tremolo. What I meant was this: a lot of people are so caught up in the expected mojo of tubes that they will use stuff based on it having a tube, even when it has no actual impact on the sound. See here for an example:

http://www.audiomasterclass.com/arc.cfm?a=will-you-ever-buy-behringer-equipment-again-after-reading-this

I'm not so block-headed that I must use tubes; if transistors will get me a better sound than starved plate tubes, I'll use transistors.


Thanks for the suggestions everyone! I'll look through these projects.

[Ripthorn]

I have a behringer tube preamp, got when I first got into recording several years back.  I'll have to pop it open and take a peeksie.  I may have a perfectly good 12ax7 not doing much for me right under my nose.

I think that what might really get you some mileage is to take a design like the Shaka tube and breadboard it and tweak until it is just what you want for your headphone amp.  I am working with submini's quite a bit and the tweaks really make a huge difference.

[petemoore]

  x7's a high gain preamp tube, that would tend to make the sharper part of the curve hit the rail [putting larger portions of it completely beyond the tubes control], for 'amp' to play through, these characteristics may not be the desired ones, terms like loose, blatty.
 30v supply is safe.
 Heres the suggest:
 Aim for low gain [slight signal conditioning? pregain ?] with option to medium gain, try putting that into something which distorts mildly [minibooster? with buffer].
 Just enough gain at the input to get the tube-boost to go slightly non-linear, drive the next stage known for slight non-linearity, match to drive the amplifier.
 If you see an impedance mismatch, try sticking a buffer there.
 That said, a different tube or some simple Jfet non-linearity could probably do well there.

[brett]

Hi
RE: impredance matching

In "starved plate" mode, it's important to drive a high impedance load.  A JFET buffer with a high value (e.g. 4.7 Mohm) pull-down resistor is ideal.

Try to use at least 30 V DC.  Power supplies for inkjet printers are often 24 to 48V.  That's reasonably safe except for setting fire to things.
cheers   

[Salvatore]

Er, I didn't mean forget tubes completely as in not ever using them in anything I play through. Note my comment above about my Univox - tube preamp, power amp, reverb amp, and tremolo.

You made no commend about an Univox - tube preamp, power amp, reverb amp, and tremolo, but never mind, I'm glad you have one, maybe it can be converted by making a headphone, ore line out directly in you're amp if thats the sound you like.

What I meant was this: a lot of people are so caught up in the expected mojo of tubes that they will use stuff based on it having a tube, even when it has no actual impact on the sound. See here for an example:

http://www.audiomasterclass.com/arc.cfm?a=will-you-ever-buy-behringer-equipment-again-after-reading-this

Don't take that article to serious, it's bashing is purely based on the fact that Beringer wants to make there equipment look good, don't we all ??
Besides mentioning 3 glamorous leds, it contains no relevant info on the equipment at all.
And to answer the question in the article, yes, the tubes are connected, I've got the mic200, the 12ax7 runs at 48 volts, and with a changing of the supplied tube it can sound pretty nice for the price.
It's predecessor, the preamp in the article, can be better ignored cause it's prone to irritating defects.

I'm not so block-headed that I must use tubes; if transistors will get me a better sound than starved plate tubes, I'll use transistors.

I hear ya about the tube hype, true, and sometimes a bit misleading, but the question what sounds better, at the end purely come down to taste, I love the sounds of my tubes, but would not want to mis my transistor, and diode distorter's.
And about the amp sound, no effects box will sound as one over headphones, I've got those Hughes and Kettner Creammachine, and Metalmaster amps with 2 12a..7's on high voltage in each.
Nice in it's own way, but over headphones not even close to an amp sound.

But to answer you're question, will an, even starved plate, tube design, sound tuby ?
Sure it does, will you like it ?
I don't know, maybe the 12ax7 you have will sound bad in you're ears in any design, I surely listened to awful designs, the devil is in the details. 

[Taylor]

Well, I looked again at the spare tube I have and it turns out it's a 6AN8, not a 12AX7 anyway. I'm not sure how the specs compare or what the deal is with this 6AN8 - much less info out there about these than the 12A_7 family. I may just buy something specifically for this project.

If I'm going to buy a tube for this, should I look at something besides the AX7? It is the highest gain of the common pre tubes, of course, and I'm not really looking for a high gain sound, just a slight grit.

[petemoore]

Power supplies for inkjet printers are often 24 to 48V.
  ...and have a primary which expects 120vac...
  That's the side to unplug when you're working on 'small' voltage WW's or Transformers.

[frequencycentral]

Well, I looked again at the spare tube I have and it turns out it's a 6AN8, not a 12AX7 anyway. I'm not sure how the specs compare or what the deal is with this 6AN8 - much less info out there about these than the 12A_7 family.

6AN8 is a triode/pentode - very interesting little tube: http://frank.pocnet.net/sheets/106/6/6AN8.pdf

[Mick Bailey]

Shaka tube with a BMP tone control. Best thing I ever built.

[george]

Shaka tube with a BMP tone control. Best thing I ever built.

+1

[Johan]

6an8 can be a pretty cool tube...for a small amp, use the pentode as preamp and hook the triod up to a champ-transformer and then youv'e got a tiny, tiny all tube amp...for powering this, look at the reall McTube..  look for fender champ 5C1 schematic for reference ( or gibson LP JR amp, )
j