Tube Pre-Amp in a box (pedal size)

[srv]

I'm thinking of building a tube pre-amp in a box, for several reasons:
1. playing through an amp disturbs the local residents
2. I want to practice into headphones but still get a nice warm tube sound
3. the PODXT gives me listening fatigue

I've been reading up a lot on a few designs and have decided on this
project, at least for the power supply sections. (using the step-up toroid design from 12v AC)
http://www.el34world.com/projects/images/TubePedalSchematic.gif

My build requirements are:
Something in a small enclosure
5 pots max.
ie: pre-amp vol/gain , treble, mid, bass and a master volume
Maybe one or two switches, one for marshall/fender (clean/boost) and a second for standby circuit
2 x 12AX7's.
No mains power supply, I'm wanting it to run on 12v AC plugpack and use toroid step up design

Taking some ideas from this website
http://www.ax84.com/static/hioctane/AX84_Hi-Octane_Options_101004.pdf
on page 5 is an option for a lead/boost switch
I thought of incorporating this into the design.

My ideal sound output would have a nice bluesy Fender clean sound
and then be able to dial up to Marshall overdrive sounds.
Think of something like the egnator tweaker front end in a box.

So my question here is actually calling on the tube gurus to see if the first circuit can be modified
easily to accomplish this and if anyone has some more ideas on this or a better design.

The finished product would look something like this
http://www.pdfelectronics.com/images/Chassis1.JPG
(Thanks to 'kurtlives')

Lastly, I hope I didn't upset anyone by linking to their circuits and photos.

Thanks.

[bassmannate]

Sounds like you're best bet would be to build something based on JFETS instead. Tubes generally can't run off just 12v and give you the output you're looking for. Even the valvecaster probably wouldn't be suitable for a headphone amp. Tubes are also difficult to fit in a stompbox. You'll fit one, MAYBE two if you're really creative with your space.

There are several popular vintage tube preamps that have been converted to JFET design that fit in a stomp box. Try looking at runoffgroove (http://www.runoffgroove.com/articles.html) and there are several circuits there that say they're an amp that's been converted to a stompbox distortion. They're basically the whole preamp converted to fets and put in a small box.

[srv]

Thanks for that link to the JFet designs. I will build a couple of those.

Looks like I misled a few people in my description.
I am actually wanting to use 2 x 12AX7's which I have already
and also the AC step up design using a toroid transformer (I also have this part already)
Also is the novelty/coolness factor of using tubes.

The tubes and transformer will mount on top of the enclosure, which means
inside the box will be the switches/pots/in out jacks and pcb components.

I guess what I'm asking is , out of the tube circuits available
what is the best one to use to get a nice clean sound, but also a good overdrive?

I like the idea of switching the last preamp stage (cathode follower?) to bypass eg. an extra overdrive stage.
Effectively using one switch to have a clean/boost mode. (see page 5 of pdf for schematic)
But will this give me the desired clean/overdrive sound I'm after,
or should I be looking at a 2 channel design (which means adding another tube and it won't fit into the box I already have)

[Bard Morons]

Another quick thought is to use a set of subminiature tubes.  Check this out...

http://amps.zugster.net/projects/subminiature

Again, they won't be running off 12v, but you can build a charge pump (DC to DC converter) pretty easily.  I used an old laptop power supply at 18v and boosted that up to ~220v for driving some of those little guys using a pretty simple circuit.  Search around the web a bit.  There are a few projects that are currently doing similar things.  Now, the tubes are tiny, but to shove them in a stompbox sized enclosure might not be a great idea.  Having them protruding out the top with a small fan though...

[merlinb]

Again, they won't be running off 12v,

He's not using 12V, he using high voltage.

SRV, practically any two-tube preamp you care to mention will do the job. Borrow the front end of a Marshall JCM preamp or something.

[defaced]

If you've never played with tube circuits before, don't try to kill all these birds with one stone.  You'll drive yourself nuts.  A simple one channel preamp is easy, but you want something versatile with different channels.  Switching all this stuff in and out, along with the interactions of the two channels is usually where the problems come in. 

Start with the power supply.  Get one up and running with the voltages you want.  There's a thread on the Mesa Bottle Rocket with pages on how to approach this sort of topic, it's worth reading because that project as a whole is a good starting block.  Once you have a working power supply, bread board (the literal meaning, not these cute plastic things low voltage devices can use) the individual tube circuits (clean, overdrive, lead, whatever) you want to build using the power supply you designed/built.  Once you tweak them to get the sounds you want, then figure out how to make the two of them play together. 

Adding/bypassing stages like shown in the AX84 PDF is done all the time.  You can do this with one stage or more stages.  The details are usually what will drive you nuts with switching stages and the like. 

[anti-idiot]

Thanks for that link to the JFet designs. I will build a couple of those.

Looks like I misled a few people in my description.
I am actually wanting to use 2 x 12AX7's which I have already
and also the AC step up design using a toroid transformer (I also have this part already)
Also is the novelty/coolness factor of using tubes.

The tubes and transformer will mount on top of the enclosure, which means
inside the box will be the switches/pots/in out jacks and pcb components.

I guess what I'm asking is , out of the tube circuits available
what is the best one to use to get a nice clean sound, but also a good overdrive?

I like the idea of switching the last preamp stage (cathode follower?) to bypass eg. an extra overdrive stage.
Effectively using one switch to have a clean/boost mode. (see page 5 of pdf for schematic)
But will this give me the desired clean/overdrive sound I'm after,
or should I be looking at a 2 channel design (which means adding another tube and it won't fit into the box I already have)

Well, i've had to control my urge to put as many switches as possible; but here it is

http://s20.photobucket.com/albums/b228/Anti-Idiot/Schematics/?action=view&current=Preamp2-ChannelTube.jpg

Sw1 is for modes (1959SLP or 2203)

Sw2 will give you a slight boost on 2203 mode. 10k in parallel will give you 5k; 3k9 will give you 2k7 (and you could add a 680n with that 3k9)

Sw3 is for this.

This is untested, but should work fine.

Another way to do this is by building a 2203 and switch between the inputs (not using jacks but a dpdt), but i can't find a schematic.

Cheers.

[srv]

I found another circuit that looks like it would be good to build and try out.
It's the 'English Muffin'
http://www.ehx.com/products/english-muff-n

I found a schematic in pdf format
http://www.talkbass.com/forum/attachment.php?attachmentid=61227&d=1181933678


Can anyone tell me what the power supply on the high voltage side is doing
going to pin 9 of the heaters on V2C(off R22/ R23 )
Interestingly it doesn't connect to V1C

I thought it might be something to do with hum reduction or standby but I'm not sure.

[defaced]

Pin 9 of that tube is being used as a sort of artificial center tap.  This floats the entire regulator circuit at whatever voltage the elevation is.  Looks like about 80 volts DC if my numbers are right. 

DC elevation of the heaters is done to elevate the heater over the cathode voltage (of a typical gain stage), thus reducing heater/cathode coupling hum.  It also decreases the heater to cathode voltage in a cathode follower setup.  H/k voltage can exceed datasheet maximums in some cathode follower designs. 

Circuit comments:

Holy wow, that heater arrangement is a little excessive.   If I were to build this, I'd nix the relay and all of the regulator circuit.  The relay would get replaced with the standard 3PDT stomp and the heaters would be elevated AC.  I'd also drop the elevation down to about 40-60 volts.  This can be tuned at the time of building if someone really wanted to.  Guys that have messed with it say they've heard significant improvements in hum after about 10 volts of elevation.  This arrangement would mean the LED would either need to be on or off, no switching, unless you want to use a 4PDT switch.  Since this is a drive pedal, IMO, a on/off status light is kinda not needed. 

Preamp decoupling caps are huge.  C18 could be 22u no problem.  C17 would need to be larger, but I'm not sure 100u is necessary.  I'd start with 22u and see what is does (you'll get power supply ripple in your audio) and increase as needed.   

Note this is a 12AY7 design.  Probably not as high of gain as you'd initially think with 3 gain stages. 

Some of these resistor values are odd.  16M?  162K? 562K?  I wouldn't get too hung up on those. 

[sst4270]

I was typing up a response when I saw defaced's response and mine was virtually verbatim.

So... What he said.

-Steve

[slacker]

Holy wow, that heater arrangement is a little excessive.  

Never mind excessive, it looks wrong to me. Anyone know what the  "+12V_return" supposed to be connected to, as it's shown I don't understand it. That whole section is floating around 80 volts as the only connection to anything else is through the pin 9 connection. If it's supposed to be grounded then the pin 9 connection makes no sense. Or am I missing something?

[srv]

Thanks for your feedback.

fyi: I found some more info here
http://www.freestompboxes.org/viewtopic.php?f=1&t=2458&sid=b34e9f4360e85b2267223f833aacfe4b

I'm going to try buiding this with 12AX7 tubes.

Stay tuned.....

[defaced]

Never mind excessive, it looks wrong to me. Anyone know what the  "+12V_return" supposed to be connected to, as it's shown I don't understand it. That whole section is floating around 80 volts as the only connection to anything else is through the pin 9 connection. If it's supposed to be grounded then the pin 9 connection makes no sense. Or am I missing something?

The whole nomenclature for that part of the circuit is wrong; the -12v return is not correct.  It should read something like 0v Ref and 12v Ref because there is only 12v between the two of them; if you referenced them to ground they'd be ~74v and ~86v because of the 80v elevation.  Since the whole thing floats at 80 volts, pin 9 acts as a heater center tap.  There is no earth connection for the regulator circuit, if you do earth it, you'll turn the heater into a fuse or the power transformer will die. 

SRV, that link got cut off.  Can you post the complete link?

[srv]

reposted link to more info

"www.freestompboxes.org/viewtopic.php?f=2&p=43256"
www
dot
free
stomp
boxes
dot org


Interesting that the site does not like that domain name being typed here, it shortens it to ... ?

[PRR]

> Never mind excessive, it looks wrong to me. Anyone know what the  "+12V_return" supposed to be connected to?

It is correct, though excessive.

Anything powered FROM +12V should RETURN to "+12V Return".

The funny thing is, there's NO loose-ends on the 12V returns, they are all accounted for. (+12V does have a loose-end, over at the relay.) That point and label could have been omitted.

Taking heater CT is useful for AC heat, not for DC heat. That's maybe leftover from some AC-heat incarnation.

Taking CT from the center of a twin-heater tube is a cheap trick, saves two resistors. If V2 is pulled you lose the CT tap. This may be moot since the whole shebang stops working without V2. Conceptually the heater supply could float to unknown voltage and zap the heater insulation; IMHO this is unlikely to cause real harm. But since heater CT is moot when using DC heat, this is a "cheap useless frill", excessive over-design.

The heater system should be held at some high voltage because some cathodes are at a few volts and V2A cathode is at +180V. Heater insulation breaks down some place past 90V, with some differences in specs for different tubes, and wild differences in what factories can reliably do. Splitting the difference, +86V, is a wise idea.

It is correct as drawn. You could just as well tie the +86V to "12V Return" and save one longish wire.

[slacker]

Thanks for the explanation, I didn't know about the heater insulation breaking down over a certain voltage I thought the reason for raising the supply was just to help prevent hum, it all makes sense now.

[srv]

@PRR "(+12V does have a loose-end, over at the relay.) That point and label could have been omitted. "

I believe this is correct as the relay is only powered on through LED2 when the footswitch is engaged S1C
and then the connection is made back to "+12v return"


Thanks for your comments on heater voltage breakdown, though still not sure why V1C has not got this as well
if it is an internal thing.

[defaced]

though still not sure why V1C has not got this as well

It does, via V2.  What I and PRR have already said is that the single elevation point at Pin 9 of V2 elevates the entire heater circuit, this includes V1, the regulator, the relay, and all of that stuff.  The only difference between doing it via Pin 9 on one tube and doing it via the heater center tap (if the transformer has one) or via an artificial center tap (2 100 ohm resistors usually) or via a hum balance pot, is just the designer's choice.  They all do more or less the same exact thing, just different ways.  Though as PRR rightfully stated, if you pull V2, the whole elevations scheme and voltage reference of the regulator circuit is lost. 

[EATyourGuitar]

someone else on this forum already posted a tube pedal that runs on 9v or 12v or whatever. its just a bunch of voltage doublers. sorry I dont have the link in front of me. I do remember it was very difficult to get enough current with all the doublers and a one spot adapter. look at the mesa vtwin or the soldano tube pre pedal or the hughes and ketner. those are real tube pedals and they are huge and run on a IEC power cable. then look at everything else that is small. all fake B/S. glowing tubes are actually orange LED's. all the fake shit that has been sold, all runs a a low voltage plate. starved plate design. just find a really nice distortion pedal and build it. like the BSIAB. or the DSL if its been reversed. wampler pedals etc...

[thomasthebuzzard]

also I know this is wierd and might be off topic but I have gotten 9au7 tubes to run on a 9v battery. This might be helpful to fit it into a smaller package if your building it yourself.