Soul-O-Tube preamp

[Mick Bailey]

Here's my take on the 12v tube preamp pedal, which includes a low insertion loss tone control circuit to increase its versatility and cater for a wide range of pickup types;

http://www.avwz35.dsl.pipex.com/soul-o-tube.gif

The intention with the design was to achieve a fairly clean output which could be used as an additional stage to push an amplifier into smoother distortion - either as a blues overdrive or to increase the gain of an already 'hot' amp. It was also intended as a booster for weaker pickups, both as a flat-response preamp, or to effectively cut or boost bass or treble frequencies. There's enough headroom to get a decent clean boost.

With a single 12AU7 it is essential to reduce signal losses; the usual tone control circuits attenuate the signal too much and there is not generally enough signal recovery for anything other than a simple treble roll-off control.

This tone circuit provides an absolutely flat response when the bass and treble controls are at maximum. You can appreciate that C5 and C6 are effectively removed from circuit and at this point the attenuation is that of the series-parallel value of the 1M bass control, R6 and the 470k treble control. The bass control is particularly effective and enables the preamp to be used as a useful treble booster. If more bass attenuation is needed, change R6 to 47k.

C6 controls the level of mid range when the bass is turned down. Decreasing this (to say, 470pf) will pull down the mids as well as the bass, but this can be too extreme.

R2 and R5 were chosen to give 1/2 B+ when measured on the plates (pins 1&6) of the tube. I arrived at these values after testing the circuit with a range of NOS Mullard and Brimar tubes.

You need to use a regulated power supply otherwise it will hum. The usual 7812 with a 470uf cap across the input works well if powered from a 400ma unregulated 12v supply.

[Minion]

You could also use a Voltage doubler to get the Plate voltage higher .... If you use a small 12v ac Transformer you can rectify and regulate 12v for the Heaters and then run a Voltage doubler off of the 12v ac and get about 32v DC for the plate , you get you more headroom and clean gain without really complicateing the design , you might have to change the Bias of the tube at the higher voltage , you could also put a switch on the Cathode bypass caps so you can go from clean to overdriven with a flick or stomp of a switch ......


Cheers

[Mick Bailey]

I run a high voltage version at 300v with a nixie supply and changes to the bias resistors (1.8k) though I kept the caps. The reason I chose 12v was that It was originally built with a 12U7 space-charge tube but these are very difficult to get hold of at sensible prices. I found that the 12AU7 behaves almost identically to the 12U7 in this circuit, so left it that way.

I do agree that increasing B+ will increase headroom, but haven't found this to be necessary with the range of guitars that I own. If anyone would like to do this then the Shaka Tube power supply could be made very compact if small caps are used and there's just about room in the 1590b box to squeeze this in.

[Renegadrian]

Nice! Seems a good project to try in the next future...THX for sharing, man!!! We needed another TUBE circuit!
The more the better!!! 

PS Have you tried it with an AT or AX?!

[Ripthorn]

Just one more project on the ever growing list of to-do.  Good thing I still have a few 12au7's waiting to be used.

[Auke Haarsma]

Thanks for posting!

[Mick Bailey]

lower output with AT and AX - almost too little to be useful.

[Renegadrian]

Made a Vero layout from the schem - it should be correct...
I haven't built it yet so take it as unverified, altough triple checked as always...



As there are two C5 in the schematic, I wrote their value too...Other components follow the schem.

[Mick Bailey]

Thanks for spotting the cap error - the last C5 (.01uf) connected to the treble control should be C7 - I'll amend my schematic.

[Renegadrian]

Thanks for spotting the cap error - the last C5 (.01uf) connected to the treble control should be C7 - I'll amend my schematic.

Right, now the 0.01 cap is C7

[Isak]

Thank you for posting this!

Can I use it also as overdrive?
Meaning preamp + overdrive in one 12au7?
If yes what should I change to a not so hard overdrive?

Thanks again,
Isak E.

[Renegadrian]

Well, I cannot answer, as a lot of other ckts had priority and this layout remained unbuilt...

[Isak]

Thank you for replying Rene.

I was thinking that if taking the values of R2 and R3 in the valvecaster project make a nice and not so hard overdrive maybe I can increase the values of R3 and R4 in this scheme will give me a kind of overdrive, hope it makes sense.

What do you think?

[Renegadrian]

it could be a clever way to mod the original schem, also give the cathodes a lower value like other similar ckts

[Isak]

I'll try that, thank you!
As soon as I will have answers I'll post

[Isak]

Hi again.

i know its been a while and the topic is old but yet again i came a cross this project and i want to build it.
but i want to use 80V instead of 12V using this charge pump:
http://www.diystompboxes.com/smfforum/index.php?topic=74729.0
my questions are:
do i need to power the 12AU7 with 80V at pin1 and pin6?
if so, do i need to change the values of R3 and R4?
should the resistors be above 1/4W...? maybe 1/2W or 1W?
as i already thought to my self all the caps should be above 80V, right?

i see the scheme is gone i upload it here for you to look at it and tell me if its possible.



i thank you in advance.
cheers,
Isak E.

[GibsonGM]

Yes, pins 1 and 6, thru your plate resistors.   I'd go with 1/2 watters, myself, and they are the standard anyway....you're not near the limit at 80V, but as you try different values you COULD approach .25W.   (Ohm's Law....voltage/resistance = current, then do the power, current * voltage).      This is just for the plate resistors, BTW - the others are fine to keep 1/4W.

If you have 1/2 watters in there, you can also safely up the voltage into the hundreds eventually if you want, they have a higher voltage rating than the 1/4 watters.

As for values?  That's up to YOU to decide, ha ha.    Look up "load lines", figure out what lowering them might do......[url]http://www.valvewizard.co.uk/index.html[/] check out "Triode Gain Stage" to see what the plate resistors do.

[PRR]

At 80V, this thing will be LOUD.

Perhaps strong enough to pop the amplifier input.

There's other ways to interface High-voltage tube boosters into guitar amps.

> do i need to power the 12AU7 with 80V at pin1 and pin6?

No, it goes to the place marked "+12V", at the *other ends* of R3 R4 from the tube pins.

> should the resistors be above 1/4W...? maybe 1/2W or 1W?

Use Ohms Law. Assume worst-case, that all 80V appears across the 100K. That causes 0.064 Watts of heat in the resistor. 1/16th-Watt is barely enough. 1/8W is quite safe. 1/4W is a lot easier to handle and may be cheaper. 1/2W is fine but are going out of style and tend to cost more than 1/4W parts.

In actual use, the resistor and tube split the voltage. Heat is even less than 0.064 Watts. However breadboard accidents happen; anyway 1/8W-1/2W parts are all cheap and plenty big here.

FWIW-- many 300V guitar-amps with similar values use 1/2W. They often survive 30-day warranty, may live years, but fry if a tube shorts. 1W is a better choice for 100K with 300V supply. However from 300V to 80V is 1/14th as much heat, so you are nowhere near needing high-Watt parts.

[GibsonGM]

At 80V, this thing will be LOUD.


Ha, I hadn't even CONSIDERED that, Paul!  I went right to the thought of "this is a front end for an amp" rather than a boost....yeah, you'll need to attenuate as well!

Honestly, you probably aren't going to get that much more 'goodness' out of it at 80V...now, if you were going to make an entire preamp, yes.  You'd want to go even higher, ideally.

What PRR said about the resistors is true.  I am old school; I like 1/2 watt+ units in case it's hard-wired and you want to throw 300V at it later - for any tube items I always use parts rated for "normal" voltages found in "average" tube equip...esp. the caps....but at 80V you have no worries (altho I'd put 100V rated coupling caps in!). 

[Isak]

hi guys and thank you for replying.

actually its not for an guitar or guitar amp or what ever you think its for
i'm building a semi modular synthesizer and one of my modules will be an tube preamp, to overdrive the sound a bit.
i tried the vlveucaster http://www.beavisaudio.com/Projects/ValveCaster/
but it was to noisy and low output.
i remember someone said that if i give the vlvecaster a higher V i will have a lot of headroom to work with, so i was figure using this preamp to give me a nice overdrive with headroom using 80V.
i dont think i will need more then that, will i?

At 80V, this thing will be LOUD.

Perhaps strong enough to pop the amplifier input.

the sound chain will be...VCO's ---> Mixer---> Tube Preamp--->VCF--->VCA
should i be worry about the VCF input?