Med-high voltage Valvecaster/Tube preamp project

[momo]

Hey there,

I have this cool film projector case that I want to use for this project.
These days we all look for small footprint boxes, as you see here, I'm going the other way!
I want to build a great 12au7 preamp with its own psu.
Been reading the valvecaster thread for a while now and my head is spinning...

So really, my motivation for this is that I have already built the original valvcaster and it farts...
As I read, it would be the bias drive pot thing of the original schematic.
I heard 18v versions that sounded great.

So anyhow, has anyone seen a build of his kind that I could reference too?
Maybe with a feedback switch and tone section, input cap choice to help with pickup choices.

thanks

[GibsonGM]

Hi Momo,

There is a LOT of info about Valvecasters; there's a link to a forum topic on it that's quite long in this link, and we can use this page for reference, too: http://beavisaudio.com/projects/valvecaster/

To my way of thinking, all you need to do for 18V, for starters, is to come up with an 18V supply (could be a wall wart; I've used one, a 1A regulated type, for a GTFO preamp).  If it's unregulated, you'd need to filter it.  Then make sure your components are rated for more than 18V (caps...).  You'd need to jimmy the heater voltage (6.3V in some varieties)...I've used a big ol 10W cement dropping resistor to do that, or you can use the shown LM317 to set it for you (on a heatsink ha ha).

If you did it on BB, you can adjust cap values to taste, as well as set up a switched cap arrangement for 'fat/thin' input tone.  Sky's the limit   I think it would be a very fun project.

I love the enclosure, tho being plastic it MAY challenge you with a lack of shielding, allowing noise to be picked up by the unit.  Even if you have to shield it inside, tho, I'd try to use it!      

[momo]

Thanks for the reply.
I searched for the GTFO circuit here and could not find it.
That might be something to try.

Here is what I'm looking for which will help reduce options.
I love the warm tube sound, that presence...
I would enjoy something with some headroom, clean sounding but can crunch a bit at the end.
Something I can plug an acoustic with a Dearmound pickup where I can EQ that a bit.
18v

Tube list I have:
12au6
12au7
12av6
12ax7
50b5(that would be 50v)

thanks for any suggestions while I continue the hunt for the best tube preamp I can do with the tubes I have..

[Fat Bob]

Low voltage tubes are tough. I like to use a 6.3v transformer
(like Prem SPW-301) and only power it (with 120v) through
one of the primary windings. The other primary becomes
your HV (at ~170V). Yuo can then use the 2 6.3v windings
together for 12V at 450ma or seperate for 6.3v tubes.

[GibsonGM]

GTFO uses a 'nixie supply', a DIY switching power supply, to jack up 12VDC to something on the order of 100-200 volts, so it's not exactly the same sort of thing as a Valvecaster!      It uses 2 12AX7s, and is a really nice addition to a home studio, IMO (I use mine a lot for recording).  It's a 'real' tube preamp, just does not require an iron transformer to get a higher voltage.   One could use one, and bring up the headroom even more. 

If you haven't deal with high voltage yet, I'd suggest what you started out thinking about...a modded Valvecaster at 18V, giving you a chance to get into tube concepts.   Coupling caps, some tone shaping...the circuit has quite a lot to offer 'in a small package'    

18V will give you a good amount of headroom, as will where you set the bias.  It does sound warm.  You could always try one at 30V if you need MORE headroom Don't think you will, though.   Feeding an amp a higher signal than what it was designed for...with a preamp...is always going to cause distortion.  Some is always present and can sound VERY good (even on 'cleans'!).   With a dual triode like 12AU7, you want to pay attention to bass content (input, coupling, output caps), biasing, and the LEVEL you feed the amp coming after it.  Stock, I am sure it will sound pretty good, and then you can tweak it all you like!

What amp will you be using this in front of?

[GibsonGM]

For later, here is some homework. You will have questions..this is an old thread, with a few diff. schematics going around.  But great to start learning about higher voltage tube preamps...

https://www.diystompboxes.com/smfforum/index.php?topic=93936.120

[momo]

Ok thanks for that.
I have this torrodial  18vac. 650ma
or this 24vac, 1.25amp

So a few sub projects in this build.
A solid clean psu.
Breadboarding the circuit to acomodate bias tweaking to the actual tube that I have ,no idea if is good or not...I have a few!..., and the rest.
Shielding the plastic case.

[momo]

What amp will you be using this in front of?

I am in the process of buying a new one, I was thinking of the Fender tone master series, either the Deluxe or Princeton, although I think the Deluxe has a wider frequency response which I like.

[momo]

Whoohoo, I'm getting exited on this build.
See my pot choices, I will probably have 3 knobs, maybe 4.
The switches will be on the side plate that I will fit in there.
I will also put a glow bulb in the front where the tube will be!
I have a collection of old lamps, 6v-12v, something like that.

:-)

[momo]

I'm sorry I don't mean to flood the forum with this thread, but  I found the transfo.
My goal is to also use the stuff I have...!
Here is a 13.5v/1amp filtered psu.
So I will forget the 18v design, will go 12v.
It will be easier for the heaters too..
I plan on adding a booster in there too instead of having two 12au7 tubes.

[GibsonGM]

I think you will be happy with 12V, and you could always increase the voltage later if you use caps that are rated higher (maybe 30V units).   Having some kind of boost built-in before the valvecaster that you can switch in/out would be a great addition!   

I suggest that you build it as designed, and then add the boost to the input (perhaps a mosfet boost, or something using a JFET).  AMZ, linked at the top of the page, has some things you can look at!  You can 'try out a boost' by inserting a Rangemaster, Tube Screamer, etc, before your breadboarded Valvy, tho they may not sound the way you want...they will give you an idea of what you can do. 

I keep a few tube sockets around that have solid leads wired to them so they will work with a breadboard  

[mozz]

Transformers look too big. I would find a 12vac, 150-200ma, use that for the heaters, rectify and use for the plate, when using a 12au7. 2 triodes will be enough gain.

[momo]

Transformers look too big. I would find a 12vac, 150-200ma, use that for the heaters, rectify and use for the plate, when using a 12au7. 2 triodes will be enough gain.

Thanks, are you saying that because of limited space in the enclosure and some chance the transfo would induce hum in the audio signal?
I have a 18vac torrodial 5VA, the 62021 : https://us.rs-online.com/m/d/aa12558a3e36db2444dd2f5d4cbc3d85.pdf
I figured out that I can get 278ma with that transfo.
150ma max on the 12au7, I have 128ma left for a AMZ booster....

So this would work with not much headroom..

[momo]

OK thanks mozz for the suggestion, I think this torrodial will work, nice and small too.

Now I took the filter, rectifier circuit off the other transfo.
This one had what seems like a center tap, see picture.
This tap is connected to the fuse which goes to positive out DC.
Now I don't have a center tap on the torrodial, so where do I connect the center tap pad on the3 circuit?

[GibsonGM]

I wouldn't do it that way, Momo...    I'd just find a 12V wall wart that is ready to go and use it...and run the heaters on DC.  Mozz is just letting you know that the trafo's you're showing are very large for this project - enough to power much more than the 1 tube.     You need 150mA for the heater, and only say 5mA per triode.    1 12AU7 has 2 triode 'sections' inside it, as it is a dual triode.

If using 'junk box' transformers, what I do is look at the current rating - mozz is suggesting 12V/200mA or so, I agree completely.   If it is a DC transformer, measure its output unloaded and tell us here what that is.   You would then likely need a dropping resistor to dial in the voltage for the heaters.  An unregulated transformer will output higher voltage if not run at the rated current (and you won't be drawing 200mA!).

2nd way, find a 15V/200mA wall wart, and use a 12V regulator to obtain heater voltage.   We want to keep it simple if we can   Easier to find the right transformer first.  Getting the heaters set up is the hardest part of tube work, IMO.

[momo]

I have experimented with my torrodial , the 18v, 5VA.
I used my existing valvecaster for the tests.
I used a 90 ohm 5w resistor on the 18v AC feed and brought down the voltage to 12v.

So here is the plan for the power section,
I would send my 18v to some filtering circuit, I would love to use the one shown that comes from a wall wart. Just wondering how to plug the center tap part on that circuit as my transfo has none.
Then would follow a 12 v regulating circuit.

I will be putting an AMZ mini boost with the J201's in front, I would like to use it on 12v as well.
Tapping off the regulated circuit.
Finally for now....hehe
My enclosure is big enough for me to separate the psu and the audio.
Psu on the back part, point to point items for the VC circuit in the front part, copper sheilding inside, sheilded cabling......
Should be Awesome!

[momo]

Here the layout so far.
Trying to minimise hum.
I could put the filter and regulator part on that brown pad.
Front Pad for AMZ mini booster circuit.
I put that golden plate on which I will have the ac prong as well as a few switches.
I can go 2 prong ac on this , I would like to use a miniature iec connector, the unit will be copper taped.

:-)

[GibsonGM]

I think it's probably more than you need to do, but you CAN make it work that way!  

My way of thinking would be to try to avoid a line power connection to the unit, if possible...by using a plug-in wall wart, with a power jack. Then it's only 18V going to the enclosure, which is much safer.  A regulator can be used internally...perhaps you are already thinking along this line, disregard if you are...

[momo]

I'm starting to get younger with this project..!...lol
Things are always evolving.
So the psu ,filter, regulator will all be on the same board, the back one.
I will use a detachable 90 degree -2 prong mini connector for ac . This connector will be on the golden side plate.
The AMZ mini booster on the front board will be modded with the 1k tone pot thing like on a fuzzface.

[GibsonGM]

Is there any chance I can get you to evolve it to bring 18V to the box from a wall wart?     

It's SUCH a simple pedal, it really doesn't need the same full PS that a 3 or 4 tube preamp would! 

And the rectifying/basic filtering would remain FAR away, and so would offer much less potential for noise or electrocution - and you'd have more room in the enclosure...just a suggestion.