Valve Bass Preamp

[Blues_King]

Ok, so this isn't really a stompbox as such, but it was the only place i could think of where i might get good responses!

I'm interested in making a valve bass preamp, based on the Hiwatt bass preamp they use in the custom 200 and 400 heads, they have schematics for these on schematicheaven.com. I'm relatively inexperienced when it comes to valve electronics, i've worked a bit on my organ but not much else. But i know how the basic preamp stage is constructed and wondered how I would go about getting just the preamp made.

So, what i really need to know is:

1. What i would have to do with the power supply to be able to run this at the required voltage (for a 12AX7 is it something like 200?)

2. How, and if i would need to do anything to the output to make it compatible with most power amps.

3. I want just your basic volume, treble, bass, master volume kind of setup (normal/brilliant volume would also be cool), how would i achieve this, how many triodes would i need?

http://schematicheaven.com/newamps/hiwatt_400w_dr405.pdf

I wondered if i could just 'pull' that apart, and use the first half (up to the arrow saying 'master volume') would that be possible?

any help would be gratefully recieved

Thanks

-James

[Ripthorn]

It looks like you should be able to just take the schem up to the master volume point, but I am don't have much tube experience.  For a typical 12AX7 I think they run about 340V.  You would need a transformer and rectifier to get that.  You also need to be aware that you will need to supply the heaters.  For a TMB tonestack, you don't need any triodes to run it, you just put them in there.  Take a look at lots of schematics and you will see the basic structure of a tonestack and then you could actually make your own (you can also use the duncan tonestack calculator).  Hope that helps, though I get the feeling more people will be coming along to correct me.  I actually think I might make a submini version of the Alembic F-2B (I already have made a version using JFET's).

[theehman]

1) I would just take the transformer out of a 220v/12VAC Ac adapter and wire it backwards.  Operate the unit from a 12VAC adapter, like EH does w/ its tube pedals.
2) I think you can take the signal straight out.  I'd put a DC blocking cap on the output, though.  Check out the output of this Alembic F2-B clone: http://moosapotamus.net/IDEAS/F2B/alembic.htm
3) You would need 2 12AX7 for a single-channel preamp.  You'd have 1/2 a tube left over so you might want to wire it as a boost or something.

[Blues_King]

Thanks Guys, that helps alot

Will defo. try the 'reverse transformer' idea, i always wondered how they ran the valves in their pedals!

The tonestack idea would be good aswell, i might look into that.

If i'm meant to run these at 340V for the optimum, what would happen if i ran them at 220? Ideally, i want this to pretty clean, only breaking up if you REALLY push it.

The output idea also makes sense now, dunno why i didn't think of that one lol

I'd need rectification if i was to do this, SS rec seems to work better for bass, faster response and so forth, but could i somehow make that switchable with a valve rec, just for the option?

Cheers

James

[Ripthorn]

Running at a lower voltage is called plate starving and causes clean headroom to go away pretty quickly.  For max clean headroom, you want to run the voltage as high as you can without overheating or exceeding specs.

[MarcoMike]

it seems to me only one 12ax7 is required for each channel... and only 2 capacitors differ in those channels... you can "easilly" make it with just one tube and eventually a switch if you need both channels' tone.

Edit: I don't really get how V3 works... but for a proper tube tone you may need more than just one tube in the chain... so maybe you should keep v3, and place a master volume at the very end...

I guess you won't notice any difference from SS to tube rectifier, having just the preamp stage...as its contribution is probably more on the power tubes, or in any case related to them, as in correspondence of power requirement peaks the tube rect shows its limits, resulting in some kind of compression of the output signal... but this is just how I imagine it... I have no experience about it...

[Nordskov]

I have made the Alembic F2-B clone as described at moosapotamus.net, but in mono with only one valve.

The capacitor sticking up is for troubleshooting 

You can see the two back-to-back toroidal trannies delivering ~280V rectified, and the low voltage DC circuit providing 6,3V for the heaters.
Both are 15VA.

Don't ask me how it sounds as I ran into serious problems with feedback in the tonestack whitch I haven't sorted out - yet.
Maybe a thread coming up soon 


Henrik Nordskov

[runmikeyrun]

I've always wanted a 405, i'd give a nut to have one (and the other nut for a marshall major  )

The higher your preamp voltage the cleaner your preamp will run, all other things being equal.  If you run it at lower voltages you could drop the value of the plate resistors to give you more headroom, at the expense of some gain.  Then again, with 4 stages you should have enough to compensate.  Hiwatts were built to run clean and loud so that is a good choice of preamp to follow.

This is a fairly complicated preamp to build.  From my experience in diving into too big a projects when i had less experience i found that my assembly and parts layout was sketchy, soldering even more so, and mistakes in building prevalent.  This all adds up to a good possibility that the "damn thing won't work" when you fire it up for the first time and then you've got a debug nightmare- did i put a part in the wrong place?  solder joint bad?  hum or oscillation from where i put parts too close together?  I got a lot of my tube experience from modifying other amps that worked when i started on them, changing one part at a time so i knew where my mistake was when i made it.

For a decent clean and easy one tube preamp check out the thread on the valvecaster- it's labelled "tube preamp running off 9v battery".  The single tube version runs pretty clean especially if you drop the plate resistors to 100k, run a higher supply wall wart (mine runs off 20v) and use a slight pad on the input jack.  There's no tone controls but it's a good start on building valve stuff.  Also no high voltages to shock you with!

That being said, don't let me discourage you, that's a rad project and definitely needs to be built.  It's just complicated, especially having the build the power supply on top of the preamp itself.

Good luck and let us know if you need help.

[Caferacernoc]

Simple one tube project would be the Alembic F2-B. I'd use the schematic from the Geofex site.

www.geofex.com

Cheapest way to power it would be the back to back 12v transformers.
Like the McTube.

http://www.dogstar.dantimax.dk/tubestuf/mchowit.htm

[Blues_King]

Cheers guys

I looked at a few datasheets for 12AX7's and most of them say that no more than 300V is the best way to go, so if i tried to get say 290 ish? I reckon the back to back torodials is a good idea, i might try that

I'd like to be able to blend the two channels, like they do on the hiwatts, so if i amalgamated the inputs and led them to each stage?

Also, on the schem there are arrows coming off the cathodes of the first 2 triodes that say 'C1' (or something, maybe i'm misreading ) what exactly does that mean? sorry if thats a noobish question

I reckon I will try this, i'm pretty experienced with soldering and the like, and i've worked on a few valve things, but i've never had to understand them properly or build one from scratch.

[slacker]

I think it means you just join the 2 points marked C1 together, so the 2 cathodes share the same resistor and cap to ground.

[Nordskov]

One advise...
Be VERY, VERY careful with the high voltage supply. 300V can be lethal!

You can use Duncans Tonestack Calculator to get an idea of the respons of the different stacks:
http://www.duncanamps.com/tsc/download.html
Though it doesn't have a tonestack like the one in the HiWatt 


Henrik