High voltage high gain tube distortion pedal

[BubbaMc]

I'm considering designing a high gain tube pre-amp and housing it in a stompbox type of enclosure. 

Has anyone done this or are you aware of any commercial designs?  I'm thinking something along the lines of a Mesa V Twin but with a higher B+ voltage (around 600VDC).  The Mesa Rectifier or Soldano SLO type of sound is what I'll be aiming for, initially anyway.

At this stage I'm thinking of using a voltage trippler and rectifying it for the required DC voltage, although this may only work for those with 240V wall sockets.  The other option would be a switching mode power supply.

I'd like to know what is the best way of attenuating a signal.  At the end of the preamp stage I anticipate quite a high AC voltage which will need to be brought down to instrument level or line level.  Would a voltage divider suffice?  Or is there a more elegant solution?

Any ideas or comments?

Cheers.

[anchovie]

The Mesa Bottle Rocket doesn't have solid-state parts like the V-Twin does. Schematics can be found through your favourite search engine.

All the commercial tube pedals I've seen use a B+ of 200-300V. I personally can't see the need for 600V at all, the preamp tubes of Mesa amps don't get that much to the plate resistors.

[Ripthorn]

There are several commercial pedals out there.  There is the Mesa Bottlerocket, the Blackstar pedals, and a number of others.  A very typical approach is to take a high gain preamp from a high gain amplifier and stick it in a stompbox form factor.  One thing to be aware of, a 12ax7 (highest gain dual triode) will get totally and completely fried at 600VDC.  Their typical operating voltage is about 300V and can handle up to about 400V as a max.  Also remember that if you use a doubling or tripling circuit, you will be sucking a lot of power.  Also, if you want higher gain, I would go for a somewhat lower B+ because a higher B+ gives more clean headroom.  The lower B+ would give more distortion at lower AC signal levels while consuming less power and requiring less expensive parts.

[BubbaMc]

Great info, cheers.  600VDC was just a guess.  What are the B+ voltages for a Mesa Recto preamp?  They'd be higher than the bottle rocket I'm sure.

A very typical approach is to take a high gain preamp from a high gain amplifier and stick it in a stompbox form factor.  

This is exactly what I'd like to do.  Start with a known pre-amp design, get it working - and then design my own.  Attenuating the signal is something I'm not so sure of though.

edit:  I just had a look a the Blackstar HT-DISTX.  That's the kind of thing I'm shooting for, but with more gain stages.  I think this idea would work nicely, seeing as a the average high gain tube amp generates it's tone in the preamp, and has a relatively clean power section.

[anchovie]

The Recto has multiple B+ supplies around to 400V mark, the Bottle Rocket has two (230V and 170-ish V). Look for schematics using your favourite search engine and you'll find the voltages marked on there.

IIRC, the Blackstar only has one tube in it - the rest of the work is done by op-amps.

When you find the Bottle Rocket schematic, you'll see that there are some passive filters last in the signal chain that use some high-value resistors to make them also act as attenuators.

Is the plan to make a tube pedal to run into a guitar amp, or will this be going direct to a power amp?

[BubbaMc]

I envision a pedal that can go into the front end of a guitar amp, a power amp, or a mixer.  I'll probably start with just the guitar amp option for now.

Thanks for the tip on the attenuation circuit in the Bottle Rocket, just the sort of thing I was looking for.

[bancika]

take a look at soldano x88r preamp schematic. It's similar to SLO which was pretty much cloned by mesa in dual recto.
Also, I'll post schem and layout for my lite version which is a single channel and no tone-stack version of x88r. Stay tuned, it may be useful

[GibsonGM]

I'm working on something like this...using (2)  120V:12V transformers set up like in the McTube.  Then a voltage doubler so at the output I am getting roughly 300V.  Tapping of the  junction of the 2 trannies to get 12V for the heaters. 
At the output, to attenuate, you could use a simple R voltage divider and a bypass cap to maintain highs....

[bancika]

You can also try to divide the plate resistor and use like 90k+10k instead of 100k (90k is closer to the plate) and the tap off the coupling cap between them. That will give you  a tenth of the output while still loading triode the same.

[brett]

Hi
The McTube power supply type is by far the best option for this type of thing.

I'm working on something like this...using (2)  120V:12V transformers set up like in the McTube.  Then a voltage doubler so at the output I am getting roughly 300V.

Voltage doublers are easily avoidable.  They cost significant $$ and are somewhat inefficient.  For a B+ of 300V, simply use a 110V:12V and 6V:110V transformers.  Rectified, you will should get between 280 and 310 V DC.  For a pre-amp, you'll only need 10 uF of post-rectifier filtering (2 x 4.7uF).  Also, put some in-line resistance before and between the filter caps (about 1k should do).

And BE AWARE OF ALL OF THE SAFETY ISSUES OF HIGH VOLTAGES.

cheers

[BubbaMc]

Yeah it looks good Brett but it won't be able to supply over 350VDC which is what I'm aiming for.  I think I'll look into designing a SMPS to keep the efficiency up and size down.

Has anyone built one of these?  http://www.musikding.de/product_info.php/info/p1347_High-Gain-Tube-Preamp-Kit.html

I wonder just how small one of these could be made (even just one channel).  Using a SMPS supply and surface mount components I reckon this could be made to be very small indeed - with an attenuator at the end of the signal chain this sort of circuit would be the ultimate in stompbox distortion pedals imho.

Here's the schematic:  http://www.andyszeugs.de/projekte/slo_preamp/pdf/Schaltplan_SLO.pdf

[GibsonGM]

Actually, the doubler I made requires a couple of Si rectifier diodes and a few caps...it can handle some mA, which for preamp use is quite sufficient...for the experimenter who doesn't want to buy transformers!  But if you should have the 12V and 6V trafo's around, by all means....
If I was looking for 300V+, I'd go ahead and just buy one, Bubba...you can use a bigger than avg. stompbox, it's ok )

[Ripthorn]

The big thing to be careful of when using a back to back transformer setup like brett mentioned is that you have to make sure you have current ratings that are high enough on the primaries and secondaries of both transformers .  Otherwise, you could hit core saturation and kill a lot of things, like youreself.

There are classic AC doubling circuits out there that are cheap and easy, just make sure all components are rated high enough.  I have a list of probably 4 or so preamps I'm going to either clone like this or adapt to submini tubes.  Should bed fun.

[bancika]

that preamp from musikding is a slone of soldano x88r I was talking about. It looks like a decent price for the kit.

[BubbaMc]

that preamp from musikding is a slone of soldano x88r I was talking about. It looks like a decent price for the kit.

It's an excellent price for what it is.

I think I'll order one.

[GibsonGM]

Not a bad price at all!

[MikeH]

I JUST finished doing this.  I built a matchless hotbox clone.  Sounds pretty good- although I'm not a fan of the tone stack, I think I'm going to convert it to a marshall style stack.  But the hot box sounds good.  There's also the Soldano Supercharger GTO- schematics are floating around for that too.

[Caferacernoc]

You can also try to divide the plate resistor and use like 90k+10k instead of 100k (90k is closer to the plate) and the tap off the coupling cap between them. That will give you  a tenth of the output while still loading triode the same.

How is this different than having a voltage divider after the output cap? Is it a better way to control output between stages when cascading? I'm thinking of a scenario where you want soft clipping in multiple stages and you hate to just dump the volume between stages with voltage dividers because it gets noisy.

[bancika]

I JUST finished doing this.  I built a matchless hotbox clone.  Sounds pretty good- although I'm not a fan of the tone stack, I think I'm going to convert it to a marshall style stack.  But the hot box sounds good.  There's also the Soldano Supercharger GTO- schematics are floating around for that too.

I replaced stock tone stack with TMB and it's better, go for it. There are couple of mods on my site IIRc, you may want to check them out...

[BubbaMc]

I just ordered the preamp kit from musikding, that should keep me busy over the christmas break.

I'll start a build thread as soon as I get it.

Anyone know of any good speaker simulator circuits?