What is a good tube distortion pedal to build?

[caspercody]

I have two 12AX7 tubes, and I want to make high gain distortion pedal. Any idea's on what pedal to build? I have also made a charge pump pushing out 35 volts. What voltage would I need on the plate to get the tube to really work? I have already tried to breadboard the first two stages of a Fender amp, but it sounds farty sounding in the bass response. Not sure if it the bias of the tube, or because it is just the first two stages only (not proper filtering).

Thanks
Rob

[anchovie]

35v won't do a lot for 12AX7s with no solid-state help before them. If you want to copy a tube preamp circuit, use tube preamp voltages. For high gain, cloning a Mesa Bottle Rocket is something to consider.

[caspercody]

Thanks for your reply! What type of solid state help are you talking about? I want to try to stay away from using high voltage. That is why I am wondering what voltage the plates need to see? I know I can get 88volts out of the pump charge, but is that still enough? I just want to expermint with this, but I do not want to waste time if it can not be done.

Thanks
Rob

[tubelectron]

Hi,

I agree with anchovie : IMHO, you would be better to stay with HV (200-250V), but I may be wrong...  As I have never tested LV tube overdrive !

You can also look to the Matchless HotBox, which is a fairly simple design, but I don't know if it reaches hi-gains (since I alway planned to test it but...). Again, it's HV.

Another interesting and compact tube pedal with HV is the Westbury W-20 "The Tube", but it only uses one 12AX7. The gain range is wide and consistent, and goes to the hi-gain (like a RAT LM308N would do, at maximum distortion setting, to get a picture).





A+!

[deadastronaut]

what is the minimum voltage that a 12ax7 would run on....? just curious..

[Brymus]

Ive heard 70v to 100v is the minimum for 12ax7 to sound like a 12ax7.
Others have said they notice a big difference between 200-300v with 300v being optimum.(F2B)

[Morocotopo]

Does anyone know how that Westbury pedal performs in terms of hum? Having the transformer inside the box and that close to the cicuitry / tubes seems like a recipe for lotsa hum...

[Astronaurt]

Well there's always this:

http://www.diystompboxes.com/smfforum/index.php?topic=63479.0

I haven't even gotten half way through that monster of a thread, but it's been a great help. I've breadboarded the Valvecaster schematic with one 12AX7 tube and I'm just running it off of 12V regulated actually, and it gets a great light tube overdrive (it sounds absolutely amazing when I have an overdrive pedal in front of it already ). Not exactly the high gain that you're proly looking for, but I've actually been on a quest for high gain tube distortion for a while now too so it looks like we're in the same boat. At the moment I'm ordering parts for a couple different FET boosts to put in front of this thing to see if they'll overdrive it more. Alternatively, you could just double the schematic and have 4 12AX7 gain stages running off of whatever voltage you'd like. I think Renagadrian had something based off of the Valvecaster that he was running off of 35V called "Tube Star" or something or other. Not exactly sure where the scheme for that is... probably in the thread!

[runmikeyrun]

build the twincaster... its basically two valvecasters in series, which is a 12AU7 low voltage circuit that sounds surprisingly good.  It wont give you heavy palm muted chugs but it will give you fuzzy distorted tones out the wazoo.  You could even build 3 valvecastersin series, i'm debating on doing that.  It's 6 gain stages from 12AU7s... bound to get dirty!

[Cliff Schecht]

Anybody else notice the rectifier diodes oriented wrong in that Westbury schematic?

[PRR]

> rectifier diodes oriented wrong

Hey, yeah!

I was wondering why tubes at 266V biased to swing ~~20V peak audio, and then a pair of 0.6V clipping diodes. THD rises to maybe 0.1% and then SMACK into sand-state. Seems like 266V-9V= 257V of wasted effort.

[Mike Burgundy]

Maybe because Marshall did it? JCM900 wasn't it?
Why indeed

[tubelectron]

Hi,

Does anyone know how that Westbury pedal performs in terms of hum? Having the transformer inside the box and that close to the cicuitry / tubes seems like a recipe for lotsa hum...

Well, my Westbury W-20 is not a "hum-free" pedal, that's right : the transformer is directly AC mains powered and is not shielded. So there is some hum induced, but it stays at an acceptable level to me, considering its construction (it could be much worse). I should shield the Xfo, but as this unit is from 1978, all-original, excellent condition, and may be is one of the first commercial tentative to built a high voltage tube overdrive in a compact stompbox, I "respect" it and left it as is... And accept the hum as is...

Anybody else notice the rectifier diodes oriented wrong in that Westbury schematic?

Yes, but I think that nobody would blame Mr R.G. Keen for this obvious mistake in the PSU section, and conversely thank him for drawing a schematic which is nonetheless correct otherwise...

THD rises to maybe 0.1%

The THD of the unit rises way over 0.1% ! Unfortunately, my THD-IMD & spectrum analyser is not available these times to make a suitable measurement, but I think that when the gain is at the maximum, we probably go over 50% THD.

If we except the residual hum, the W-20 sounds something like between a LM308N RAT and a Tone-Bender MKII Professional, when all at maximum drive. It also sounds smoother, even at low to medium settings, and stay touch-sensitive. A good pedal... If you are not too much "hum-obsessed" !

A+!

[anchovie]

What type of solid state help are you talking about?

Circuits like the Tube Driver and Shaka Tube use an opamp for the first gain stage to boost the signal before a low-plate-voltage tube. The tube becomes more of a clipping/filtering stage rather than being responsible for most of the gain.