New Pedal, sort of. EH Transformer and tubes used.

[william]

I've been working on building a distortion pedal using the EH transformer and a pair of 12AX7's.  The design is based off the lead channel of a Soldano Super Lead Overdrive amplifier.  I'm planning to build this in the near future.  My question though, is should I be concerned about providing the Jfet better filtering to prevent noise?

The schematic can be seen here.  
http://209.240.71.22/circuit/rectpedal/rectpedal_schem.gif

Thank you.

[spongebob]

Nice work!  

My question though, is should I be concerned about providing the Jfet better filtering to prevent noise?

The FET has almost no power supply ripple rejection, filtering would be a good idea! If you don't want to use a regulator, you should at least add a RC filter (or more than one) after the rectifier diode to cut down the hum.

[Peter Snowberg]

Very cool!

Without question that JFET stage would give you some hum.

I'll second what spongebob said.

The typical tube amp approach is to use several RC stages in series. As you only need a small amount of power in this case you could easily use a TO-92 cased 9V regulator like a 78L09 which will give you MUCH cleaner power with only a fraction of the capacitor requirement of the RC stages.

You could also use a zener diode based regulator here, but I prefer the 78L09 approach generally (less waste current).

[william]

I've updated the schematic with the changes suggested.  I've added a 9v regulator.  Anyone see anything that would be a show stopper?

[Peter Snowberg]

That looks great!

Build away!

[toneman]

Don't think the biasing is correct on V2 pin 7.
Grid is coming directly from plate of previous stage.
Puretube??
afn
tone

[puretube]

it`s OK;
(but a little more filtering against hum @ V2 tomorrow....)

[william]

I look forward to hearing your advice on how to better filter the unit.  THanks for the input so far.

[puretube]

Don't think the biasing is correct on V2 pin 7.
Grid is coming directly from plate of previous stage.
Puretube??
afn
tone

http://www.tubecad.com/october99/page3.html

http://www1.korksoft.com/~schem/marshallamps/jcm_900_preamp.pdf

http://www1.korksoft.com/~schem/fenderamps/bassman_5e6a_schem.pdf

the grid of a cathode-follower already "sits" on a high voltage -
common examples in above linx....

I`d suggest:

1.) add a "T"-filter between the HV-rectifier and C18:
4k7series - 22µ/350Vshunt - 4k7series;
2.) Move R16 left of R9;
3.) Insert 4k7 between R14 and C17;

[changes]

that's a great idea william but i'm just curious why did you use a jfet instead of a tube for the first stage?

[william]

The preamp design is influenced by the slo100/dual rectifier hi-gain amps, and those use 5 gain stages.  I didn't want the cost, added current, or waist of half a 12AX7 tube.  So I'm taking a small leap in assuming the first stage of the amp provides a gain boost for the following stages.  As such, the first stage shouldn't add any clipping, but boosts the signal to clip the following stages.  My thought is, if I don't need a tube for that "tube" sound, and it will save time, money, and space don't use a tube.  Now the design fits nicely into two 12AX7 tubes and fits on a 4x6 inch board.

[elberto]

hey william, just wondering if there was any update with you build!  I'm particularly interested in the idea of mixing a jfet stage with tubes, how's it sound?

[lovekraft0]

Don't you have the filaments wired for 6 volts? I usually wire them that way whenI use a 6.3 filament supply - don't you have to wire from 4 to 5 and leave 9 floating to use 12 volts?

[william]

I haven't begun building it yet.  I'm hoping to order the parts in the next week or so.  I have a PCB designed and for the inital build I'll skip the tonestack.

Yea.  The Filiment symbol I used doesn't have the layout for the 12v version.

[changes]

boy i wanna hear samples as soon as you build it.......

[aron]

William,

What are you going to use for the enclosure?

I have to start building mine too.

[william]

Always you guys with the hard questions.  I've been thinking about that alot recently.  The PCB at this point is about 4"x6" so I'm thinking the enclosure needs to be about 8x5 or so.  Its going to be a big pedal.  Most of the size is coming from the filter caps.  I haven't decided on the enclosure yet.  If the pedal works out, I want something cool though, thats all I know for sure.

[puretube]

:?:



:?:

[william]

The thing is I don't know how well the pcb I have designed would fit into that enclosure.  I don't know where the tube holes are located, or where the mounting posts would need to be.  I'm providing a picture of the pcb so far.  

[puretube]

can`t you find vertically mounted caps? saves space immensely! (cheaper,too...).