Pencil Pusher Build Report

[Gladmarr]

I got the bug to make something with little tubes a few weeks back, so I acquired some 6021 tubes ($2.70 each  ) and went to work on our own mlabbee's Pencil Pusher - just the preamp version. 


Other than the tubes, I found all the parts in my stash of odds n ends, so the overall cost of this one was $2.70.  I didn't enclose it, so right now it's an electrocution hazard waiting to happen, but it works for experimentation sake.  The schematic is here.  I had to substitute a few parts.  I took apart two 9vDC wall warts to get the power supply.  I used the transformer out of one to get the B+ supply and ended up with only 150VDC in the end, so I may make a voltage doubler to get a higher plate supply.  I used the rectifier and filter caps out of one of the wall warts to get about 15vDC through a 5v regulator with a voltage divider on the ground tab to get between 6.1 and 6.5 vdc for the filament supply.  Here's a clip from the datasheet for the regulator that shows how to get a higher output from a LM78XX regulator.

I could only find 22uF 450v capacitors for the B+ power supply, so that may have let a bit of noise escape the power supply.

It sounds pretty good as a clean boost as is.  Admittedly, I have the wrong B+ supply at the moment, so once I get that resolved it'll probably sound a little more impressive, but I'm not much of a balls out distortion guy anyway, so that aspect doesn't bother me much.  I do have a bit of noise in the circuit, and I think that's because I didn't have any shielded signal wire, and the circuit isn't encased in metal on all sides.  I'll post more as I get further with it and make it sound nicer.  I might even find a way to make decent soundclips to give everyone an idea what it sounds like.  Thanks for the schematic, mlabbee.  I don't know if he'll see this as it looks like he hasn't posted here in about a year.

Oh yeah, I must be getting better at watching what I'm doing while I build stuff, it worked right away when I plugged it in!   

[spudulike]

Seen quite a few schems Id like to try with those tubes, anyone know of a reasonably priced supplier ?

Nice build btw, and thanks for the report.

[Gladmarr]

I got mine from Antique Electronic Supply www.tubesandmore.com  They were $2.70 each.  That's by far the cheapest I've found.  I'm going to shoot for getting my (re-)build into a hammond box so it's more like a pedal than a hunk of bent metal with some junk on it.

[spudulike]

Thanks

BTW you can also connect a 1N4007 across pins 1 & 3 of the regulator, cathode to pin 1, for better output.

[Gladmarr]

Hey all, for what it's worth, I finally got to revisit this thing.  I tried to make the plate supply using a voltage doubler, but I still only ended up with 150VDC in the end.  I did clean up my filament supply, because I was using very low value resistors to get the 5v regulator to give me a regulated 6.3v.  I just went to higher value resistors so I wouldn't be burning a bunch of current just to set up my voltage.  I have to say I'm not amazed with the circuit at this point.  It can sound nice as a clean boost, but it doesn't get very dirty, and there's not much boost to be had.  So far, I have some nasty noise coming into the signal chain presumably from the power supply still.  I'm going to have to look into that.  After that, I'm going to mess with the gain of each stage to see what more I can squeeze out of this thing.  The EQ works quite well, but it should since it's a baxandall tonestack and I like those best.  I'll add more as I get somewhere with this thing.