guitar pentode overdrive preamp

[neilium]

Hi,

I just finished a X-mas gift for a bandmate. It was almost all junkbox parts, but it came out sounding good. It's a guitar overdrive with 2 stages of 6ak5 (EF95) followed by a 12au7 CF. The schematic is is a pile of hand-generated slop,but if you want to download it (168K):

http://www.newcontrol.com/ndj/pentodepre.jpg

Component choices were driven by what was available. The 12VAC filaments don't make a lot of noise, to my surprise. I assumed I'd be putting in a DC filament supply once I had everything else working, but for now I'm going to leave it.

This thing sounds real crunchy with strong bass and is great for chords. It gets pretty distorted, but it doesn't fart out when you hit a diminished chord. One thing it doesn't do is singing leads, but there are already a lot of boxes that do that all to well.

In my 6ak5 shoebox I found a couple of Mullards, and they sound great. Other 6ak5s I tried fair pretty well, too.

Here's some pics.

Now, do I really want to give this away?

-n

 

[NewYorkDave]

Properly wired AC filaments can be very quiet, at least with indirectly-heated tubes (separate heater and cathode). People assume that you need DC for low noise, but low-noise recording gear with AC filaments was built for many years.

 

[Gus]

The heaters just float and you don't have Hum? Looks like a fun circuit . How does it sound compaired to something like the variadrive tube box?

 

[NewYorkDave]

Hey, good point! I failed to notice that. Yeah, I'm surprised if the preamp is quiet with a floating heater supply.

Neil, just for laughs, take a cliplead and ground pin 9 on the 12AU7. If anything, it will get quieter still.

 

[neilium]

Error of omission on my part: There are two 100 ohm resistors from pins 4 and 5 of the 12au7 to ground. Sorry about that. I'm sure that's not all that's missing in the schematic.

-neil