Tube Overdrive Running on +9v (but not really)

[trad3mark]

Recently, I made my first tube overdrive, running on 9V, but not really. It worked really well, so after some argey bargey, which is over and done with, i thought i'd share the process i went through, in case anyone else wanted to have a go!

Allow me to explain.

Courtesy of frequencycentral/rick, I used a nixie power supply to get +125V out of the 9V. As many of you will know, that's a pretty nice voltage to have for tubes. The power supply can go higher, I think rick got 199 out of it before. But, i limited mine to 125, cos the Inductor couldn't handle anything higher. I must give credit to rick for this, so if he's ok with it, i might let him share the nice schems and perf layouts he did for it?

So the other part of the power supply was +6.4V. There's a few ways of doing this. Initially, i started off with a 5V reg, and 2 x 1N4148's between it's ground and the actual ground to bring it up to 6.4V. Unfortunately, this got REALLY hot, so, with some help, i put 3 x 1N4004 between the +9v input and the 9th pin of the tube, which did the trick too. Not the best solution, i know, but it worked REALLY well!

So, I now had the required voltages i needed: +125V, and +6.4(ish)V. Now i needed the gain section. I was working with an old Philips 12AX7 i pulled from an old turntable. I think the date on it was about 1963? Anyway! Before i started building anything, i had a look at some of my favourite amps, or at least the schematics i could get hold of. There's actually a lot of similarities between brands, which makes for some interesting research. The amps i looked at were:
Vox AC30 (the topboost section)
Vox AC100 (some slight, but interesting differences)
Fender Deluxe Reverb
Orange 120
Badcat Hotcat 30
Mesa MK IV
Silvertone 1263
Silvertone 1485 (Cos it's Jack White's afaik!)
Soldano SLO 100 <<-------- Very important, i'll get to this later!

Right, so, at this stage, it helps to understand a standard cathode gain stage. I read two things. It's pretty good to read them in this order too:
http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf
http://www.aikenamps.com/ (click tech info > advanced > designing cathode something)

There's some other interesting things, particularly about the AC30, but we could be here all day giving links lol!

So, Once you've an idea of what makes up a gain stage, i'd suggest starting SUPER simply. Use standard values as recommended by Merlin's Chapter (the first link). At this stage, you can start to take ideas from other amps, to try to get a similar sound. For example, the Soldano SLO 100 uses a 39k resistor at the cathode in the 3rd gain stage. I tested out putting in a 15k resistor with a 25k pot, and it gave serious clipping, very crunchy! Similarly, when i was going for something smoother, i put a 4.7k pot, with a 22uF electrolyte at the cathode, and got a fairly blusey sound.

Anyway, what's good to keep in mind is that the tube will provide you with two gain stages, so you can really experiment. In my own build, i put low-ish gain in the first stage, influenced by the silvertone, and REALLY high gain in the second (even higher than the soldano), so, i use the first stage for clean boost, and the second for super angry clipping. The best part is, it's a really nice tubey sound. I've a marshall jackhammer too. Now i know that's a pretty mediocre OD anyway, but compared to tube sound, it's got nothing.

My next plan is to basically take a number of gain stages from lots of amps, and run two tubes to get a very versatile OD, sort of like a fancy tube preamp.

Im not sure if all this was any help to people, but it'd be nice to share these things and see if anyone else has done anything similar.

[MichelP]

Well, this was sure an interesting read for me.  Thanks a lot !

[ddr]

Thanks for posting this, it is an interesting read.  Some days I am looking for "paint by numbers" instructions for building something without knowing what I am doing, and some days (like today) I am more interested in learning something.

Have a good one,
Dan