So.. with a transformer on the way, it’s time to start fooling with the calculations. The article by Fred Nachbaur gives you a lot of info, but not a full analysis.
I’m not gonna do a full analysis either, but I want to cover some of the basics, and then measure once we are built. I’ve done some back of the envelope already and I’m pretty sure I don’t know what I’m doing.
12AX7
The tube is 12AX7, a very popular pre-amp tube, so knowing how it sounds and how it used in a circuit will be very handy. This is a dual triode. It houses two triodes in one glass bottle.
Power
Each triode is rated for 1 watt of power. Power is P = VI, so we can draw that curve on the standard datasheet just to keep ourselves safe. The datasheet shows voltages from 0 to 500 and currents (in mA) from 0 to 4.0. All we need to do here is pick a number, do the math and plot the point.
I’ll start with 250 V for no reason at all and find the current that creates 1 watt of power. Please see the datasheet below
1 W = 250 V * I -> I = 1/250 A = .004 A = 4.0 mA. Isn’t that handy? Right at the top of the graph. Wow, what luck!
Lets do some more
| Volts | mA |
|---|---|
| 250 | 4.00 |
| 300 | 3.33 |
| 350 | 2.86 |
| 400 | 2.50 |
| 450 | 2.22 |
| 500 | 2.00 |
So just draw those points on the graph and connect them with lines to get a good idea of the safe ranges for this tube :
12ax7 data sheet with max power curve
Click on the sheet for a better picture, but there is no way we are gonna get close to the max power rating, it’s not even in charted territory. Recall from the real McTube article that the max voltage is 140 V. That would be over 7 mA of current and this is a stomp box meant to go in front of your amp. I don’t think we need to worry.
Next up.. where on this chart are we?
any update?
I looked for a cheap transformer for a while. It looks like you did better than me. I finally decided on the Triode PA522 which is a 120/240 pri. 330-0-330 / 12.6 sec. winding PT. I wired the primaries in series as if it were a 240 supply, but only give it 110 (USA)., so the end result is 165-0-165 / 6.3v. rectified and with a load of a single 12ax7, I think I’m around 140vdc at the plate…
I’ve been recording so a bit slow to hit this blog.
I did the first calcs. Drawings are done.. I did realize anyone was reading! I’ll wrap that up soon.
The only reason it was so low before was Fred’s insistence on keeping the PT cheap. You should get good results.
Oh yeah.. and I’m looking for a supplier for the little boxes for my “Amp Calculator”.
More soon..
How is the project coming along?
I’ve built a preamp much the same, but used two 12au7 tubes at 38 volts DC. I will be starting the real Mctube preamp soon going to run it at about 100volts DC.
Glen
Slow.. but I need the ability to do direct tube driven in to my audio interface so this weekend I’m gonna try and get the McTube rolling.