Critic my design

[paisley_tele]

Here is a design , well I stole the preamp from a spitfire then add the tone stack and PI along with the power amp from a stangray except made it 2xel84. I am learning on what I like so not sure on what it will sound like. I took the stangray for the cleans it has (looking for ac30 cleans). Then I took the preamp from the spit fire cause it was easy and didn't need much to get it going. Just need to figure out the power section. Wanna stick with a 5y3 and no choke. Any suggestions?

[IMG:799:597]http://i131.photobucket.com/albums/p299 ... 077a69.jpg[/img]

[Cliff Schecht]

Protip: Run some calculations.

Figure out what the current draw of each stage is and design your B+ dropping string accordingly. For example, if the power tube grid and all subsequent preamp stages are at 20mA, the B+ is around 300V and you want to drop 50V for the power stage screens, you do (300V-50V)/.02A which gives a 12.5k resistor. Power rating? Well, .02^2*12500 gives you 5W, which is the static dissipation. Usually doubling the value you get from your P=I^2*R or P=V^2/R calculation is enough. Do this for each stage of the B+ filtering, reducing the current from stage to stage as you have to power less devices. For a standard 3 tube setup like in an Express or similar, I find about 10-15mA is a good guessing point for all three tubes being on and working. After a while you get good at making educated guesses and running napkin-math to get the values quickly as you are just working with Ohm's law, not Maxwell's equations.

Edit: My apologies for the originally incorrect math.. If you're going to be a dick, don't make yourself look like one .

[paisley_tele]

So if I choose 250v for the plate for the el84s 200v for the plates on the PI and 150 to the plate on the preamp the resistor string would look like this
1.5k-3.3k-3.3k that's using .02 draw for the 84s and ..015 for the PI and preamp. Of course the filters to.

[Cliff Schecht]

I guess it's also good to mention that the B+ filtering lines are a good place to tweak an amp for various sounds. For a lower screen voltage on the output stage, the grid is easier to overdrive and you can get more power amp distortion (I love this sound personally). A lot of octal Fender designs ran relatively low screen voltages and they have a great sounding uniquely sweet distortion. Tweaking the preamp voltages will also change how much distortion you can get out of the preamp. Lower plate voltages means easier clipping and more gain (conversely, higher B+ gets you a cleaner amp). And of course the gain of each stage can be tweaked while you muck with the B+ as well, but then you start getting a bit tweaky for my tastes. It's better if you can predict how the stages will react before you build something so you don't end up with BUMS (blind urge to mod syndrome). You can do this predicting with studying past designs, hand calculations, running Pspice simulations, "just knowing" (i.e. lots of experience), by tweaking or by whatever method you choose.

[paisley_tele]

Ok so I would need high volt cause I want cleans. But when I go over the calculation for some other amps it ends with .00131818 amp draw. Not under standing that?

[Cliff Schecht]

1-3mA per triode is usually about right.