Saturday, June 23, 2012

Brute Force of Jekyll & Hyde



Guts of the Pre+Amp, TB-PPA1:

Here are the current audio circuits I am using in the Akai Pre+Amp and Ampex 601/602. First up is the Brute Force preamp circuit with optional XLR input and XLR dual output circuits. I use this circuit in the Pre+Amp and 2xPre units as well as in slightly tweaked form in the Ampex 601/602 Tape Machine Amps. Like the 2xPre Akai/Roberts monoblock mod, I can fit two channels in each 601 or 602 unit, the only difference between the two is the tube rectified/AC heaters in the 601 and solid state rectified/DC heaters in the 602.


If it is not clear, the first preamp stage's coupling cap connects to the switch of the 1/4" line in jack. V1 is a 6267/EF86 or you could use a 5879 which is basically the same tube with different pinout. The line amp is a 12AU7 type and you can use anything compatible you want here including 5963, my favorite. Also, the plate resistor on the top triode of the white cathode follower is 220R or 220 ohms. There are two outputs, one is transformerless at about 3k and impedance balanced while the other is 600 ohms transformer balanced. They can be used simultaneously. 


And then here is the Jekyll Hyde Amp. It has two distinct modes that give it two very different sounds. One tame and one wild, but both rich with tone. Some stuff might not be so easy to read. The cathode on the second 12AX7 stage has 27k feedback from the output transformer. At the cathode there is a 3k bias resistor and in parallel with a 47k resistor in series with a 25uf 50v cap. The mode switch effectively shorts the 47k resistor and erases the negative feedback. So: switch open is cleaner with NFB. Switch closed is wide open and driven with no feedback.

1 comment:

  1. If you plan to build my Brute Force circuit (the one that there are two of in the 2XP1) then as long as you are an experienced tube builder, it should be very easy. What you want to do first is strip down your Akai monoblock to its bare chassis. You will wind up reusing the metal chassis, tube sockets, tag strip panel, power transformer, some of the tubes, and choke. Nearly everything else will be new parts. I like to get most of my parts at either tubedepot.com, mouser.com or tedweber.com The power supply electrolytic caps that Weber sells (the small-body 20uf, 450v and 40uf, 450v) are the perfect size and while you are there you can pick up the cathode bypass caps, power cord, switches, and solid-core wire as well. Over at tubedepot, I like to buy my knobs, pots, mogami shielded cabling, and solder. Then at mouser I get the Switchcraft L12A and L12B jacks, all the resistors (I like the Dale CCF60 series for 1/2 to 1 watt duty, Xicon for the 5W stuff),and coupling caps (I like the new Mallory 150 series in 10% tolerance). Oh yeah, and finally I like to go to hoffmanamps.com for the chassis-isolation black "shoulder washers". Also, check out my blog post here for a schematic and more info. Let me know if you have any other questions. http://xfmr.blogspot.com/2012/06/brute-force-of-jekyll-hyde.html

    Whoops, almost forgot. For input transformers I like the ones in the early Shure M68 and M67 mixers (NOT the M68A or M67A!!). If you get the M68FC or M67FC you are alright (FC stands for female connector which is the mdoern standard for input XLR) and even better, you get high quality Switchcraft inputs that usually just a need a little cleanup with DeOxit. For output transformers (which are not necessary, but nice to have) I like the Edcor WSM10k/600. OF course those are both for budget builds, if you want to get fancier than that, the sky int he limit. The input can be anything from a 1:10 to a 1:15 and the output can be anything from a 4:1 to a 1:1

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