Tuesday, April 9, 2013

Magnecord PT6-J Made into Preamp plus Vari-Mu Tube Compressor / Limiter "Pre+Limiter"

Tablebeast Magnecord PT6-J TB-PPL1 Pre+Limiter:

Whoohoo! I finally got this one finished last week and it turned out marvelous! The new owner is tickled with its amazing performance. I remember when he first asked me (based on this blog) if the new circuits I installed would be better and/or more useful than the stock Magnecord PT6-J with a simple restoration. I laughed a bit and told him that OH YEAH it will be better! Now that he has it back in his hands, he knows what I am talking about.

So, I start out with a simple preamp circuit using a single 12AV7. It is a rather popular DIY design made popular at "The Lab" and originally designed by a fella named NYDave. I have tweaked his circuit out a bit but it is still the same basic idea. One of my changes is that I am using the 18v volt tap that originally was used to power the 5879 and 12SJ7 with DC heaters. I reconfigure it to have two filter stages and knock the voltage down to 12 volts to run the single 12AV7. With the amazing triad input transformer that was already on-board I change its configuration a bit by pulling the primary center tap from ground, making it a floating input. Then I add a 1/4" DI input, a -20dB pad, and a phase reversal switch. The unbalanced, transformerless output of this circuit goes to the tag strip on the rear of the unit.

Then to this lovely preamp circuit I add a Vari-Mu Compressor / Limiter that started out as a Federal AM-864/U circuit. I performed quite a few mods to this circuit and thought it really isn't much of a Federal any more, it still retains the special flair that the Fed has, just with a LOT more adjustable parameters. I moved the sidechain from the first stage to the second, giving it a lot more control over threshold. I also added attack and release controls as well as a compression and hold switch. The compression switch turns the mu-line on or off so that it an be used straight up as a line amp or as a compressor. The hold switch is very cool. It essentially holds the compression at its highest amount by removing the release function entirely. This can be cool for keeping stuff that is crushed, fully crushed full-time. I have also added pin jacks and locking trim pots to match the two 6SK7 tubes for optimum operation. I have added a really sweet transformer to the venerable Magnecord lineup as an input/interstage. Magnetic Components of Chicago have a ClassicTone line of transformers and one of their models is 40-18093. It is a designed to be a clone of the interstage/phase splitter transformer from the Fender Musicmaster Bass amp. It has a nice high impedance primary which couples very well with the preamp stage. The secondary has a slight stepup with a center tap. The mu-line goes to this center tap and the two secondary lines each feed to the input attenuator which goes to the pair of 6SK7 remote-cutoff tubes. I will be using this iron on all my vari-mu compressors going forward because it performs perfectly in this capacity. Plus, it doesn't hurt that it is a nice beefy unit with decent shielding and attached wiring. It fits right in with the massive Triad iron. Speaking of which the output transformer, which is designed as an output ready to handle the power of a pair of 6V6GT tubes, it has a balanced 600 ohm output in addition to the single ended 4 and 16 ohm taps meant for hooking up to a speaker. Well, here I have disconnected the 4 and 16 ohm taps, grounded their common tap and floated the 600 ohm taps. These have a 604 ohm resistor strapped across them to knock down the gain just a bit, but there is also a -15dB output pad on a switch that knocks it down even further. This switch is pretty much on any time I hook up to a digital system, but the switch is there in case you're feeling brave or are smacking the shit out of some high headroom tape! The limiter output goes to the XLR male jack originally used as a tape recorder interface. The input is hooked up tot he tag strip and I have it set up so that the output of the preamp circuit is patched directly to the input of the limiter circuit. The beauty of using the tag strip like this is that if you WANT to decouple the two circuits and use them separately (or add another device in between the two circuits, OR just bypass the preamp circuit for line-level work), this is as easy as turning a few flathead screws.

I am very proud of this new creation. I made a few of these before with a stock Federal circuit and though they were great, this new iteration is something special. This PT6-J is MADE for this type of conversion and I would put this Federalish mod up against ANY stock Fed and wipe the floor with it. Plus, it comes with an amazing microphone preamp on board as well and that adds a whole 'nother dimension to what it can do as a standalone channel strip. The iron is really what puts it over the top. Check out the puny transformers on the original and you'll see what I mean. Where the original was not really designed to be a studio tool, THIS one certainly IS! I may put one up on evilBay soon, so if you are interested in getting one for yourself maybe we can bypass that whole process and deal directly. OK, so here is a pile of pictures showing off the unit from all angles.