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.


Thursday, February 28, 2013

Ampex 602 2 Ch Brute Force Preamp Conversion

Ampex 602 Tablebeast Modified TB-602A 2CH Brute Force Preamp:

Here is my latest Ampex conversion. I tell you what, the Brute Force circuit really loves the 602. While the 601 and the Akai/Roberts monoblocks make amazing foundations for this circuit themselves, there is just something about the 602 that outshines them in my opinion. I think it may be the solid state high voltage PSU or the DC heaters that give it that inky black noise floor. It is a such a 3D sound that you wind up with something that sounds finished the way it does when you hit tape just right. But the best part is that it doesn't have the tape hiss or the hassle of messing with those aging machines. With this unit you get a brand new set of electronics built from the ground up with all new wiring, caps and resistors while using the best vintage parts to fill it out. I am very proud of my design and so far everyone who has been lucky enough to get one of these from me has been head over heels for them. Get one while you still can. Here are some more pictures.

Friday, January 18, 2013

Now Taking New Orders to Fund the Station

Tablebeast Modified Akai Roberts TB-2XP1 Preamp and 2x TB-A436 Vari-Mu Compressor:

 If you like this blog and would also be interested in getting something like what you see here for yourself, then now is your chance! I am beginning production on gear builds for the fundraising phase of the station. So, after a few months of catch-up on pre-XMFR orders, I have now have limited time available for new orders. Available will be my regular turn-key devices: (Akai / Roberts monoblocks, Magnecord PT6-J, Ampex 600, 601, and 602), trade-in orders (where you send me the donor to modify), and custom orders (pretty much anything under the sun: Bogen, Knight, Masco, LaFayette, Magnasync / Moviola, Heathkit, Dynaco, Sony, Teac, etc.) As you can see here on this blog, I make these old relics into new beast in the form of: guitar amps, microphone preamps, hi-fi stereos, audio effects, synthesizers and more. I can install add-on transformers of nearly any type as long as it fits the donor from brands like Jensen, Cinemag, Sowter, and Shure. I'd prefer to focus on what I am familiar with (which is a lot as you can see), but I am always looking for a new adventure as well. So let me know what you've got! Prices start at $399 for full rebuilds and can go on up to much more depending on the complexity of the mod and cost of additional parts. So, if you like something you see on this blog, drop me an e-mail at ::: xfmr AT xfmr DOT org ;;; and we can discuss what you're looking for.

Wednesday, January 9, 2013

Ampex 601 2 Ch Brute Force Preamp Conversion w/ Altec Iron

 Tablebeast TB-A601 Ampex 601 2 channel with Altec 4722 and 15095:

This is my new king of the hill. What an awesome, AWESOME preamp this turned out to be. I started with an Ampex 601 reel to reel tube electronics module. I then gutted and dismantled the whole thing. I separated the parts from stuff I planned to reuse and stuff I planned to discard. Of the discarded some parts, some are kept for other projects and spares while others are just thrown away. Sprague bumblebee caps? Yeah, I keep those when the values are not right for the conversion, you bet! Old cap cans and ALL the wiring? In the garbage it goes!
The next step is to prepare the chassis for my intended design. I rearrange the tube sockets in my needed layout and add a few sockets as needed. Then I install the new Weber cap cans with mounting brackets. The back panel gets all the Switchcraft chassis-isolated jacks, XLR input and pad/phase switches for channel 1 and pad phase switches are also added to the channel 2 input on the front.
When considering iron for this unit, I chose some of my favorite affordable and readily available vintage transformers. The Altec 4722 and 15095 were what I went with. I wired the inputs so that they will take the 4722 of course, but it will ALSO accept a 15095 or 15335 as an input depending n application. The outputs get 15095 plugs, but can also accept 15335 units as well. This makes the octal plug-in format function as Altec originally intended, where you can swap out the transformers easily to modify gain staging and impedance matching depending on what you stuff in there. Oh yeah, they also sound freaking INCREDIBLE!
The tubes in here are the 5879 Pentode for preamp duties, 12AU7 for line amp/output duty and the classic 5Y3 providing voltage rectification. They are set up as a pair of my Brute Force preamps which have insane amounts of gain. They are perfect for ribbons with low noise and the high gain, but really I have yet to find a mic that I don't like with it. Of course you can just turn the gain down when using a hot mic and if that doesn't get you in the sweet spot, engaging the pad and turning back up will get you there.
The inputs and outputs give you a LOT of different options. Of course it has balanced XLR inputs for mics and 1/4" DI inputs for instruments or other hi-z sources, but this unit also has dual outputs per channel. One output is tapped before the output transformer and is connected to an impedance balanced TRS 1/4" output jack. It can be used simultaneously with the transformer balanced outputs that are also on TRS 1/4" jacks. Now the impedance balanced outputs are HOT and I LOVE hitting an actual tape machine with this output as it will roast that tape if desired. The transformer on the output has a stepdown that knocks the signal down while balancing and adjusting impedance as is my preferred output for getting maximum coloration as it allows yo to crank the circuit a bit more.

Sunday, January 6, 2013

Projects Out the Door Before the END of the World!

OK, well, it didn't happen, but I was still determined to get certain projects out to folks before the Mayan deadline for our civilization, JUST in case!

Pair of Tablebeast 2xPre TB-2XP1 Microphone Preamplifier:

First up is a four channel set of mic pres that I was sad to see go. These are outfitted with Jensen inputs, have impedance balanced outputs, and were stuffed with some very special tubes. All four have GE 12AU7A tubes on the line amps, but the bottom pair has some fantastic German-made RFT EF86 pentodes and a GE 6X4 rectifier instead of the normal NEC-made valves that the I normally install. The first two channels do have these NEC tubes and they are simply fantastic of course, but the RFT and GE in the second pair just seem a bit more magical. Also, grainy pics I know. I took them at night in a hurry before I boxed her up. The apocalypse waits for no one.

Tablebeast TB-HR16, a Modified Alesis HR-16:

 This is a bit of a rare bird these days, at least in my hands. I don't do much circuit-bent stuff any more, but when tempted with this Alesis I couldn't resist. This is a fantastic drum machine even without the mods, but it takes to the patchbay style mod so well it almost seems like a shame to keep them stock. I hadn't done one of these in years, but it all came back once i started and I am very please with the results. Now that I have become familiar with what the guts of this guy actually ARE, I am really impressed with this advanced-for-the-time drum computer. So impressed that I am on the lookout for one of these to keep for myself. Oh yeah, the back panel for this guy was missing when I got it, so it got some hand labels on there. I was bored and there was an empty space ont he face, so it got a little unplanned beast drawing. What the heck.

Tablebeast Pre+Amp TB-PPA1-R 3U Rack w/ Jensen:

This is a rack mount Pre+Amp. Of course I have built dozens of these by now, but this one was particularly cool. It had the Jensen inputs which I just can't say enough good things about. Don't get me wrong, I love the Shure inputs that I usually use. They have a great utilitarian quality that I try to compare to denim when folks ask me to explain the difference between the Shures and the Jensens. What do I relate the Jensens to? Silk of course! So the Shures are jeans and the Jensens are a kimono. The Jensens really are smoother and less aggressive so I think it is a decent comparison. Being a man of many tastes, I like them both in any case. Anyway, this unit was awesome and is finally on the other side of the pond and hopefully in its new owner's hands by now. Enjoy!