Copyright 2000 R.G. Keen. All rights reserved.
Lots of people have reported good sounds from the Mini-Tubes and Mini-Tubes II JFET preamps as originally designed by Jack Orman. The original can be seen at his web site, http://www.muzique.com/amz. To make it easier to get one of these things running, I have done a PCB layout for replicating the circuit, as well as adding some extensions and cleaning a few things up. Well, actually I played a little bit, and had some fun with PCB layouts. I did the layout for the Mini-Tubes II and then modified it just the tiniest bit so that you can build the Mini-Tubes I on the same PCB layout. It's not as complicated as 3D chess, but satisfying two similar circuits simultaneously on one copper pattern is a fun mental stretch.
This is the .pdf file of the layout for the Mini-Tubes II, showing parts population diagram and wiring hookup. The Mini-Tubes I is here. The actual size printable layout, suitable for toner transfer techniques is available now.
In answer to your many email requests - no, I'm sorry, I can't provide either ready to iron toner transfers or ready to solder printed circuit boards. Jack Orman, who did the original design has firmly refused any use of his work that gets sold, and I'll respect his wishes.
Note that the same copper pattern is used for both, so if you can print two copies of the layout on copper, you can build both effects.
There are a couple of minor things that I thought needed cleaned up in the circuit as published. These are not really mistakes, more in the class of polishing. I have changed the original circuit to:
add an output "pull down" resistor to prevent pops if someone uses this as a switchable stomp box effect
automated the "channel switching" for the II; the Mini-Tubes II uses the preamp architecture from Kevin O'Connor's London Power Standard preamp. While a DPDT switch does work as shown for switching, I have converted to shunt muting switching with transistors so an SPDT switch will do the channel switching, making the thing a bit cheaper
allow use of 2.2uf ordinary aluminum electros back to back instead of 1uF non-polarized electros. This is usually cheaper, it's certainly easier to find; and if you can get 1uF films, you can put those into the same pads on the board, just bridging the leads to the appropriate holes. These places are noted by "Cnp" for C-sub-non-polarized.
I have added a power converter section to the board. This uses two LTC1044A charge pump voltage converters to make 30+ volts from a single 9V battery, and automates power switching to the converter with a PNP transistor turned on and off by the usual connection to the stereo lug of a jack. This circuit is pulled from the application notes of the maker of the chip; however, it must be considered experimental until proven in service.
About the board
The board was first laid out for the Mini-Tubes II, which is a more complicated circuit, having two tone/volume stacks. The Mini-Tubes I conversion is done on the same board by un-populating certain components, changing the values of some components to match the earlier circuit, and inserting six jumpers. This makes for a maximum of flexibility. It can get confusing if you think about both populations at the same time, so when you build one, print out ONE layout and population diagram and work from that. At least that's what I'd have to do.
The basic board is designed to be usable in a Hammond 1590DD box. This box is the big brother of the 1590BB, and is about 4.7" by 7.1", but only 1.3" tall. It has enough panel space to get those eight controls onto the top surface, and to hold a channel switching stomp switch and a bypass stomp switch if you like. In this mode the voltage converter section can stay with the preamp.
Huh? "can stay with the preamp?" Why would anyone separate the converter?
It's like this - if you cut the board off at the dotted line on the layout, the remaining preamp circuit will fit inside a Hammond 1590BB if you want to take the challenge of getting eight mini-pots, a board, and two stomp switches into the box. If you can, you'll probably want to use an external power jack, as there is going to be little room inside for a 9V battery. In this case, the converter isn't much use. There is a pad on the preamp section of the board for bringing external power onto the board for this version of the board.
If you're the adventurous type, you might make the whole board, cut off the converter, patch the one wire in the converter that was cut away, and place the converter separately in your box, running wires for +30 and ground to the preamp. That works too.
I have recently come up with three more converter techniques that I will be posting; these range from a 6/9/12Vac wall wart to a common 555 timer to make the higher voltages.
It's worth noting that the converter section shares exactly one ground connection with the preamp section, and NO other traces. That was not an accident. That minimizes the switching hash that might otherwise get conducted in. It's the printed circuit version of star grounding like is used in tube amps.
You'll need to put a jumper from the converter side to the preamp side in the (thoughtfully provided!) space to carry converter power to the preamp. If you use the converter, the battery connects to the converter section only, and the converter is turned on and off by that PNP transistor with its base connected to a stereo grounding lug. DO NOT defeat that by jumpering the transistor and switching the ground to the whole circuit by a stereo ground lug connection. That will guarantee that switching hash will ride into the inputs on the ground line. Trust me on this one.
Use a 2N3906 for the PNP, 2N3904's for the channel switching NPN's; otherwise, just follow the schematic. The transistors noted as "DSG" are the JFETs, of course, and the ones noted CBE are bipolars used for switching. There are three resistors standing on end, in the source-to-ground of the second JFET section, the "pull-down" resistor for the output of the Mini-Tubes II, and at one channel switching transistor's base. Sorry. That was a lot more expedient than reflowing things so those could lie flat. It's a fairly full board, although not the tightest I've done. It's axiomic that the difficulty of completing a layout increases exponentially as you get closer to completely full on the board. Laying those three resistors down is probably possible, but could well take as much as ten times as many hours as getting to the degree of completion that the board has taken so far - so I weenied out.
You can probably use 10K's for the three "Rsw" base resistors to the bases of the channel switching transistors.
That's it for now. Let me know if it looks good to you. I've posted the real size printable layout, suitable for making toner transfer sheets. Comments are welcome.