Furniture Update: Studio Desk

Here's an update on the Foray into Furniture:

The first photo is an overall shot, including the CPU, Digi 003 rack interface, monitors and wall-mounted display. All the wood is cherry; the top is 3/4" plywood with veneer banding on the edges. The sides and back are 1/4" plywood. The CPU shelf is 1/4" solid cherry. The rest is solid cherry 1×2s. I used a tongue & groove router bit to make the simple rail, stile & panel construction. The slot cut for the panel also serves as a mortise for the rails. Some dowel joints were also used for the frame under the CPU and to attach the top. The finish is Tried & True oil finish, which is a polymerized linseed oil and beeswax product.

The drawer is made with birch plywood prefab drawer sides from Rockler (they come precut to width, with a routed groove for the bottom panel). I used an inexpensive Wolfcraft dovetail jig to make the drawer box. Side mount drawer glides attach to the upper rails. The only screws used are on the drawer glides and to attach the drawer front to the box.

A rectangular hole was required for the cables to pass through the top. The DVI connector for the video monitor is the longest and the IEC power connector was the widest. I dressed the opening with a custom rectangular grommet (somewhat visible in the photo) made from a scrap of cherry.

Mostly, everything works as planned, except that our low-noise CPU enclosure has a front door to cover the CD/DVD drives. The CPU tower has to be pushed forward so that the hinge of this door doesn't interfere with the leg of the desk.

This last photo is unrelated to the studio desk. This was a mini-project from the summer. I bought the clock movement with the intent of making some kind of desk clock. An ECC832 triode I ordered for an amplifier project arrived DOA and the dealer didn't ask for the defective one's return. I fashioned the clock loosely after a vintage radio and epoxied in the triode as a decoration.

Teaser: Lama Kazu 12PP

It may be a while before concrete evidence of the next electronic project shows up here, but here's a little teaser and progress schematic for the next amplifier, the Lama Kazu 12PP. It will be a 15-ish watt push pull with 6CM6s for the output (they're 6V6s in noval bottles). The preamp will be:

Input -> 6AU6 -> Gain control -> 12AV7 gain stage (DC-coupled between sections) -> Tone stack (Probably BM-Π) -> Volume control -> Sunn/Dynaco style 6AN8 pentode recovery dc coupled to cathodyne.

I'm going to use a gain control that rolls off the bass as drive is increased to smooth out the distortion.

Coming soon: Repairing my friend Ray's Pearl King concertina.

Foray Into Furniture: Studio Desk

Have you ever seen a piece of furniture for the home recording studio that meets the following criteria?

1. Compact footprint for use in a small room.
2. Work surface elevated for comfortable use of keyboard & mouse while standing (with guitar strapped on).
3. Computer CPU elevated so that a vacuum cleaner can go under.
4. Enclosed sides to help contain CPU fan noise.
   
5. Includes a storage drawer.
6. No particleboard.
7. Looks nice.

Well, I'd never seen anything like that either, and we even had one of our equipment dealers hunting for it. Even relaxing requirements 6 and 7 left us with nothing. This means I have to build it.

I chose cherry wood and decided to do a simple rail, stile and panel construction for the sides and back, with an edged plywood top. The CPU shelf will be thin solid lumber.

A gap at the back will allow cables out to items at the floor level (power, audio monitors) and a grommet at the top will allow cables up to the video monitor (wall-mounted) and Digi 003 interface.

I've drawn it in CAD and started construction. Right now, all that's left to build is the drawer. Photos will follow soon.

Search for Tubes

You already use Duncan Amps and NJ7P to find tube datasheets. Here are a couple browser search plugins I made to make it easier:

• NJ7P plugin.
• TDSL plugin.

Preamp: Prairie 10LS/1LB

I started out with the idea to make a tube DI box for the '63 Dano Baritone guitar I ordered. It turned out that the Dano was long delayed and the order was eventually canceled, due to some manufacturing problems. I continued with the project as a bass DI. I threw in a microphone input for laughs. The circuit evolved.

A starting point was the Altec 1566 preamp. I wanted a pentode input, so I chose the 5879, which is well known from Gibson guitar amps and is not too hard to come by. The input stage went through several revisions: Initially, it was diode-biased using two 1N4148s in series. In the original configuration, I also included a version of the pentode screen "squish" control from the AX84 Blues Preamp. In the end, the cramped layout (see below) meant that I had to abandon the squish, as the screen picked up loads of noise. In the end, I also dropped the diode bias in favor of a resistive network. The idle current keeps the screen and cathode a bit stiffer than bias and screen dropping resistors alone would have, but there's enough sag and cathode degeneration to make it interesting.

Instead of the parallel 12AX7 for the cathode follower I used a 12BZ7 which is very close in performance to two 12AX7s in parallel. This left the other half of the 'BZ7 for a gain stage.

I used two 6 VA Hammond 229 flatpack power transformers, which are really intended for printed circuit board use, but I mounted them on a turret/eyelet board. I used eyelets as a substitute for solder pads. There are a few unusual thing about these transformers (compared to more typical EI cores): They're semi-toroidal, so there's less radiated EMI; their voltage regulation is pretty high, so at low loads like this (or at open circuit) the output voltage is much higher than rated. I could have used a 2 VA model for the plate supply, but cost was nearly the same, and the lower-rated model had the same core and footprint. It was just 0.2" lower in profile.

Input and output transformers were 8:1 from Edcor. The input (MX8cs) was mounted on a small turret/eyelet board similar to the one used for the power supply. The output transformer (TPC104) came on its own PCB. The preamp circuit was built on a prefab turretboard from AES. The photos show earlier revisions of the project. When the squish control was removed from the front panel, the extra hole was used for a "thru" jack, which connects to directly to the instrument input when one is connected, or to the secondary of the mic input transformer when no instrument is connected.

What do you do (with a Kalamazoo)?

I won't spend any time with the background on the Kalamazoo Model 1. It's all available at Miles O'Neal's Kalamazoo Field Guide. Here's what I did to improve our Model 1 and why:

I added a power input module. This includes an EMI/RFI filter, and moves the power switch to the back panel, and off of the tone potentiometer. The original front-panel fuse and neon remain.

Volume and tone potentiometers were replaced; they had been abused and had intermittent open circuits at certain points of their travel, which made some nice high frequency squeals.

All resistors were replaced (with metal film) to reduce noise. All values were kept the same, with the exception of the 6BQ5's cathode bias. I measured the old one at 160Ω (schematic shows 150Ω) so I put in a new one at 160Ω... I had to keep some "mojo".

Terminal blocks were added & replaced to make it possible to use modern passive components (which have shorter leads than the originals).

Wiring was replaced to improve layout and grounding and reduce hum. (Original even had signal ground returns through grounded heater wires!)

The original multisection filter capacitior was replaced with new electrolytics on a new power supply turretboard that also holds the dropping resistors and a heater elevation voltage divider to reduce hum. An additional LC filter stage was added on the power supply, using a Weber 9H/120Ω choke.

Shielding was added on the cabinet, using copper foil tape from Stewart MacDonald.

Input input jacks were replaced with new from Neutrik.

No major cosmetic work was done. Just glued down some loose tolex.

Amplifier Tweaks: Final Lama Kazu 8SE

The tweaks to a simple single-ended amp could be endless. I think this will be the last adjustment to the Lama Kazu 8SE, however. (Note: It wasn't! Some edits to this post were made in October.)


I decided to try a 12BZ7 in the second gain stage/cathode follower. Of course the bias had to be adjusted for that. Not knowing how exactly the 12BZ7 would act at clipping, I decided to add a small (3.9kΩ) grid stopper on the socket.

After using the amp for a couple months with the 12BZ7 installed, I ended up switching back to a 12AX7, but kept the hotter bias caused by the 1.5kΩ cathode resistor I put in for the 12BZ7.

I added a 560kΩ resistor in parallel with the drive potentiometer. This serves to cut the bass response as the drive is increased, and really reduces the muddiness at high gains.

I also decided to go over the whole thing and check my original calculations; it appears I chose the wrong tap on the output transformer when I built the thing. I changed it over from the 2.5kΩ to the 5kΩ tap. It's now actually capable of producing the 8-or-so watts of power I originally expected!

Now with the 'BZ7 there's not a "normal" tube in the whole setup!

Coming soon: Sound samples of everything...

Stompboxes II — Stupid FET Trick

I started out wondering: "How variable are JFETs? How many parameters must be measured when selecting them?" I ended up with a grungy overdrive/distortion: the Stupid FET Trick.

Looking at the 2N5457 datasheet from Fairchild, you can see that drain saturation current (I_DSS) and gate cutoff voltage (V_GS(OFF)) are correlated: I've seen other methods on the web for selecting FETs to use in audio, and they generally involve selecting for gate cutoff voltage, since this affects the operating point. I_DSS is much easier to measure, or at least to select for. In my case, I was designing a circuit using the stock CircuitMaker model, which has an I_{DSS of 3.859 mA at 15V.} (The range of values in the datasheet are 1.0 mA to 5.0 mA.) _{The model's behavior was determined with this little test circuit simulation:}The device test circuit is not much more than that. It needs a supply voltage somewhat greater than the reference voltage in the datasheet (I have an 18V regulated supply), has gate and source tied together, and has a drain resistor set to drop the supply voltage to about the reference voltage with saturation current flowing:
You don't really have to have a close-tolerance on the resistor; just measure its actual resistance and use that to select the range of voltages it will drop for the range of currents within your design tolerance. I needed five 2N5457s that were within 5% of the Circuitmaker model. I bought 100 (for 8 cents apiece at Mouser), and I tested 40 before I found six that were within my range. Most of the others were under and a few were over (not surprising since the selected value was in the upper portion of the range). My circuit was nothing earth-shaking, just a few ideas taken from typical tube guitar preamp design with the JFETs substituted in, and component values adjusted to get a meaningful operating point at battery voltages. I didn't do a thorough scientific verification of the gain in the real circuit, but the measured operating points were all within 10%. (Even though I selected the JFETs at 5%, the resistors were 10%). It doesn't really sound like a tube amp, but it is better than the last distortion unit I built (some 25 years ago), the Tube Sound Fuzz from Craig Anderton's Electronic Projects for Musicians. I continued my tradition of natural aluminum enclosures, and aside from the name of the unit, I only used pictograms for the legend (Skull & crossbones = gain; heart = tone; quaver = volume). I'll have to design the next JFET circuit around a device with a lower I_{DSS (and} V_GS(OFF)) to use up the ones I have left.

Coming soon: Interstellar Overdrive!

Stompboxes I — The Buffalo Box B175

While performing inventory of my parts bins last fall, I came across a couple ancient-looking TO-1 metal cans. I have no recollection of their origin, but the leads were rather short, so I presume I cut them out of a junked radio or similar item, probably when I was a pre-teen. At first I presumed I acquired them at the same time, but it turns out they're from different manufacturers, so maybe not. One was labeled B175, the other was B186. A bit of searching showed they are both PNP germanium transistors intended for audio amplifier use. Word on the street was that should try one in a Rangemaster clone.

I breadboarded the crazy-simple circuit, and the B175 had a higher gain when plugged in, so I chose it for the finished project. I also added a bypass switch and an "anti-pop" 1MΩ input resistor. I assembled it on a Radio Shack "Bug" PCB (276-159) and crammed it in a Bud econobox enclosure. A "vintage" knob controls the gain. The unadorned "natural aluminum" look I used with the LK8SE continues, and is even refined in my choice of functional legend on the unit.

I've never encountered a real Rangemaster, so I don't know if it sounds like one. It does work nicely with the LK8SE for "screaming solo" work. You need to crank up the bass control on the amp to get a crunchy sound, though.

Eric Barbour or Fred Nachbaur?

Recently, I mentioned in on an online forum my idea for a 15-ish-watt amp based only on Compactrons with a 6AS11 preamp and a pair of 6LU8s for the power amp and phase-inverter (each tube contains a beam tetrode and a medium-μ triode). One response was: "You should get a job with Metasonix". I suppose I could have taken offense at the comparison: While I think there is a place (and probably a need) in the world for a guy (Eric Barbour) who makes an amp called the F*%@ing F*%@er [grawlixes mine], that's not me or my style. Also, amusing as it is, I don't think I could bear to build an amplifier that uses 800 watts of power for just 30 watts of output.

I think the Metasonix comparison really came from the idea of using new-old-stock tubes for other than their original purpose. It wasn't Eric Barbour that gave me the idea, though; it was the circuits and writings of the late Fred Nachbaur. Fred's pages were one of my key resources when I decided to move from repairing & tweaking amps (like a Kalamazoo model 1 and a "Blackface" Fender Bassman) to designing & building from scratch. His design philosophy was mostly rooted in classic methods, but he loved to use oddball tubes, especially ones with atypical heater voltages arranged into series/parallel combinations. That approach seemed to border on obsession in some cases: Just look at the power supply for his "Spunky" guitar amp. It's a pretty cool arrangement that eliminates the need for a transformer with a heater winding. A bit of goofiness though is seen in the two 10-watt power resistors needed to set the voltages right, and the fact that there's an entire separate transformer for the bias supply. I guess it works if you have the parts on hand.

In any case, the all-compactron amp is not going to happen, at least not right away... Since I'm on the waiting list for a new Danelectro reissued '63 Baritone guitar, which will need an amp, I'm going back to a previous idea for the 15-ish watt amp. The power section will be a pair of Electro-Harmonix 6973s. Just like when I chose the JJ 7591S as the power tube for the Lama Kazu 8SE, (7591s are rarely seen in single-ended amps— they mostly turn up in old Ampegs) I couldn't bear to use the typical EL84 or 6V6 power tubes that are more common in amps of this size. More to come on that project.

Amplifier Tweaks: Lama Kazu 8SE

The Lama Kazu 8SE was revised last fall; I never got around to mentioning it here until now. The revised schematic is at right. Most of the changes were to reduce the gain, smooth out the overdrive character and clear up some blocking distortion.

• V2 was changed to an ECC832, (JJ's equivalent to 12Dw7) with the high-μ triode at the input and low-μ triode at the driver stage.
• V1 was changed to an ECC81/12AT7.
• The volume potentiometer was changed to 1MΩ and the attenuator resistor was removed.
  
• Coupling capacitors into the tone stack and power amp were reduced to 22nF to drop some of the "mega bass" muddiness.
  
• Cathode bias resistors were changed to 4.7kΩ at the second stage and 3.9kΩ at the driver stage.
• Load resistor on the driver stage was reduced to 39kΩ. This, combined with coupling and valve changes really reduces the output impedance so that grid clipping at the power stage is more graceful.
  
• The 68kΩ dropping resistor in the power supply was reduced to 39kΩ, since the second preamp stage is using more current with the ECC81.

Sound files will be posted eventually. This amp works really well with my Rangemaster clone, known as the "Buffalo Box" and based on an old B175 PNP Germanium transistor I found in my junk bin.

Treble Peaking Filter Online Calculator

A beta-test version of my Treble Peaking Filter Calculator is now on-line (graciously hosted by dianemariekloba.com)! What it does is calculate the resistor and capacitor values required to make a high-pass shelf filter with specified turnover frequencies.

If you use this in the early stages of an amp or distortion circuit, before any overdrive or clipping actually occurs, the concept (and circuit) is essentially that of preemphasis like the RIAA equalization used in record cutting. In amps & overdrive/distortion, the idea is to reduce the bass frequencies that tend to flood the distortion stages.

Somewhere later in the amplifier system there needs to be complementary deemphasis. This often happens in the tone stack, speaker and speaker enclosure.