Makin’ holes in stuff

Making holes in wood and metal is a big part of the DIY tube amp building hobby, but practical construction strategies aren’t something that get a lot of attention on forums or websites. We (myself included) probably spend 90% of the time in thought experiments and circuit analysis and 5% of the time on fabrication (the last 5% is chasing math and rounding errors).

I usually build enclosures from raw materials: 3/4″ hardwood and 1/8″ aluminum. While this is by no means the only way to do things, here are some of the tips and tools I’ve accumulated for my style of construction. The focus here is on making holes (especially in metal); for tips on making a simple wooden box, see this page.

Drill Press

drill press

You do not need a drill press, but it makes many things easier. A drill press is more stable than a handheld drill and easier to setup for repeatable depth or consistent spacing. Drill presses are more powerful than handheld drills and have more settings for speed, both useful features when using different types of bits and materials.

Limited throat depth is a disadvantage of the drill press. A press advertised as “10 inch” swing or throat depth can drill to the center of a 10 inch piece of material. This means the distance between the chuck and the vertical support is 5″. In my experience, 10 inches is the minimum size press that will be practical with tube amp top plates. Even better if you can fit a 12 inch or larger in your budget and work-space.

Drill presses work best on flat stock. While this isn’t necessarily a disadvantage, it is something you need to plan for while building enclosures. Drill first, then glue and assemble!

In most cases a press is slower to setup for cuts than a hand drill. I still have a good quality battery powered handheld drill when I just need a quick hole for mounting bolts/screws, when placement isn’t critical, or when I don’t have clearance to use a press.

Drill Bits

drill bit pilot point

You don’t need anything super fancy or expensive for drilling in wood and aluminum, but you should invest in a decent set of bits. Regular twist bits with a pilot point have worked great for me in both presses and hand drills. The narrower end on a pilot point also helps with hole placement when cutting to precise locations on the press. I suspect that brad point bits would not hold up well to lots of aluminum/metal drilling and I’ve broken a lot of cheaper standard point steel bits. A bit set with a good coating on it will definitely stay sharp longer.

Bi-metal Hole Saws

holesaw

For cutting out socket holes and holes for mounting large capacitors, I use bi-metal hole saws. I’ve tried punches, but I like working with 1/8″ aluminum and I haven’t found a punch that works for this thicker material. If you’re working in thinner steel, a good set of Greenlee punches may be your best friend.

I cut octal holes with a 1 1/4″ hole saw and 9-pin holes with a 3/4″ (sometimes it’s necessary to enlarge this to 7/8″ depending on the socket and tube). Both of these sizes will work with a handheld drill, but larger sizes for motor run caps really beg for a drill press.

I’ve been very impressed specifically with the Milwaukee Hole Dozer series. They’re easy to find at the big box home improvement stores, the arbor can be swapped between saws, and they’re easy to clean out shavings and stuck plugs. I’ve drilled a lot of octal socket holes without serious dulling of the saw.

Unibit

unibit

Unibit is your BFF for drilling out grommet holes to run transformer wiring. It is also very handy for a quick 9-pin socket hole. I’ve found the 7/8″ size to be perfect for most tube amp needs (anything bigger than this is a hole saw job). Tip: use painters tape to mark your depth on the bit so that you don’t overdrill to the next size larger than intended.

The unitbit has a tendency to grab the stock you’re drilling into. Whether using this with a handheld drill or a press, be sure to super clamp your stock and avoid spinning top plates of death.

Unibits bits aren’t cheap, but they’ll last forever if you get a good quality one.

Countersink Bit

countersink

This one isn’t the most used bit in my toolbox, but I do think it adds a nice touch for top plates. I use these to put the head of mounting screws level with the top plate and to clean up any holes drilled for chassis air flow. These work on wood and aluminum.

Forstner Bit

forstner

The forstner bit is your hole saw equivalent for wood work. A forstner bit allows you to drill circular holes in wood, removing the material to a specific depth. Most jacks and pots do not have much of a bushing on them. If you want to mount jacks and controls in the wooden portions of your chassis, you’ll probably need something like this to reduce the wood to a manageable thickness:

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I use forstners to remove wood from both the inside and the outside of my enclosures, depending on where I want to locate the recess. A good forstner bit leaves a very clean edge so I use them wherever possible in wood. Don’t try these in aluminum.

Jigs

Because I incorporate solid state parts on heatsinks in many of my builds, I drill a lot of holes to vent the chassis:

A thin and flat ruler taped to a straight fence makes a really simple and useful jig for making sure the hole spacing and placement is consistent on the drill press. Pictured below:

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  • First, layout the lines along which you’d like to drill the holes in your plate with a square and a permanent marker (remember to use the mirror image if marking on the underside of the top plate).
  • Second, mark your material along the edge using a square. Exact location is somewhat arbitrary.
  • Now, line up the material and bit to cut the initial hole at your desired starting location (but don’t drill yet).
  • Adjust the fence to line the mark on the material’s edge up with any whole inch mark on the fence/ruler (you see the marked line at the 10″ spot on the ruler in the photo). The material should be flush against the fence at this point.
  • Finally, clamp the fence/ruler combo down securely and recheck that you are drilling the first hole where you want and that the mark on the material’s edge is lined up with a convenient mark on the ruler.
  • Now you can drill the first hole and move the material along the fence to keep the rest of the holes in a perfectly straight line. Using the mark on the edge and the ruler, you’re able to very precisely control the distance between holes.

 

Fender AA864 Blackface on the bench

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Came with an assortment of tubes. The power tubes are mismatched brand (I imagine not matched for bias, Gm, etc). The “ECC083” does not have a brand marking but the internals look like nice quality construction. All tubes are visually fine.

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The fuse in the amp is a 3A fast blow type. Back panel calls for 2A.

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Bolt heads show it’s been opened up before. We’ll see why later.

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Very clean chassis. Sockets are in decent shape.

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Gotta love vintage point to point and turret/eyelet construction. Again, nice and clean here. Fiberboards are in good shape, minor warping from age/storage. Cloth covered wire. Rectifier diodes and bias network in the top of the pic will be replaced.

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Power filter caps. The larger orange cap on the right appears to be the reason it was opened up in the past (probably a long time ago). Non-original, non-spec, not that it matters. These will all be replaced.

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Old tag. Not sure about production number. Maybe May of 1967?

What’s on the bench

I realized I have mentioned a bigger project in the works for a while now but I haven’t provided any real details. Here’s the basic schematic. This is a two stage push pull amp using a long tail pair shunt cascode phase splitter up front (that’s a mouth full). The schematic does not have all the details yet as it is untested, but you get the general idea of what I’m going for.

I made two PCBs for this project due to all the little TO92 and other SS parts. The first PCB is a fairly simple MOSFET series regulator using a LR8 as the voltage reference. This will feed the input stage of the monoblock. The input stage will consist of a pair of shunt regulator PCBs with (probably) a simple 10M45 tail CCS. Bartola Valves has some great articles on the shunt cascode design, in turn based on experiments by Rod Coleman.

I’m shooting for about 20W in triode mode without feedback. I’ll likely also experiment with UL and NFB at higher output levels, too. The output transformers are Hammond 1650N. If the project turns out well, the next layer of complexity will be a direct coupled MOSFET driver for AB2 operation. For the first build, I’ll keep it nice and easy cathode bias though.

 

Schitt is getting into DIY, except they’re definitely not

Pic from Schitt product page

If you’ve missed the recent hubbub, Schitt is launching a line of drink coasters. It just so happens that these coasters double as unsupported DIY projects with scarce documentation.

The schematic shows a 6418 sub-miniature triode direct connected to an AB push-pull transistor output stage (cap coupled). Power is a blistering 30V to the tube and 15V to the output stage, cleverly derived with a bridge doubler and regulated with LM317s.

Schitt is hedging the product with extra coy advertising (AKA the Schitt Shtick) and reinforcing in several places that they are not a DIY company. Hence the product is a coaster, not a miniature hybrid amplifier. It will come at no surprise to Schitt when DIY documentation is created by early adopter hobbyist communities, I’m sure.

Some of the quoted specs:

  • Frequency Response: not terrible, but not exciting (like 10-100K, -1dB or so)
  • Power Output: much less than anything else we make (like, less than Fulla 2, maybe 400mW into 32 ohms, all in, 10% THD or so)
  • THD: about 0.5% at 1V RMS (6418 tubes) or about 1.5% at 1V RMS (6088 tubes)
  • IMD: didn’t bother measuring, this amp ain’t about measurement
  • Output Impedance: about 8 ohms (yes, 8, not 0.8, not 0.08), in case you didn’t get the memo, this ain’t a high-performance amp

What is it about Schitt’s non-committal and self-effacing copy that gets so many people so excited?  Why did I just buy a small lot of 6418s? Why are coasters already on the way to me?

Schitt, you sly dogs.

Letters to WTF: is tube linearity just a matter of lower gain and higher operating voltage? Or it is inherited with the medium – vacuum vs semiconductor?

Generalizations are difficult to make, but you can look at the theoretical mechanisms of operation for some interesting nuggets.


In a tube, current is transferred between anode and cathode by the space charge in a vacuum. Child’s Law states that current in a vacuum is directly proportional to anode voltage (to three halves power) and inversely proportional to the distance between electrodes (squared). The speed of electrons depends solely on the applied voltage.


In semiconductors constructed of doped sandwiches of semiconductor material, Child’s Law doesn’t apply. Here we use the Mott-Gurney law. This states that the current density in a semiconductor is directly proportional to anode voltage (squared) and inversely proportional to the thickness of said material (cubed). The speed of the electrons depends on both the electron mobility of the semiconductor (assumed to be constant) and the applied voltage.

There are extra constants for calculations in either case, but notice the similarities. In both cases, we can assume distance between electrodes (whether separated by a semiconductor or a vacuum) doesn’t change. The difference here is the power for the voltage term. The generalization is that current density through a vacuum is less affected by changes in anode voltage than is current through a semiconductor.

Is this the theoretical mechanism to explain the heuristic that vacuum tubes are more linear than transistors? Maybe…at the very least it’s an interesting observation. In reality, geometry, application, and other factors matter, aside from just materials. Some tubes are more linear than others just as some transistors are more linear than others. There are bad ways to bias and operate tubes just as there are good ways to bias and operate transistors.

One little project comes to fruition

Early this year, I wrote a post about simple RIAA correction with opamps. Although it doesn’t involve tubes (yet), I recently completed a PCB-based build based on this post. This was both to test the calculations/theory as well as good practice in PCB design.

This ultra-simple phono preamp runs on just a pair of 9V batteries for power and utilizes a mix of feedback and passive EQ for RIAA correction. The batteries should last about 24 hours (playing time), but a bipolar AC-derived supply could be substituted without trouble. Gain is easy to adjust with just a couple of resistors (set at 40db in my build). The bill of materials runs about $25 with 5532 opamps and 5% tolerance WIMAs.

I’m planning on building a couple of these with coworkers and basing build instructions and any revisions on the experience. I do have some extra boards from this first run. Shoot me an email if interested!

New Page: Phase Splitters

It’s no secret that I’ve been working on a larger push pull project, but it may still be a surprise because progress has been so slow. Don’t get me wrong though, dad stuff is the best.

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Writing time is sometimes easier to come across than bench time, so there’s a new overview of phase splitters page for your enjoyment. Happy holiday!