The LR8 in tube circuits

The high voltages required for many tubes rule out or complicate integrating many otherwise useful solid state parts. The LM317 and TL431 are ubiquitous regulator solutions, but they’re limited to 36-37V. Too low in most cases for a simple one-chip B+ supply.

The LR8 (datasheet here) is a lesser-known TO92 high-voltage regulator. The maximum input voltage is 450V and the minimum dropout voltage is 12V. Output voltage is set with a simple resistor divider. With just a handful of passive parts, you can use the LR8 to create a regulator for tube B+:

LR8 simple

As a little TO92 device, dissipation and current are limited of course. The circuit above might work for something like El Matemático (one per channel), but higher current applications require the addition of a pass device. In this case, a MOSFET uses the LR8 as the voltage reference on the gate, in turn setting the source voltage just a few volts lower: LR8 compound

While zeners and VR tubes also make a good gate reference in similar series regulator applications, they come in fixed values. The great thing about the LR8 is that we can set the output to any value we like, alleviating the need to keep a bunch of zeners or VR tubes on hand.

I have PCBs of the series circuit made up and will be testing in an upcoming build. In the meantime, this isn’t so complicated that it couldn’t be done on a proto board.

Western Electric 300B back in production?


You may find a brand new production pair of Western Electric DHTs under your Christmas tree this year according to a recent press release that updates the release schedule from the Georgia-based company. According to WE, new 300Bs will be shipping in December of this year. You can find detailed specifications on the product page here.

Previous press releases reveal that modern manufacturing will achieve a better vacuum in the new production tubes and that cathode core material will be the same used in vintage tubes. Note that this is the core, not the emissive coating. Average lifetime is given as 40,000 hours (4.5 years of continuous playing).

Though these new production tubes will not be cheap ($1299 per pair), new old stock WE 300Bs sell for eye-watering prices online. Provided these new production tubes demonstrate a good track record, the price for made in America tubes adhering to WE’s original quality standards may not seem so exorbitant to tube enthusiasts (who are a bit exorbitant by nature).

The Western Electric brand name and trademark was revived by Western Electric Export Corporation. The current CEO is Charles Whitener, who was also a founder of Tube Depot (a Tennessee tube and parts retailer).

DIY DHT filament strategies

Having a small stash of #26 tubes and always being curious about it as a preamp tube, I’ve embarked on some preliminary research of successful implementations. There’s a huge thread on, but some choice references are Ale Moglia’s iterations and Kevin Kennedy’s classic implementation.

In general, the prototypical 26 preamp is a fairly simple single tube grounded cathode gain stage. Perhaps this topology simplicity is why there is so much experimentation in the support circuits. Gyrators, current sources, and line output transformers all make an appearance as the anode loading strategy. The B+ supply is similarly diverse: SS regulators, tube regulators, VR tubes, etc. It seems the popular consensus is for fixed bias: using the filament current drop across a resistor to set the cathode current. But there are fixed bias and traditional cathode bias implementations as well.

All of the above is fairly comfortable stuff coming from the general tube world of 9 pins and octals. The filament (AKA heater) supply, on the other hand, is something new for those used to indirectly heated tubes. In indirectly heated tubes, the cathode is a sleeve surrounding the filament heating it; this mechanical separation helps prevent heater hum (50hz or 60hz AC) from entering through the cathode. In a directly heated tube, on the other hand, the filament and the cathode are one and the same.

Depending on the circuit, AC filament power with balancing resistors and/or a ‘humdinger’ pot may be enough for an acceptable hum level. With the higher Mu (8-9) of a #26 and the need to keep front end noise/hum to a minimum (because it will only be amplified by everything following it), a DC heater solution is in order. We are looking at a requirement of about 1A at 1.5V for a #26 tube. The general approaches I have found are:

A low voltage SMPS and a dropping resistor requires no explanation if you know Ohm’s Law and can find something quiet with the right ratings. Voltage regulation and current sources/sinks have been covered in principle a few times in projects and general information pages as well (see links above). Mixed strategies are what have piqued my interest the most.

Kevin Kennedy’s article suggests a 7805 followed by a LDO CCS to supply #26 filaments. From what I can gather, this is the principle also behind the Ronan Regulator (which I see mentioned frequently but I can’t seem to find the ‘official’ schematic). In these strategies the voltage regulator makes a first pass at cleaning up the raw DC and absorbs some power dissipation. The constant current source follows and sets the filament current to a fixed value (in turn setting filament voltage as per Ohm’s Law). Including a CCS to limit current has a protective side-effect as well: cold filaments are otherwise eager to soak up a lot of current, potentially stressing the power supply and filament/cathode itself.

Rod Coleman also has a very interesting approach to DHT filament regulation. You can find boards/kits for sale here (no commercial interest, just admiration for the design).

coleman regulator

This circuit feeds the filament from the ‘positive’ end with a gyrator, also known as a cap multiplier in this configuration. The transistor Darlington pair sees a low-passed capacitor at its base and works to amplify this smoothed signal at its low impedance emitter (effectively making the cap seem much bigger than it really is). This doesn’t regulate voltage because the gyrator doesn’t have a fixed reference, but it does reduce ripple drastically.

The ‘negative’ end of the filament is connected to a constant current sink. This is a ring-of-two CCS design which will have a lower operating voltage requirement than a cascode CCS or many ICs. Because our current is relatively high, low dropout voltage is a benefit in reducing overall power dissipation.

The filament is fed a low ripple voltage with a CCS setting the current. It’s simple, but reports seem universally positive for Coleman’s regulator approach. Whether filament bias or cathode bias, the filament supply should be left floating (it finds ground through the bias resistor or grounded cathode).

There’s little reason to reinvent the wheel here as far as I can tell. Although I’m still casually reading, I’ll more than likely try one or more of the above approaches to powering the filaments in my upcoming #26 project. More to come on this project as parts arrive and the ideas ferment.

P.S. here’s a FET version of the same gyrator-CCS one-two punch:

FET filament reg

Big PP problems

approximate top plate.png

Hoffman’s Iron Law impacts all systems, regardless of the type of amplifier. It states that speaker designers may only optimize for two of three performance goals: efficiency, size, and frequency extension. Modern speaker design goals trend towards slim and minimally-intrusive boxes. Because few are willing to give up low frequency ability, this aesthetic trend has resulted in lower-efficiency speakers, requiring ever more powerful amplifiers.

When you have a set of bookshelf speakers or less-efficient towers, a single-ended triode amplifier may not cut it for power. Larger push pull (and parallel push pull) amplifiers are capable of using lower turns ratio output transformers and delivering more power to a load. That could be the difference between realistic dynamics and a more compressed musical presentation. Of course, there are some things to overcome when you upsize your tube amplifier.

  • The power supply – If building an amplifier with (parallel) push pull power tubes, you’re going to need a lot of heater current. You’ll also need a lot of high voltage current. This means solid state rectification is the way to go. Using a bridge rectifier (four diodes) rather than a full-wave (two diodes) also saves some efficiency in the transformer.
  • Size and weight – More current demands from the transformer(s) directly translates to a larger size and weight. Again, bridge rectifiers will help reduce the power transformer size slightly. A switching buck converter for heaters is also something worth looking into for efficiency’s sake. Building as monoblocks is a good solution, but you’ll probably spend twice as much on chassis, power supplies, etc.
  • Current sharing – To keep standing DC currents from saturating the output transformer core, we want all our output tubes to share current equally (or at least balance per phase in each channel). Bias servos, Blumlein garter bias, individual fixed bias, and individual cathode resistors all have their advantages and disadvantages. Careful consideration here is key.
  • Driving Miller capacitance – With a bunch of parallel output tubes, the input and driver stages will need some grunt to keep Miller Effect from rolling off high frequencies. This is especially true if you’re driving triodes in the output stage. A follower of some type may be needed to ensure a low enough source impedance.

If you haven’t already gathered, I’m a bit preoccupied with how I’ll utilize the big old chassis I picked up recently. Clearly something large is in store. The present question is octals or DHTs and two or four output tubes per side. The chassis originally held some monstrous iron, so there’s space for just about anything.

room for activities.gif

Swap Meet Gold

As if I didn’t have enough project irons in the fire, here’s a humongous old PSU chassis that is begging for an over-the-top power amp build. Eight octal sockets and very large transformer/choke footprints (mounting holes at 3.5″ and 4.75″ spacing).

More to come on parallel push pull power amp design challenges…

BF chassisBF chassis 2

More Opamp RIAA

I’ve detailed some very simple RIAA math for opamps in a past post and even did a little PCB board project to test the calculations. The image above is from a Patreon patron who built a battery powered phono from the same batch of PCBs. I’m very happy with the beginner-friendly nature and sound of this 9V-powered opamp phono preamp. The $25 bill of materials is nice, too. But, it doesn’t have a tube.

Now that I know the RIAA math and combination of passive and active equalization works, I’ll move on to phase 2. The battery powered two-stage preamp has about 40db of gain (60db if you count what’s needed for the RIAA correction). What if we only asked the opamp to perform the equalization (without the extra gain)? Having an opamp-based RIAA correction module eliminates the pesky RIAA math, but still lets us roll our own for the rest of the circuit.

Here’s a quick take on the circuit:

unity riaa signal

This brings the low frequencies from the phono cartridge up and the high frequency levels down to create a ‘flat’ signal. All that’s left is to make up the 40db or so of gain to get around 1Vrms output. A stage or two of grounded cathode tube amplification is the simple answer. There’s no urgent need for high Mu here, either: just about any tube could work. Note R16 still allows for some gain to be set at the opamp, so even a single tube stage can get a little help.

Keeping with the theme of simplicity, the opamp circuit would be powered from a common 6.3V winding:

unity riaa power

The heater supply is voltage doubled and regulated with a common IC. We can also use a rail-splitter to create a virtual ground and improve the performance of the single-supply opamp circuit.

In theory, the above looks like a fun and simple way to build a tube phono stage. The tube type(s) used would be extremely flexible and the RIAA portion adds no real complication to the build. The builder needs only focus on their tube fundamentals.

This is on my short list for the next batch of test boards!

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


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 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


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


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:


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.


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:


  • 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


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.


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


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


Very clean chassis. Sockets are in decent shape.


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.


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.


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.