Thomas Mayer’s new site

I’ve followed Thomas Mayer’s hobby website for a long time. He uses a tantalizing mix of high quality transformers and DHT tubes to build some beautiful audio devices. The tube of the month series is also a must-read review of odd-ball tubes and applications.

Now we can all see how much Mayer charges for his impressive tube builds. It’s about what one might expect based on the craftsmanship he clearly puts in and what the audiophile market supports in other products.

Check out vinylsavor.com here. If nothing else, browse the galleries and drool over the very Scandinavian glass, wood and metal work.

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There is now a NuTube portable amp kit

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Some recent digging around Pete Millett’s nutube.us website turned up this interesting page. What we see is a pocket size amplifier powered by AA batteries.

At first glance, it looks similar to a project you can find on this archive of Audio Mania magazine (Japan). Here’s the schematic showing the DC booster power supply:

nutube

The Nu:Tekt kit appears to use an opamp driver on the output rather than the FET combo shown above. With the exception of the small daughter board which appears to come assembled (I assume this is the booster), the kit looks to be completely through hole.

Little is revealed on Millett’s site, but I’ll be following this kit with interest!

Solid State Phase Splitters

The phase splitter is a critical step in a push pull (differential) amplifier. Because tubes don’t come in “p-types”, we feed the output devices signals that are inverted relative to one-another in order get one to push while the other pulls.

I’ve been finding solid state concertina-style phase splitters crop up here and there recently. A couple of days ago even the great Pete Millett got in on the action. Millett employs a JFET concertina splitter in his hybrid amp (a must-read, btw), but MOSFETs are also a good option for this application if you use parts with reasonable input capacitance.

Here’s a push pull schematic I’ve been marinating that illustrates the MOSFET concertina:

mosfet concertina pp

By using the MOSFET we’ve reduced the twin-triode count in a stereo push pull amp by one. The MOSFET will also let the splitter swing closer to the power rails, though in this particular case the 10BQ5 doesn’t really need a lot of voltage swing at its grid. The tubes shown are odd heater standards: 407A is 396A with a 20V heater and 10BQ5 is 6BQ5 with a 10V heater.

You can find a lengthier explanation of the RC step network between the 407A and MOSFET in Morgan Jones or buried in this diyaudio thread. In brief, the resistor divider sets the DC voltage at the gate of the MOSFET while the 0.1u cap bypasses the upper portion of the divider at AC frequencies so that we don’t lose any gain due to the divider.

Phono Preamp Heaters

Heater supplies, even with indirectly heated tubes, are a potential source of hum with high gain circuits like phono preamps. In a grounded cathode gain stage, the tube will amplify any signal it sees between the grid and the cathode. The tube doesn’t particularly care if that is an audio signal or an induced signal from some other part of the build. Indirectly heated tubes have a cathode sleeve around the filament heating it. The close geometry creates a happy little environment for coupling between the two. Eliminating this source of noise may require running heaters on DC rather than AC.

Here’s a simple schematic adapted from something Eli Duttman suggested for his modified RCA phono preamp:

12V dc heaters

This circuit (now on a PCB waiting for a phono build) uses a voltage doubler to turn a common 6.3Vac input into ~16Vdc which is then regulated to 12Vdc by a LM7812. The regulator is limited to 1.5A, but this is probably enough for any sane phono preamp’s heater demands (the pair of 12AX7 in the El Matemático require only 0.3A). This is one way of producing a DC heater supply.

I was recently discussing truly budget-oriented tube phono preamps with another builder. They proposed a $100 parts budget. The first place I’d look to start cutting costs in such a build is on the relatively pricey purpose-built power transformer needed for tube projects. In the case of a simple phono preamp like El Matemático, I’d try the following cost-cutting measures to the power supply:

  • Solid state 1N4007 rectification
  • Use a 115/230V isolation transformer like Triad N-68X in reverse (115V in, 230V out) for B+ @ $16
  • Use a 12V SMPS like Meanwell EPS-15-12 for heaters @ $7
  • Triad C-1X choke @ $10 and 220uF 350V+ caps @ $4 ea as CLC filter
  • Add RC to end of CLC filter to lower B+ and/or clean up residual ripple

We can greatly lower the cost of the B+ supply with the isolation transformer trick but it leaves us without a heater supply. Rather than a separate 6.3V or 12.6V transformer followed by a regulator circuit like the one shown above, I’d be tempted to experiment with a switch mode power supply like this Meanwell unit:

EPS15-12

The EPS15-12 supplies up to 1.25A at 12Vdc with 80mV of ripple (peak to peak). One need just supply it with mains voltage (85-264Vac). Power supplies like this switch at a very high frequency, which is why their transformers can be made so small. If that switching is audible, capacitively coupled between cathode and heater, additional filtering may be needed. Meanwell does not specify the switching frequency, but it’s very likely well above the 20hz-20khz range.

The final, potentially very affordable power supply, would look something like this:

very cheap psu

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?

300b

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 diyaudio.com, 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 (here’s a good read on this topic). 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

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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.

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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…

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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!