Letters to WTF: Holiday Gifts for the Tube DIYer?

Q: A simple question – is it possible to purchase a kit from you? If not, are there decent kits you would recommend for a first-time tube build? My husband has built a couple of solid state preamps and power amps and is intrigued by tube ware.

A: Your husband is a lucky guy to have someone encouraging his hobby!

I don’t sell any kits for my builds at this point (though I do send out prototype PCB boards to my Patreon subscribers on occasion). I am happy to provide some recommendations for beginner-friendly tube projects and/or gift ideas, though. Some of these are PCB boards that require you to select parts (or leave your husband to do so afterwards). Many builders enjoy the process of picking out and sourcing parts, so this isn’t necessarily a bad thing  and you might include a ‘parts budget’ as part of the gift in that case. 

Tubecad.com makes some of the best documented and flexible board kits you can find for tube hobbyists.  In particular, the Aikido and CCDA designs have a great following and lots of user support on community websites like diyaudio.com:

Aikido Noval Stereo
TubeCAD Aikido (click to go to listing)
9-Pin CCDA PCB and User Guide
TubeCAD CCDA (click to go to listing)

Note the above let you add parts or order boards by themselves. Adding parts might be tricky for you to do without your husband’s input, though TubeCAD does a good job keeping the options and confusion to a minimum.

Here’s another PCB board (no kit) for a phono preamp (for turntables) that I can also recommend. The designer of this one is another well-known author on tube topics:

Valve Wizard Phono Board (click to go to listing)

Bottlehead is one company that gives you everything you need in a full kit. They have a lot of tube kit options at different price points. If your husband also listens to headphones, this company is especially well-known for their headphone amp kits (two options below, but explore the site to find more). 

Bottlehead Crack (click to go to listing)
Bottlehead Single Ended eXperimenter’s Kit (click to go to listing)

Bottlehead’s kits are pricier, but the documentation and the all-in-one nature add a lot of value for beginners. 

Lastly, Elekit is another Japanese company that does all-in-one kits. These are available through the diyAudio Store.  I don’t have personal experience with Elekit kits, but I have read a lot of good things (and the manuals I’ve seen look very well-done).

Elekit TU-8500 (click to go to listing)
Elekit TU-8100 (click to go to listing)

Hopefully you find something in your budget in the above links.  I think anything you do to show an interest in his hobby will be very well received!  

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

Muchedumbre XL PSU details

These two buffers are very similar. In fact, the XL came about as a simple mutation of the regular version when the cost/size of an extra tube was not an issue and there was the possibility of driving a lower impedance load (long cables, solid state amps, etc). In theory, the power supply requirements are identical down to the B+ current draw. “In theory.” Although if you study the schematics carefully and read the write ups, you’ll see that my steel-trap mind is missing a spring or two.

The original Muchedumbre called for a Triad C-3X (500 ohm DCR). On the schematic for the XL, I seem to have spec’d the Traid C-7X (270 ohm DCR). The inductance rating for these two chokes is identical, but the C-7X has about half the DCR. Is that going to be a problem?  In short: no. I love tubes; they make up for all my shortcomings.

Current draw between the two configurations (XL vs vanilla) is the same on paper, despite the extra triodes in the XL (the added triodes are in series, so the same current flows through both). If using the same choke, the B+ should end up the same. With the modest current in the pre, however, you have plenty of leeway in DCR before you have to worry about big B+ changes.
ΔVdchoke = (500 ohms – 270 ohms) * (10mA x 2) = 4.6V
No problemo.
Say you have a big old Hammond 193M at 10H and 63 ohms DCR. You could very safely use that, too. The lower DCR gives you a little more room in the B+ to increase the series/sense resistor if you’re amps’s input impedance suggests it. You gain some B+ in the choke DCR and drop it back down across the sense resistor. Current across the DCR is twice that of each channel, so assuming 10mA bias per channel:
ΔVdchoke = (500 ohm spec – 63 ohm part) * (10mA bias x 2 channels) = +8.75V
ΔVdsense = (1k5 ohm spec – 3k ohm part) * 10mA bias = -15V
Net result is a 6.25V deviation from the ‘on paper’ spec’d parts. That’s less than 5% of the B+, so better than parts tolerance in a lot of cases (e.g. low frequency chokes). The x-axis divisions on most tube datasheets are 20-25V, so they’re only so precise. Generally, 10% changes in B+ based on available part specs is probably about where I would return to my load lines to recalculate. In practice, I sometimes build anyways and then measure actual B+ before worrying about anything.

Solder and wire beats paper and pen.

 

Muchedumbre XL

The regular Muchedumbre is an ultra-simple buffer with few parts and straightforward operation. It’s a great beginners circuit for high voltage tube applications.

If you’d like something with just a little more nerd sprinkled on top (and an extra 12AU7), try the following. This XL version uses a White Cathode Follower buffer for about half the output impedance of the vanilla version. It requires a few extra resistors and caps and the heater reference voltage is tweaked just slightly so I can sleep better.

For ideally symmetrical drive ability, the series resistor in the anode of the upper 12AU7 should be calculated as:

Rseries = (Rp + 2 * Rload) / Mu

Plugging in values for a 12AU7 and a (worst case) 10k input impedance gives you about 1k5 (rounded so that it’s an easy to find value). You can optimize this for the input impedance on your amp using a plate resistance number of about 7k and Mu of 20. Just keep in mind that the resistor is in series with the tubes and so it drops B+ voltage based on the current at the bias point.
I am assuming a low input impedance on the amp and so the calculated value is also on the low side, but this preserves operating voltage and overhead. The actual drive current required in a typical preamp output stage is very small, so even a loosely optimized WCF is plenty capable. When in doubt, use smaller values for the series resistor for line level. If we were trying to drive something needing lots of current swing like a bunch of parallel output tubes or headphones, we’d be pickier about Rseries.

Muchedumbre XL

If we break down the circuit into a cathode follower (upper triode) and a grounded cathode amplifier (lower triode), we can see that this creates a nifty push pull circuit. The cathode follower is non-inverting, so it’s pulling the output in the same direction as the input signal. The lower triode is a grounded cathode amplifier and so it inverts the input that it sees. But the input that it sees is from the anode of the upper triode, which is already inverted. You invert the inverted and you get non-inverted (same ‘direction’ as the upper triode). Tada!  Push (lower) pull (upper).

 

The difference between a headphone amp and a preamp

This is a question that, as a beginner builder, confused me quite a bit. While it isn’t too hard to understand why a preamp cannot drive power-hungry low-impedance headphones, it’s less obvious what separates an amp that can drive headphones from a low gain line stage. Headphone amps and preamps often share the same small signal tubes, usually Class A, and often single-ended.

Here are the modifications I would make to the El Estudiante headphone amp to make it better suited to line stage duty. While a purposely designed line stage might perform better, I can’t think of a way to do a halfway decent tube line stage any cheaper or simpler. If you don’t go mad on caps, this costs less than the headphone version.

Output Stage

Power requires both voltage and current. How much voltage or current required for a given amount of power depends on the load you intend to drive. Remember:

Power = Voltage x Current

But also:

Power = Voltage² / Impedance

AND

Power = Current² x Impedance

To create power into low impedance headphones, we need current. This drives a lot of design decisions in tube headphone amplifiers. Common approaches to create power are push-pull output stages (eg SRPP, White Cathode Follower), output transformers, and solid state power buffering. The Estudiante creates the power required for low impedance headphones using the latter approach: a single-ended CCS-loaded MOSFET buffer. At a 100mA quiescent current, it can make about 150mW into 32 ohms:

0.1A² x 32 ohms x 1/2 = 150mW

(note RMS = Peak / √2)

On the other hand, with a 10,000 ohm input impedance on an amplifier, this current is unnecessary because the maximum ‘power’ is limited by the voltage, not the current:

24V² / (10,000 ohms x 2) = 25 mW

Now we don’t really look at power output per se in line stages and we’re rounding up the peak output voltage as half the power rail voltage, but it’s obvious that we don’t need all the current to drive the input impedance of an amplifier because we’re limited by voltage anyways. Consequently, we can lower the current in the MOSFET output stage to something that doesn’t even require a heatsink, making a preamp build that much simpler and cheaper.

With the LM317 CCS, we calculate the needed set resistor as 1.25V / Iq (where Iq is the idle current). A resistor of 100 ohms will give us 12.5mA idle current, which should be plenty for a reasonably low output impedance, but not enough to need a heatsink (I would probably still bolt my TO220 parts to the chassis though).

linestage estudiante

In addition to lowering the idle current in the MOSFETs, we can change the big nasty electrolytic cap found in the headphone amplifier to a higher quality film cap. Electrolytics are great where you need a large capacitance in a small and affordable package, like the output coupling cap in a headphone amplifier, but electrolytic capacitors have been shown to create distortion at low frequencies (see Douglas Self’s Small Signal Audio Design) and exhibit leakage current that creates a thump on power down (which may just be annoying on headphones, but potentially damaging on a high power speaker amplifier).

For an input impedance of 10,000 ohms and a -3db point of 5 hertz, Our new cap size in microfarads (uF) is calculated as:

1,000,000 / (2 x Pi x 10,000 ohms x 5 hz) = ~ 3uF

A film cap of this size at a rating of only 63V+ is not hard to come by. I’d probably buy an assortment just to see if I could hear a difference. We should also increase the size of the loading resistor on the output from the 1k in the headphone amplifier to something like 100k or 1M so that we aren’t rolling off the bass or unnecessarily loading down the MOSFET output stage.

Finally, because we’re reducing the current in the output stage, our power supply requirement is relaxed, maybe opening up more wall-wart options to power the project. So if you’re looking for a simple, low-voltage, and cheap tube preamp option, modifying a headphone amplifier like the El Estudiante may be a good option. I’ve even used the headphone amp to feed power amplifiers in a pinch and it sounds surprisingly good.

Letters to WTF: why doesn’t anyone include tone controls?

Tone controls get (an undeserved) bad reputation in a lot of DIY hifi circles. They are very difficult to get close to technically perfect (eg exactly Xdb boost at all frequencies above Xhz) and they’re math heavy, so you don’t often see them fully detailed in audio DIY.  And in principal all the equipment we’re building is supposed to be perfectly flat and transparent, right?  Well that’s what the engineers say, but others might say that transparent is the enemy of fun. I would say that you don’t see tone controls because that’s just not how hifi “is done.” No, that’s not a good reason. And maybe the world needs a simple preamp design with bass & treble…

Here’s some good reading from Baxandall, the papy of modern tone control:

Here’s a good article from John Broskie on his Tilt Control board/kit:

That Tilt Control is a different take on tone controls, but I think it’s pretty elegant.  Broskie’s boards and kits are top notch too (not affiliated, I just have a engineering crush on him):

Ideally you’d sandwich this kind of tone control between two cathode followers (or one low gain stage and one follower).

Another Muchedumbre lives

I finished another Muchedumbre build with some slight variations.  This has two outputs and two inputs (easily switchable back to the 1+3 arrangement).  The power supply CLC filter uses all motor run caps instead of a mix of motor run and electrolytic. Other than these small tweaks, it is built as designed.

The wood apron is a very nice piece of walnut with a lot of prominent grain motion and color variation and the panel is inset rather than sitting on separate interior spacer boards. This is going to live a very happy life in Madison, WI.