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!  

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.

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!

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!

Another batch of boards

Another small batch of prototype boards for projects has arrived. I’ll test these as soon as I find the time to stuff and test the last batch of experiments…

dc heaters
Simple 6.3V to 12V regulated heater board (LM7812 based), good for phono builds

mini booster
A miniature DC booster board with on-board USB charger for battery power B+

opamp phono
A simple opamp phono preamp powered by a couple of 9V batteries just because