The Nuvistor and Bob Katz’s Audio Blender (via Inner Fidelity)

Bob Katz has been writing a series of articles over at InnerFidelity for several years and they’ve recently taken a turn down a more experimental path. His most recent article details a device that mixes a transparent solid state signal and a Nuvistor signal biased to provide a distortion spectrum with just a small percentage of second harmonic. Check out his write up here!


A Nuvistor is a small metal and ceramic tube released by RCA just as transistors began supplanting vacuum tube technology in most electronics. They are a true vacuum tube with familiar triode operation and characteristics and an indirectly heated cathode. The most common Nuvistor in consumer electronics was the 6CW4 (high Mu) though there are several triode flavors and even a couple of tetrodes.

nuvistor cutaway.png

Because they were originally intended for radio and TV usage, Nuvistors enjoy very good bandwidth, low noise, and high gain (high Mu variants).  The metal envelope is integrated with the basing and the tube plugs into what RCA dubbed the Twelvar base. You can probably guess how many pins that had. With the Nuvistor, RCA also introduced the RCA Dark Heater, a lower temperature filament that guaranteed higher stability and less AC leakage. Despite this innovation, most Nuvistor heaters require around 1W to light (e.g. 150mA @ 6.3V).

The 8056 used in Bob Katz’s project has the following characteristics:

8056 characteristics.png

And the following very respectable plate curves:

8056 plate curves.png

With a modest Mu, low plate resistance, and very low B+, it’s no wonder Bob decided to marry this interesting tube to a solid state partner for his Blender. The 8056 heater requires 6.3V at 135mA. At this voltage and heater requirement, it’s close to being practical for modern portable devices. In their heyday Nuvistors were used in battery-powered and efficiency-critical applications like the US Space Program and military radios and communications equipment.

Would I ever build something with Nuvistors? It’s tough to say.  I’ve been on a casual hunt for tubes that might be suitable in a portable battery-powered application. Other candidates are the Korg Nutube or the sub-mini 6088. Like all things in this hobby, there are trade-offs. The Nuvistor 8056 heaters are hungry relative to these other options, but the other characteristics are very attractive. In all likelihood, I’ll try them all eventually. This is why I DIY.

Top 10 tips for PCBs on EasyEDA

I’m a believer in point-to-point construction, but not because I think it necessarily sounds any better. Point-to-point is simply the quickest and easiest way to try a new circuit. With tubes, high voltage caps, and through hole resistors, building without boards is straightforward (once you have some understanding of properly grounding circuits). When incorporating TO92, TO220, heatsinks, etc, a PCB starts to look much more appealing.

If you’ve been following my adventures, you’ve seen that I’ve been experimenting with PCBs recently. Most of my schematics are drawn in EasyEDA. EasyEDA is part of the JLCPCB family, where you can quickly import and order boards (you can also export Gerber files for purchasing elsewhere).  Based on my experience so far, and the advice of a cool dude name Matt, here are some thoughts and tips for working with EasyEDA/JLCPCB:

  1. Take advantage of the grid/snap spacing for layouts: I set my grid to 125mil (1/8″), the snap to 31.25mil (1/32″), and the alt snap to 12.5mil.eda grid
  2. Create m3/4-40 mounting holes: I make my holes 125mil (1/8″)
  3. Make all connections on one side of the board (where possible)
  4. Use the reverse side as a ground planephono.png
  5. Don’t be shy about creating your own parts (this is the PCB Lib function)!
  6. Don’t use standard TO92 packages; use TO92 ammo package footprintsto92
  7. Enlarge holes for wire-to-board connections and make the pads generous where you can
  8. My preference is to keep tube sockets off boards to prevent footprint issues, board flexing, and keep heater wiring flexible
  9. Avoid parallel tracks for anything carrying AC (signal or power): see Doug Self for good reading on coupling
  10. Add multiple hole connections for film caps (options for multiple radial and axial types)film cap

April/May has been a whirlwind in the WTF Amps household, but we should be returning to actual building soon. I’ve got about 30lbs of transformers and aluminum plates impatiently waiting for some actual hobby time to shake out of my schedule. Big boy mono blocks.

Tube news? Gibson files chapter 11

Gibson, the largest US-based company in the musical instruments business, is having a come to Jesus moment. After its early 2010’s binge on A/V properties (KRK, Stanton, Cerwin Vega, Onkyo, TEAC, etc) and subsequently ballooning debt, Gibson never really seemed to dig in with a new strategic direction to leverage the diversified catalog. It just doubled down with the Guitar Centers of the world (who is also not looking too hot these days). One has to wonder exactly what the strategic play was. In any case, several product/line missteps and steadily declining revenue from 2014 onward has brought us here.

As someone working in the MI industry, it’s difficult to resist the urge to wax poetic about Gibson having lost its way. Whether it was a miscalculated pump and dump deal or Juszkiewicz spending too much time in the varnish room, there’s a silver lining. Gibson has vowed to re-focus its efforts on guitars (and maybe amps). In an economic environment as interconnected and multipolar as today’s, that’s probably the right solution.

So I’m taking bets on whether Gibson will be relaunching their amplifier line. Here’s a great selection of schematics. We’ve seen PRS making similar moves to complement their guitar line in recent years. Will we see a new Skylark from Gibson?





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.


Class A FET power buffers

I’ve been kicking the hybrid amplifier can down the road for quite a while. In essence, I’m looking to do a bigger EL Estudiante. An amp capable of driving speakers to a dozen or two watts using a MOSFET follower output stage for current gain and a tube handling the voltage gain. While this is not especially difficult on paper, making something that is an interesting and practical alternative to tube output stages is not necessarily so straightforward.

On one hand, one should consider the target user. Most tube enthusiasts do not need so much power, so we can bias in Class A and avoid a whole lot of AB headaches and worry about bias adjustment, crossover distortion, etc. This has to be balanced against heatsinking and thermal considerations, of course. Coming from the world of tubes, our audiophile anti-bodies have already pretty well encysted any commonsensical tendencies we may once have harbored, but the smoke point of drywall hasn’t changed. That is to say solid state does not magically make Class A cool, efficient, or pragmatic, but why make something hotter, more wasteful, and more burdensome than vacuum tech? We’re hoping to make something that is more than the sum of its parts.

ccs load se

The Aikido Hybrid 16W SE by John Broskie has about as much power as most probably need. Sixteen is two to the fourth watts, so four times three decibels per doubling of power for a 12db increase over nominal speaker rating (eg your 90db speakers peak at 102db). The quiescent current is a very serious 2A. It is a single-ended MOSFET source-follower loaded by a current source. There’s little not to like other than the coupling caps (contrast this with output transformers for AC coupling in full tube amps). See also Rod Elliott and Pavel Macura’s CCS-loaded follower here.

inductor load se

We can make single-ended more efficient with an inductive load.  Just like a choke load with tubes (eg Luciernaga), an inductive load on a MOSFET lets it swing voltage past the power rails.  The MoFo is an example of a single-ended source-follower MOSFET with a simple passive choke load (50-150mH and very low DCR). This is much more efficient than an active CCS in an absolute watts dissipated sense (note the much lower supply voltage), but chokes ain’t cheap and you still need a good dose of current.

diode bias pp

If you want to lower the quiescent current needed, but stay in Class A, push-pull source-followers are the way to go. Papa Pass’s F4 power buffer does exactly this to cut the quiescent current needed in half. The need to match FETs may be unappealing, but compared to his single-ended F3, F2, or Aleph J, the power delivery into 4 ohm loads is much improved. Note Broskie suggests the same push-pull MOSFET approach (with a different biasing scheme) in the Moskido amp design.

Any of the above would probably sound pretty good: like a tube feeding a very transparent solid state amp. If the amount of power you need is modest (and it probably is if you’re a tube enthusiast), these approaches have made very nice speaker amps. Hopefully I’ll have my own design to contribute soon. MOSFET followers would also make a great multi-watt amp for low sensitivity and low impedance headphones, like the HIFIMAN HE-6.

The HE-6 are rated at a sensitivity of 83.5db at 1 mW.  With 1000x more power (1W), we’d make 103.5db (a 20db increase). Around 5W input makes it a cool 110db. Coincidentally, this is the power rating of the amp HIFIMAN recommends as a pairing.  The HE-6’s 50 ohm impedance lowers the quiescent current needed in a MOSFET output stage, though still requires a voltage rail high enough to prevent clipping. A quick approximation for voltage would be:

power = Vrms²  / impedance

5W x 50 ohms = 16 Vrms²

16 Vrms x √2 x 2 = 46 Vptp

So about a 48V power rail (or +/- 24V) gets us in the neighborhood. Doing the same for current:

power = Irms² x impedance

5W / 50 ohms = 0.32 Irms²

0.32 Irms x √2 = 0.5 Ipeak

We only need about a 48V power rail and 0.5A quiescent current per channel to get us 5W into a 50 ohm load if using a CCS loaded MOSFET. If we choke load, cut the 48V in half. If we use Class A push pull, cut the current in half. The heatsinks aren’t going to be tiny, but a desktop size amp isn’t out of the question.





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


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.

DIY amplifier top plate easel

I wanted a better way to wire my amps and had been lusting after Decware’s amazing assembly room for a while.  While I can’t afford the custom extrusions and equipment that Decware has, I can get creative with common materials.  I decided to try a simple easel based on a couple of rails of t track and some standard angle and rod extrusion.

The basis of my easel is two 2ft sections of t track. Finding a 4ft section with bolts and knobs on sale at Rockler was what pushed me over the edge to build this daydream. The t track is 3/4″ wide by 3/8″ deep. Rather than trying to route a channel in a thick board, I sandwiched the t track in scrap with 3/4″ thick scrap under the track and 3/8″ to form the outside. These were glued and clamped face down so that I could be sure the t track would be flush with the top edge. 

My bench is cantilevered from basement joists with 2×6 vertical supports. I ran a 1/2 aluminum rod between the supports to provide lateral movement and adjustment to the t track rails. The rails simply have a 1/2″ hole through which the rod passes. By mounting the rod 12″ from the surface, the 24″ rails give a 30 degree angle. This leaves plenty of clearance for transformers and is comfortable to work on while standing or sitting). I purchased some 1/2″ washers and collars, but they may not really be necessary (I’ll find a use eventually). 

My horizontal ledges are 1/2″ by 3/4″ aluminum angle (3/4″ side is flat against the t track rails). I used self adhesive cork sheet to protect plates in the easel from scratching on the support. Depending on your knobs or wing nuts, you may have to trim some of the horizontal rails so that they can be completely tightened. I cut my rails to fit a 18″ wide top plate, but aluminum extrusion is cheap if I ever want to build something wider. 

All in all, this is a handy and relatively simple addition to my tube amp building bench. And it is a lot cheaper than custom extrusions or lab fixtures!


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.