No workshop, no time, what do?

Having recently moved and had a second kid, tubes should be the last thing on my mind, right? Maybe. Then again,personal hobbies should always have a place in day-to-day life, and especially when that life gets a little chaotic. Such hobbies are about exploration and continuously challenging oneself. And yes, sometimes a little bit of dorky obsession.

Hence, with all my tools in boxes and precious little spare time, I have a great opportunity to revisit quick, cheap, and cheerful tube design (at least this is what I tell myself).

Last spring I posted about a sleeper tube that looked to me like it had potential in a simple linestage. The single triode has modest B+ requirements, low Mu, and respectable transconductance, exactly what we might look for in a bare-bones capacitor coupled preamp. To boot, the tube is called 6AF4. AF. How could I resist?

Here are the plate curves, highlighted where it looks pretty good to me:

It looks like we could work with as low as a 100Vdc supply here, but plate resistance appears lower and more consistent if we have 150Vdc on tap. With 150V B+, a 7.5k loadline passes right through the middle of my target zone of operation, intersecting the y axis at 150V/7500 ohms = 20mA. A 220 ohm cathode resistor looks like a good place to start and should allow a dozen or so milliamps through the triode. The result, fleshed out, looks something like this:

Back in my original post on the topic, I mused about 48V switching supplies wired in series to generate a B+. That’s a monkey I still haven’t gotten off my back, so I drew up a little PCB based on the XP Power VCE05 module. This is a $12 encapsulated AC/DC converter that puts out 100mA+ at 48V. Three in series gives ~150V in a very compact footprint and at a cost comparable to a small Hammond EI transformer.

Although a pair of 6AF4s will only need 25mA or so, a pair of these boards would (hypothetically) be capable of powering a small power amplifier (300V @ 100mA). It’s worthy of some exploration.

A cheap sleeper tube thought experiment

One day I was combing through tube characteristics using the parametric search function in the Tube Data Sheet Locator desktop app and I came across an interesting 7-pin triode. As luck would have it, I happened across a couple at a swap meet about a week later when the tube type was still fresh in my mind. I bought them, thinking they might be interesting to experiment with.

The 6AF4 is a 7-pin indirectly heated triode. I found the 6AF4 interesting because of its fairly low amplification factor (Mu of 15), decent transconductance, and high perveance. These characteristics suggest that a really simple and fairly low voltage preamp may be within reach. Check out the datasheet here (link to PDF).


I’d power this with a pair of 48V SMPS in series and wire the two 6AF4 heaters in series to be powered by a 12V SMPS. This would bring PSU costs and size down significantly. Voltage gain here would be about 10 and output impedance should be a little under 2k ohms.

Because resistor loading will still provide a lot of gain, even with a low Mu of 15, this version uses a matching transformer to step down gain and output impedance. Voltage gain here would be 2 or 3 and output impedance should be a couple hundred ohms. Edcor makes affordable candidates for output transformers in this application (WSM series). I’d also look for second hand matchers in the 2:1 to 5:1 range.

As far as a loadline and bias point, the blue blob area looks pretty good to me. Note how low the supply voltages are here. These low voltages make parts both cheaper and smaller. While I like the lower output impedance and fanciness of parafeed, the resistor load would probably sound a little tubier. It would also be simpler, cheaper, and more compact.

Whelp, I seem to have talked myself into it…