A Papa Rusa lives and I got a new toy

Reader CF sent me some pics of a gorgeous Papa Rusa build he recently finished:

I love the sleek case with functional heatsinks for the 5C45’s anode CCSs. It is a pretty compact build, judging from the proportions. CF used red LEDs to bias up the tube cathodes, too, and reports back that they sound great. Sweet build that I hope makes lots of great music for a long time to come.

I also recently got a new toy of my own for the shop:

This 14″ swing Delta (circa 1950s?) replaces a much more recent 10″ Craftsman drill press. The Craftsman had a lot of projects under its belt, but I’ve been wanting just a bit more swing and a longer quill travel for a while now. The extra reach will be a help with larger top plates and the extra travel means I’m not moving the table so frequently as I switch from twist bits to hole saws and forstner bits.

With just some setup and test cuts so far, I’m loving this older machine. Everything is made of metal and on a nice beefy lever or set screw (of course that means it’s also heavy AF). Recently rebuilding the shop was a great excuse to finally pull the trigger on a sweet vintage press. Now it just needs a modern table and fence…

A (gorgeous) Papa Rusa in the wild!

I just received some pictures from builder MG of his Papa Rusa amplifier. He’s used some beautifully figured wood to build a striking enclosure for his Papa Rusa 6S45Pi parafeed amplifier. It has a cool instrumentation vibe to it with the subtly angled front panel and exposed transformer and heatsinks.

Great build, MG!

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…