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.
I used a 48V switch mode power supply in the El Estudiante headphone amp and am pleasantly surprised with the quiet background and relative simplicity. When it comes to higher voltages though, you will not find many AC/DC switch mode power supplies at vendors like Mouser or Digikey. While a beefy low voltage DC supply could feed a DC/DC booster (see Millet’s 10W booster project and various eBay listings), I’d like to find something that is more easily repeated by others with parts from major PSU manufacturers.
I came across the idea of stacking low voltage SMPS supplies in a couple places and the idea intrigued me as a scalable and affordable approach to creating B+ (see here and here). The Meanwell EPS-15 48 are regulated and isolated AC/DC supplies that sell for about $8. Stacking six of these would supply 300mA at about 300V.
This schematic shows the general outline of what I’d like to try with a single-ended amplifier. The V1 supplies produce the anode to cathode voltage for the output tube. These are referenced about 100V above ground by the V2 supplies. The input tube’s B+ is a combination of the V2 and V3 supplies. All in all, this would call for seven 48V supplies, plus another low voltage supply for heaters. The final cost would be somewhat less than a traditional transformer and CLC filter, but there are more interesting reasons to try this.
All the supply gymnastics make it very easy to direct couple the two stages. In this example, Q1 is a gyrator load and Q2 sets the reference voltage. This would also allow us to drive the output tube into A2 operation. My choice for output tube here would be a 6V6: a really sweet sounding triode that otherwise doesn’t produce much in A1 operation. Power would still be low (around 2W), but that would be plenty for headphones or enough for high-efficiency speaker systems.
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:
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:
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:
Pete Millett’s Starving Student was one of the first amps I ever built completely from scratch. Unfortunately, the 19J6 tubes have become rare (or at least no longer dirt cheap) due to all the bright eyed DIYers scooping them up to build amps. I think the world needs another <50V tube amp for beginners, so I’m designing one. Like the original, it’s an oddball tube with a MOSFET buffer and an off-the-shelf power brick (same brick, in fact).
Millett is one of my personal tube heroes. This is a tribute. Full write up coming soon (and parts values subject to change once tested).