Not that I’ve had any shortage of projects lately, but they’ve all been workshop or home related. Now that the workshop is “done,” I’ve finally found a little time to return to tubes.
Many blogs ago, I posted about using switch mode power supplies (SMPS) in multiples to provide B+ for more typical tube operating points. That’s ‘typical’ as in 300V+, not the 48V of El Estudiante or other similar low voltage designs. I don’t see anything wrong with low voltage if it sounds good and meets a design goal, but it does hinder the tube and topology options.
A few posts later, and still stuck on the stacked SMPS idea, I proposed a tube that might do well as a linestage with only 100-150V B+. The 6AF4 is a 7-pin triode with a modest Mu and low enough plate resistance to drive amps with a 20kohm+ input impedance without the assistance of transformers or followers. In short, it could make a really simple preamp with a really simple power supply for a really modest cost.
And that’s what I did:
The power supplies V1-V3 are XP Power VCE05 48V modules on a small PCB with some small capacitance filtering on the output (10uF). The heater supply for the tubes (one per channel) is a small 12V Meanwell IRM-05-12. Total cost as of this post is about $45. Current cost on a Hammond 269BX (300V CT) is $55 and you’d still need to rectify and filter to make a workable supply.
With the low parts count, sleeper tube, and inexpensive (and simple) power supply, this is a great cheap and cheerful project. It also sounds quite nice in use so far. I’ll update with a more complete write up including operating points, construction tips, and listening impressions soon!
wauwatosa tube factory 
Nice Build, in terms of sound how does this compare to the el muche dumbre? I’ve been getting ready to build one so I’m curious.
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So far, the gain is obviously much higher and there is more warmth in the yet-to-be-named design. If I were picking something to be used with headphone systems (where gain usually isn’t needed), I would go with Muchedumbre. For speaker amps that like a full 2Vrms+ and/or phono-based systems, I think the new one has a lot of promise. I’m still listening and tweaking, but those are the initial impressions. I think the new one has more potential tweaks to the circuit as well if you like to fiddle (output transformers, negative feedback, SS follower output, etc).
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Thanks for the reply that helps, I would ask about it compared the Sofrito, but I already know I want to build that one because it has giant tubes of magic that look pretty. I just want to started with a simpler design first and then get to the Sofrito at a latter date.
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What do you mean by “sleeper tube”? What that it doesn’t amplify the signal?
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I’m using “sleeper” here in the hot rod sense. To put it another way, the 6AF4 is a tube that gets little attention but has some notable specs (~2k Rp, Mu <15 make it appealing for a simple single stage preamp).
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Excellent, Tim! Could be a good front end for any number of applications, even 30k input impedance chip amps. If one were into that sort of thing.
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