6SN7 push pull flea amplifier project

[Shef]

Yeah, back-to-back-with-1M I tried in my previous project, those were 1000uF, matched. I specifically tested this poor man's trio against a non-polar 82uF x 100V, the latter won giving slightly less THD, about 0.1 difference, no brainer.
82uF turned out be sufficient up to 10-12Wa output. The voltage should be twice of cathode Bias value for the particular used quiescent current.

[Lingwendil]

Got my chassis yesterday, and pulled out the tangled mess of wiring prototype to see how the transformers fit. The box is an aluminum 10x8x2.5" Bud industries AC-1418, I had some credit on Amazon, so I grabbed it. Looks like aa good size box, and if using smaller tubes with top mounted output transformers, you could probably squeeze an EL84 or similar push pull amp into it without too much fuss.

I see I will have enough room for sure, but unsure how to go with the transformer orientation. With such low power levels, would I be fine with the transformers all on the same plane, or should I plan on mounting the outputs sideways as shown? Sideways would be easy to do, and give more room.

Also, power switch will be one of these-



Would it be better to hug the chassis with the wiring the same way I would my heater wiring, tightly twisted, or would it be better to use shielded cable and just go straight to the front panel? I plan on putting the switch center left or dead center on the front panel.

[Shoog]

In my spud I actually mounted the two OT on the same bolt stacked wise. No noticable crosstalk can be detected. These toroidals are robustly immune from stray pickups.

Shoog

[Lingwendil]

Originally Posted by Shoog

In my spud I actually mounted the two OT on the same bolt stacked wise. No noticable crosstalk can be detected. These toroidals are robustly immune from stray pickups.

Shoog

Hmmmm, something like the attached picture then? Maybe I can squeeze an IEC inlet in the ccorner between the trafo and the chassis...

Picture turned sideways for whatever reason

[Shoog]

Originally Posted by Lingwendil

Hmmmm, something like the attached picture then? Maybe I can squeeze an IEC inlet in the ccorner between the trafo and the chassis...

Picture turned sideways for whatever reason

Yep, works a treat.

Shoog

[Lingwendil]

Quick layout for sockets and power switch. I like this size for the chassis. Definitely going to grab another one for another project.

[Sodacose]

Oh man, this is the kind of project that makes diyaudio awesome. Thank you for sharing!

If you've got measuring gear, I'd love to know how those output transformers behave at the extremes. I've thought about this kind of thing several times (also inspired by Shoog's crazy exploits), though never with the micro watt tubes.

[Lingwendil]

Originally Posted by Sodacose

Oh man, this is the kind of project that makes diyaudio awesome. Thank you for sharing!

If you've got measuring gear, I'd love to know how those output transformers behave at the extremes. I've thought about this kind of thing several times (also inspired by Shoog's crazy exploits), though never with the micro watt tubes.

One of my goals for this project was that it needed to just work without adjustments, it needed to be reasonably affordable, and it needed to use easy to acquire and substitutable parts.

So far the cost is at-

Resistors- already had on hand, junkbox (maybe $2)

Capacitors- already had on hand, junkbox (maybe $5? Depending on how crazy you get)

Sockets- already had on hand, junkbox (about $7 shipped, eBay)

Chassis- $22 shipped (Amazon, mouser, digikey)

Transformers- $55 shipped (Antek)

Tubes- $21.50 shipped ($2.20 each before shipping, rutubes.com used for example)

Power switch- $3 shipped (Amazon)

IEC socket- already had on hand, junkbox ($2? I take mine from dead computer PSU)

RCA jacks- already had on hand, junkbox, ($2)

Speaker jacks- already had on hand, junkbox ($2)

$121.50 if my math is correct, not bad considering, but that's the amount you would be spending to just go grab everything now. The only parts I had to go out and buy specifically for this build are the output transformers, power switch, and chassis. Everything else I already had on hand, as the power toroid I had on hand from a project I switched directions on. I'm using Russian equivalent tubes and they work great at these voltages, but if you wanted to crank more current through the output or run much more voltage, I would go with 6SN7GTA/GTB types so you can run higher idle current at a higher supply voltage.

For anybody wondering, this would make a killer headphone amplifier, especially for power hungry 16-32 ohm headphones...

[Lingwendil]

Bored at work, but have my laptop with me, so I spent a bit of time playing around with the Push-pull Amp Cad program from TubeCad. Interestingly, my estimates were pretty darn close! looks like just under a watt, and from previous experience, this program is surprisingly pretty accurate, usually gets most values within 5%. Both of these are running on 300 volt supply, and using a shared cathode resistor, but that shouldn't make a terribly large difference, and if anything the individual cathode resistors of the finished amplifier should give lower distortion than a shared resistor.

This was simmed using the 16928 ohm transformer values, and interestingly doesn't reduce 3H when doubling the transformer impedance. I of course will try to get real measurements once I am able, but it looks promising so far. I have a feeling once the feedback is wrapped around the little guy it will measure better than simmed. I've also attached the sim results from a 6V6 triode connected pair with an 8k OPT, and signal levels adjusted to give similar output power. Not too shabby for a single dual triode to do this well against the venerable 6V6 for a low power design. I will play with higher voltage too, as it looks like just around ~2 watt is possible with 375 volt supply. I might grab a pair of GTA type tubes to play with running them at higher dissipation, as the russian tubes are more or less equivalent to the 6SN7GT types, and not the higher spec GTA/GTB types.

[Shef]

In my flea-PP project I got 3rd almost disappearing with the output of about half watt per channel, but the tubes are smaller. At first I used the TALEMA toroids as OPTs, but they sounded acceptable ok with headphones only.
But then I wanted to make it able to drive speakers and came across the very inexpensive but well-German-made TrafoBlocks FL6/9 about $4 apiece on evilBay (took 4) incl shipping, and those produced about 0,3 non-distorted Wa on speakers - already enough for background listening at the office as that was the purpose.
I'll create the topic once I get evr together to some presentation level.

BTW Trafoblocks are C-core-like design (laminated though), i.e. identical primaries and secondaries well separated and 2KV hot-pot tested vs bifilar TALEMA's windings, think of this. No wonder these sounded better.