Evolution of a tube headphone amplifier

[BuzzK]

Layout

I set the parts on the top for general placement. I'll put down some graph paper and make a hole template.

 

[kward]

THAT is looking sharp.

 

[GordonW]

Is there a 9 pin electrical equivalent to the 6SL7? I know some folks use a 12AT7 but it is hard to find quiet ones.

Buzz

5751. Gain of 70, like the 6SL7, similar transconductance and plate impedance too. Lower capacitance is the only significant difference (better).

Jon, I have some of those, I think, if you need one.

Regards,
Gordon.

 

[EngineerNate]

Subbed. I love my little Millett hybrid. It'd be fun to hear something 100% tube powered some day.

 

[BuzzK]

5751. Gain of 70, like the 6SL7, similar transconductance and plate impedance too. Lower capacitance is the only significant difference (better).

Jon, I have some of those, I think, if you need one.

Regards,
Gordon.

Thanks for the idea and your generosity. I have some 5751s on hand. It'll be interesting to compare the plate curves. I kind of like the larger tube flanked by the two smaller ones.

 

[BuzzK]

Drilling & punching the chassis

I checked the curves for the 6SL7 and the 5751. The plate resistances are different and the 6SL7 looks a lot more linear. I'll stick with that part.

I made a rough drill template and drilled the holes. The largest holes were punched out with a cheap punch set (I like Greenlee but didn't have them in the odd sizes). The capacitor hole is a bit too big, but I think that I have a fix for that. Although the cap measures 1.2" diameter, and the punch is spec'd at 1.25", the hole it makes is larger.

I also used a progressive bit to ream out some of the holes and to chamfer. It's one of the most useful sheet metal tools for this kind of project.

Everybody hates sheet metal work, me included. ;-)

 

[BuzzK]

Supplies Day

Today I'm planning to go to the local hardware store to get an O-ring or other kind of gasket for the capacitor cut-out.

I've ordered some capacitor clamps from eBay via Hong Kong to fit the 30 mm / 1.2" can size. They will come in handy for other projects, too.

Just realized that I'm out of shielded (coax) small signal cable, to connect the input jacks to the input potentiometer, etc. I contacted Surplus Sales of Nebraska (a great source for lots of stuff) and they are in the process of moving.

So I'll just have to do what I can until the rest of the parts arrive.

 

[gadget73]

I have an assortment of grommets that I got at Harbor Freight that work nicely for this sort of thing.

 

[BuzzK]

Let the Wiring Begin!

Doing the amp section before the power supply.

I think I have all of the parts now.

 

[BuzzK]

Slow and steady on this one.

The audio section is nearly finished. Next, the power supply and then cosmetics and final touches.

 

[BuzzK]

Finished!

It's surprisingly quiet - it has over 1K µf of filtering in conjunction with a 12 Hy choke. The output is 250 mW at 0.68% THD.

Thanks again Dave G. for your help.

 

[boza]

That looks awesome.
Great job

 

[BuzzK]

Just for fun, see are some pics of the "original" version made from junk box parts. (This is why I call it the "evolution" of a headphone amp).