6sn7, 6j5, 76's?

[ALL212]

After careful consideration of making a two tube unit into 4...I'm a gonna stick to no more than 3.  I'll leave the 6080 alone.

Maybe the next build!

[Tom-s]

The 76 has a very nice sound to it! Holo-graphic, filled, easy listening. Pleasant bass, spacious mids, soft highs. Like it a lot! I've tried the Sylvania and RCA (VT-76 JAN version) variety in my Crack. I'm not good at this kind of descriptions, they have a "bigband" kind of sound to them.

Not the most Hi-Fi type tube ever heard in Crack, but a very pleasant surprise!

Beware, very susceptibel to trafo hum, environmental sounds, some are a little microphonic (but all very listenable).
Must be because of the adapter stack  .

Edit: I have to note that i also tried the 37 tube (RCA). And this one aint bad either (little more "dull"). The 76 has something extra and is real nice!
It could be the operating points differ. Will check later on.

[attmci]

 ::)The crack is still a crack no matter how many tubes you installed. 

[ALL212]

Would someone check my homework?

If I go to dual 6J5's from the 12AU7:

12AU7 Pinout             6J5 Pinout

Plate A      1              3   Plate Tube 1
Grid A       2              5   Grid Tube 1
Cathode A 3              8   Cathode Tube 1

Heater     4,5,9         2,7 heaters, tube 1 and 2  (Split the wiring at 6080 socket)

Plate B      6              3   Plate Tube 2
Grid B       7              5   Grid Tube 2
Cathode B 8              8   Cathode Tube 2

[Tom-s]

You are correct.

https://www.radiomuseum.org/images/tubeenvdiag_klein/6j5.png

Maybe PB has some insight on connecting pin 1 of the 6J5's to ground.
Might help in getting all as silent as possible.

[Paul Birkeland]

I think pin 1 connects to the metal ring around the base.  I would certainly ground it if you can, and certainly don't connect it to the plate!

[ALL212]

Roger that!  Ground pin 1.  Stay away from pin 3!!!

Thank you.  Kit is expected any day now.

[ALL212]

After thinking this over a bit I think I'm going to stick with the 6sn7 in the pre spot and leave the 6080 alone.

I can use adapters to go back to 12AU7's and also another adapter to go to 6J5's.  Could even be adapters for 76's.

What I've found is that all but 1 of my headphones are 300 ohm.  And the odd one is 30 ohm or less so we're not going to talk about him.

I've read on this forum about increasing the last power supply cap - maybe doubling it  - so that the coupling caps can be much smaller than 100uf?  Maybe down around 50 uf?  That would assist in the budgeting aspect of putting film caps in that spot.

Is there an optimal value in the last power supply cap and the output caps for tuning the Crack to 300 ohm cans? 

At this time I've got a choke here so power supply will probably be CRCLC.  I previously found a post with a chart showing the 3db roll offs with different loads and output caps but of course, I can't find it today.

I have purchased the Speedball but I'm planning on leaving it out on this build until I've had plenty of listening time.  I've not done that before and I'm curious so it might even stay out.

I also stole this from a 2013 post - still applies?
"Now, if you're going to decrease the output coupling cap, you can increase the 2.49K loading resistors a bit.  If you're going to have about half the capacitance, then let's push those up to 4.7K resistors, which gives you a 530 Ohm load (yay!). This gets you down to 60uF, though I suspect anything from the mid 50uF-70uF range is going to work well."

So...short version.  Is there an optimal power supply and loading resistor setup for 300 ohm cans that involve lessening the size of the output caps so that I can save some room and possibly put in higher grade boutique caps?

[Paul Birkeland]

It certainly wouldn't hurt to put a 470uF 250v power supply cap in as the last cap.

The Crack-a-two-a will accommodate just about any 100uF film caps you'd like to use without any space constraints. 

[ALL212]

thanks Paul!

Can I use a 50 - 70 uf cap for output then?

I've got the Crack-a-two-a and I did max it out - Clarity PX 100uf caps take all available space!!

[Paul Birkeland]

70uF should be OK on the 300 ohm cans.