Building my Crack - Finally

[davew]

I've had the Crack kit with Speedball upgrade since last June (yes, over a year ago :-[) and I'm finally getting around to building it. I'll post my experiences here with pictures as I go. I've enjoyed listening to my home built tube amp for a few years and felt it was finally time to add a headphone amp. I'll start with a pic of my power amp (that could be another thread discussion entirely). I'll follow that up with pics showing my Crack build progress. As soon as I get an image converter to shrink the jpg files I'll post these pics and further updates.

I plan to start with a modded PSU. I'm going to go with a CLCLC setup. Assuming I can get both chokes in there. The chokes are Hammond 158M, same specs as Triad C-7x. The 20uF caps are Mundorf TubeCaps. The lead-in 470uF cap is a Nichicon PW series. I've worked this up in PSUD2 using the T1 and load R values from a post by grufti(?).

 

[davew]

Ok, here are some of the pics I promised. The first is my home built tube amp based on a JE Labs circuit once posted on angela.com. The second is the glue-up of my Crack base. I plan to use Cherry Watco Danish Oil to finish this. The third is a pic of the powder coated (textured black) chassis plate. I had the transformer bell housing powder coated as well. I found a local shop which only charged $25 and turned them around in 2 days.

 

[Paul Birkeland]

Wow, you got a deal on the powder coating! 

Let us know how the bits fit in, that's a lot to get in under the hood. 

-PB

 

[davew]

The powder coating was cheap since I chose one of their normally stocked popular colors. If I wanted something non-stocked, like black-RAL9005, that would have been a $100 min charge. I asked about the RAL9005 since that is also the normal color used by Front Panel Express for black powder coat.

I know fitting the 2 chokes and the 2 Mundorf caps may be a challenge. The dimension specs indicate the chokes will "just" fit across the transformer using its 4 mounting points. I'll use different length standoffs so the chokes are offset vertically a bit from each other. The 20uf Mundorf TubeCaps are rectangular bars, not cylinders, so mounting those with strong high temp double sided tape or velcro to the chassis plate should work. Fully protecting the leads with heat shrink in case of drop off, of course. One cap at the top between the safety ground and RCAs and the other on the left of the transformer below the RCAs should do the trick. I'll be sure to post a few pics when I get that stage completed.

That should still leave room for a future output cap upgrade. Although with my upgraded PSU and the speedball I suspect that upgrading the output caps may only be a small improvement, if noticeable.

 

[Nubtam94]

Either just the chokes or the caps... I have yet to find a way to fit a choke into my cramped Crack at the moment after i put a crossfeed circuit in as well...

 

[davew]

I did a first test fit of the chokes and caps for my upgraded power supply. Choke #2 mounts to the transformer forward bolt and the safety ground bolt. This is a tight fit on the spacers since the spacing isn't exactly correct. Choke #1 mounts the same way on the other side with a hole drilled (at the correct spacing) to mirror the safety ground hole. The chokes don't protrude past the box boundary using 1 3/8" spacers. Cap #1 (470uf 250V Nichicon PW) will be secured with a cable tie down bolted to the panel (leads insulated, of course). Caps #2 and #3 (Mundorf 20uF film TubeCaps) will mount with the same method but stand(hang) vertically (again, leads insulated, of course). Cap#2 slides right in between the two chokes and sits against the box. The caps could easily lie horizontal if I used slightly longer spacers. So, now three more holes to drill for the power supply cap tie downs. And there is still room for big output film caps so I may drill four holes for their tie downs as well to be ready for that future upgrade.

Yes, that is a PEC pot peeking from the bottom of the photo. I plan to use a shielded cable from the RCAs.

 

[galyons]

That looks like a very neat, workable layout.  I am going with a choke on my build...may consider 2!!

Cheers,
Geary

 

[Paul Birkeland]

I like that 470uF@ 250V cap in there, A+!

 

[davew]

In the PSUD2 sims I ran the 470uF knocked down the turn-on current spike (already drastically reduced by using chokes rather than resistors) through the first choke by 20% over the stock 220uF.

 

[Paul Birkeland]

In the PSUD2 sims I ran the 470uF knocked down the turn-on current spike (already drastically reduced by using chokes rather than resistors) through the first choke by 20% over the stock 220uF.

Are you using the stepped load feature to just analyze the capacitors charging?

Remmeber, the tubes won't start conducting for several seconds, so the power supply ramps up more-or-less unloaded.

-PB

 

[davew]

I didn't use a current sink. I was just using a resitive load as shown in my original post above and observing the current through the first series element to assess the stress on that component. Would using "Soft start" with the resitive load have the same end result as your current sink with stepped load? I did have soft start unchecked. Now that I think about it I probably should always be using soft start for tube circuit loads in PSUD2. Rerunning the sim with soft start shows no stress with the stock smaller cap. What parameters would you use for a current sink here?

 

[davew]

I've finished all the mechanical assembly. All the needed standoffs and tie-downs are in place for the mods/upgrades I plan to install. Power supply caps, PS chokes, Speedball and maybe even film output caps are lined up. I'll start soldering tomorrow. Still deciding whether to start all stock first or jump right in with all the mods.

 

[Grainger49]

Ah, yes, the all charcoal powder coated Crack.  It looks like it is going well.  I'm looking forward to the finished "works" side.

 

[jrebman]

Dave,

If I were in your shoes I'd build it pretty much as you want but without the speedball.  So do the psu and caps mods, get the thing working and play it a while, and then when it is working to your satisfaction, go back and add the speedball -- much easier to trouble shoot once you know the basic amp is working.  Besides, you'll get two pleasant surprises instead of just one .

HTH,

Jim

 

[Mosez]

Hi Dave - just wondering if you had made any progress building your Crack. I for one am very interested to learn how you ended up liking your power supply mod. Cheers.

 

[davew]

Here's a update on my progress. I made some changes to the PSU mods shown in my original post. I decided not to use the 470uF electrolytic cap in the PSU. I used all Mundorf MTubeCaps. A 47uF is in position 1 followed by a 30uF and then a 20uF. The 47uF is larger than the 20 or 30 but everything still fits! The Hammond 158M 10H chokes are still there.

I used the Mundorf M-Cap MKP 100uF 250V film caps to replace the output electrolytics. These took a couple of weeks since Parts Connexion had to order from their supplier.

I've completed assembly up to the output stage cathode resistors. I've attached 2 new pics. The first shows the chokes and PSU caps "spread eagle" at the moment to give me room to work. The second has the chokes secured and the caps sitting in place. I haven't secured the caps yet since I'll need to spread things out again for the final resistance and voltage checks. I am saving the Speedball for the future.

You'll see in the pics that I've used my own cloth covered wire throughout. I wasn't comfortable using just red, black and white everywhere. I tried to use a somewhat standard color scheme.

 

[Paul Birkeland]

Looks nice!  I tend to use that wire in certain projects when I want the vintage look.

I'd suggest installing 47K resistors at the input jack between the solder cup and the hot for each channel that will help the balance of the PEC pot at lower levels.

-PB

 

[davew]

I finished the build today. All I can say is OMG! Simply amazing. Absolutely no hum. There was a very noticeable background hiss though. That hiss turned out to be due to the stock Russian 12AU7A. When I pop in any other input tube the background hiss disappears and the amp is DEAD SILENT! So the stock 12AU7A goes to the junk pile/trash.

Now the real fun begins. Listening to all my music again.

I have several input tube choices to audition:
Sylvania JAN 6189
RCA Clear top 12AU7A
Amperex Bugle Boy ECC82
JJ ECC802S
and several 6SN7GTA and GTB as well. I will also soon have a RCA red base 5692 I just scored from eBay.

For the output tube I have several pieces of pristine 1955 RCA coke bottle JAN 6AS7G.

I just listened to a bit of Diana Krall - Live in Paris, JS Bach - Suite for Solo Cello #1 - Fournier and Gregorio Allegri - Miserere Mei. Nothing is broken in yet and it all sounds great.

I added a few new pics. First shows the stuffed box. Everything did fit. ;D The second shows it running with the stock tubes.

@PB - Thanks for the 47k resistor suggestion. I did notice an imbalance at the very low end of the pot. It's only the first 10 degrees or so. If I find myself listening at those low levels I will then add the 47K resistors.

 

[davew]

And here's a pic running with the RCA 6AS7G and Sylvania 6SN7GTB.

Now to just find time to finish the base.

 

[Doc B]

I think you may be giving up on that 12AU7 too soon. New tubes often need some run time, apparently to get the cathode emission up to the designed level. Try running that tube for 20 hours and see if the hiss is reduced.