Crack w/SB - 50K stepped pot.

[NightFlight]

Possibly getting my hands on a Khozmo Acoustics 48 position stepped ladder attenuator as borrow/gift. The pot is worth as much as the retail Crack, so I'm apt to make it work in the circuit if possible. Can I get away with it, or do I have to make a change to get this load to work correctly on the output?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg.canuckaudiomart.com%2Fuploads%2Flarge%2F511711-khozmo_acoustics_48_position_stepped_ladder_attenuator.jpg&hash=91c1b9b33e8aea3be2ca013f155145c437a1f455)

[galyons]

50k won't be an issue.  I use a 50K Goldpoint in mine.  The size of the Khozmo may present some challenges with the tight'ish corner location.

Cheers,
Geary

[NightFlight]

Thanks for that input. Which headphones are you using?

And... yes indeed it may be snug. Getting the dimensions soon to see if its even possible.  However, the fight may be worth it.
I've been working it over in my head and coming out the face of the unit breaks the ease of future mods.

[galyons]

With the Crack, I am vintage....AKG K240 Sextett's and K140's.  Also have AKG K702's, but not a great match with the Crack, so they get a transformer output 396A based amp.

Cheers,
Geary

[NightFlight]

Well, I must report this Khozmo 48 step ladder attenuator is a huge success.  I've only a couple hours of listening on it, but the difference (for me) was nothing less than jump out at you and smack you in the face type experience.  Nothing changed in the sense of overall parameters, but the detail and realism top to bottom is fantastic.  Whole new instruments previously buried by the noise floor came up for air!  Its so clich-eh, but there you have it.  I couldn't be happier with this upgrade.   

On the other hand, it wouldn't be the type of upgrade that someone who wasn't familiar with my amp would necessarily notice without a prior reference point.  Still, I wouldn't hesitate to recommend a good stepped ladder pot for every BH product. I think best appreciated after getting to know the crack for a few months like I have.

It was a bit of a challenge to fit into the existing pot position, but it IS possible. Without the speedball it would have fit without issue, but the left channel A/B board for the speedball is in the way and had to be moved about 5mm left. I'll have to find a way to custom mount it, which shouldn't actually present too much of an issue.

[JamieMcC]

And I thought squeezing in my Valab was tricky.

Congrats for getting it to fit, I am pleased your efforts have been well rewarded with the improved performance.

[NightFlight]

I stuffed a couple Mundorf 250V 100uF MKP in there too. Now I have hold it by the transformer and drop it back in the wood case straight down. It's snug so I have to shove it in the last bit of the way. Ummff.... ummff. 

That and a Rogers 12BH7A D-getter in the input have really started things cooking.

I'd post a couple pics, but I'm too busy listening.   

[JamieMcC]

Awesome keep us posted on your impressions of the mods.

[NightFlight]

I think the Mundorf have provided significant improvement in terms of dynamics. In some cases they're even a bit much.  The panasonic electrolytic caps that were stock weren't to bad as the improvement the film caps bring is nice, but not a huge leap. However, that could just be the break-in period since I've only managed to get ~20hrs in on them.   The 12BH7A I had in there (I've reverted to my CV4003) is most definitely out of spec in a crack even with the speedball. While nice - the bass is bloomed and distorted - very 'tubey' sound - lush. The difference in overall cohesion once I returned to the CV4003 was very obvious.   I wish I knew how to tweak the 12BH7A parameters, its otherwise a very promising tube, it seemed more tactile and musical. The CV4003 has a fantastically 'correct' presentation - but its just not as engaging.

As for the Khozmo... I've adjusted to it and now it  just seems normal in terms of the added resolution it brought.

Additionally I've added a 12v cpu fan running at 7v (to slow it and run quiet) under the crack. The internal ambient temp was hitting 80-90C and my film caps are rated up to 85C. The fan has dropped the ambient temp to 40-45C.

[JamieMcC]

I wonder why your Cracks runs so hot. Mine gets no where near those temperatures. Do you have the Speedball installed which does away with those big hot resistors?

[Paul Birkeland]

Do you have the Speedball installed which does away with those big hot resistors?

Those big, hot resistors are replaced by big, hot heatsinks that dissipate the same power.

90C is pretty toasty, this could partially be attributed to the BH7, depending on the voltages present. 

[JamieMcC]

Hi Paul out of interest I just checked with my infrared thermometer.  My Cracks been running now for 3hrs 45 mins and the maximum temp on the hottest part of speedball heat sink was 61

[Paul Birkeland]

Yeah, if the metal tab on the TIP50 is 60C, that's dandy!

Seeing 90C inside the amp would make me scratch my head about where all that heat was coming from, though perhaps removing the rubber feet and other unusual circumstances could drive the temps up.

[Doc B.]

I'm curious - how was the ambient temp measured?

[JamieMcC]

With a cheap indoor outdoor temperature and humidity gauge which has a wire with a probe you can stick outside the window funny thing when both sensors are inside it always read half a degree difference. For ambient I was using the rooms air temperature.