[MilesDavis2]

My crack is stock and I changed the 6080 amperex to Tungsol 5998 and there was an improvement.  Also, with the 6080, there was a subtle hum at volumes beyond 1:00.  With the 5998, the hum was gone.  Just a quiet background.  Also with the 5998, noticed that the amp was just warm not hot on the top plate.

 

[Doc B.]

I would remind everyone that the way to do mods is to first determine what it is that you want to change sonically and then figure what you need to do physically to make that happen. I admit to having gone through many hobbies, buying some new toy and then reading all I can about how to mod it. I've messed up a lot of neat stuff in the process and also made a few really neat personalized toys. The ones that came out the best were ones that I took my time on, changing one thing at a time and trying it out before I changed anything else. The worst ones were often things where I piled on every mod imaginable from the start. In that scenario you can end up with mods that work against each other and you can't determine what went wrong unless you go back to the start.
 
The Crack is intended to be an educational journey as much as a (hopefully) enjoyable amp, and I encourage everyone to take that journey a step at a time and savor the process.

 

[RingingEars]

Quote:

No Speedball in yet, right? I understand the role of larger caps, but would the pot improve SQ as well, or just L/R balance?

Were both upgrades very easy, or quite a bit more complicated than the base build?

Right. No speedball yet. Not really sure what I'm waiting for to be honest. I was going to save it for winter weekend project, but that seems pretty silly at this point.
As far as the SQ of the pot? That's a good question that I can't really answer as I did the pot/cap install at the same time. I can tell you that the pot did make a difference in balancing the L/R at lower volumes so that's a plus.
As far as ease of the upgrades? Yeah, they're pretty easy. The pins on the pot are kind of a PITA so I recommend you hook the wires up to the pot before install. Also you will need to dremel out the key hole on the chassis a little as the stock pot key and the Alps key do not line up.
The caps are simple. I took a page from Beefy and installed "fly leads" when I installed the new caps. I should have done it when I first built the amp. So do like Beefy did and install them during the initial build.

 

[RingingEars]

 
Quote:

I would remind everyone that the way to do mods is to first determine what it is that you want to change sonically and then figure what you need to do physically to make that happen. I admit to having gone through many hobbies, buying some new toy and then reading all I can about how to mod it. I've messed up a lot of neat stuff in the process and also made a few really neat personalized toys. The ones that came out the best were ones that I took my time on, changing one thing at a time and trying it out before I changed anything else. The worst ones were often things where I piled on every mod imaginable from the start. In that scenario you can end up with mods that work against each other and you can't determine what went wrong unless you go back to the start.
 
The Crack is intended to be an educational journey as much as a (hopefully) enjoyable amp, and I encourage everyone to take that journey a step at a time and savor the process.

That's what's so nice about the crack Doc. It's enjoyable to build and to listen too.
I personally feel quite a bit of satisfaction every time I look at it and know that I put it together and when I strap on the cans it just amazes me. I listen to this more than my main stereo.
 

 

[captouch]

Question for all those that did the Axon cap grade. . .
 
Why 91uF?  Was there any theoretical calculation behind picking this value, or is it just the largest cap that would fit within the form factor?
 
I'm not sure what the default/stock capacitor value is, but I'm guessing the theory behind the larger cap is to have more stored charge immediately available for fast/large transients that require a lot of energy very quickly?
 
I'm just wondering whether this is way overkill for the application and another lower value cap (say 41uF just for example) would more than do the job.

 

[diego]

It's not for the energy stored. The cap forms a filter with the headphones and cuts off the bass. For instance, if you are using 300 ohm headphones,  you will have no bass below 5.83 hz with a 91uf cap but you'll have no bass below 12.94 hz with a 41uf cap.
 
As a reference, I have an amplifier with a similar output stage and 66uf caps and in my opinion it has very good bass with HD 800 but almost nothing with k701s.
 
Regards,
 
Diego

 

[MrDavis]

Because 100uF was not available at Parts Connexion.   And because that is what Beefy did...

 
100 uf  capacitors come with the kit.
 
Quote:

Question for all those that did the Axon cap grade. . .
 
Why 91uF?  Was there any theoretical calculation behind picking this value, or is it just the largest cap that would fit within the form factor?
 
I'm not sure what the default/stock capacitor value is, but I'm guessing the theory behind the larger cap is to have more stored charge immediately available for fast/large transients that require a lot of energy very quickly?
 
I'm just wondering whether this is way overkill for the application and another lower value cap (say 41uF just for example) would more than do the job.

 

 

[captouch]

Thanks guys for the education on caps. 
 
So if the higher uF is desirable for low bass extension and 100uF already came with the kit, then what's the benefit of going with the humongously sized, but lower valued 91uF caps?
 
Quote:

Because 100uF was not available at Parts Connexion.   And because that is what Beefy did...

 
100 uf  capacitors come with the kit.
 


 

 

 

[RingingEars]

 
Quote:

Thanks guys for the education on caps. 
 
So if the higher uF is desirable for low bass extension and 100uF already came with the kit, then what's the benefit of going with the humongously sized, but lower valued 91uF caps?
 


 

The size difference stems from the different materials used in the caps. The stock caps are electrolytic where as the AXON (or others brands everyone is using) are film caps. I couldn't tell you the science behind it, but evidently it takes more of the film than the electrolytic to reach equal values ie: microfarad and/or voltage.  
 

 

[Doc B.]

Quote:

 
As a reference, I have an amplifier with a similar output stage and 66uf caps and in my opinion it has very good bass with HD 800 but almost nothing with k701s.
 
Regards,
 
Diego

This is dependent upon the operating point as well as the cap value, since the op point determines the output impedance of the amp. As an experiment we just doubled the current through the 6080 in our lab Crack in order to lower the output impedance and we got quite nice bass with K701s. Before everyone jumps on this mod - we are letting it run for a while to see if the semiconductor devices and heat sinks we use in the active loads can handle the additional power dissipation.
 
 

 

[Beefy]

 
Quote:

Before everyone jumps on this mod - we are letting it run for a while to see if the semiconductor devices and heat sinks we use in the active loads can handle the additional power dissipation.
 

 
Must be pretty tough on the transformer as well?

 

[RingingEars]

I know it's already been done, but here's a pic of the cap/pot mod I did to mine.

 

[captouch]

Looks nice RingingEars. 
 
You still have the Speedball to go, right?  Anything else planned?
 
You may have said before, but what tubes are you using?

 

[RingingEars]

Quote:

Looks nice RingingEars. 
 
You still have the Speedball to go, right?  Anything else planned?
 
You may have said before, but what tubes are you using?

Yeah. Still need to do the speedball. Other upgrades will include a Neutrik locking jack, RCA jacks, bypass cap or choke, maybe a jewel light indicator. I would like to get the PCB style sockets too, but we'll see. I would also like to rewire the whole thing. My wiring is kind of helter-skelter.
I'm using an NOS RCA 6AS7G that I found cheap on that auction site and the original 12AU7 that  came with the amp. I'll probably start rolling others here soon.
 

 

[ShibaInu]

Off-topic, but where can one go to test tubes to verify they're good before putting them into the Crack?  I'm in Bay Area, CA (San Jose).  Radio Shack doesn't have tube testers, do they?