Tube Rolling w/Crack

[Laudanum]

It should be the 7236 that is the source of the higher gain (or less loss of gain) compared to 6080 or 6AS7.  I believe the 12BH7 is about the same as the 12AU7 or close.

I dont have much experience with them but I never thought that stepped attenuators would be a great idea for heaphone amp unless they have a lot of steps.
Anyway,  this link from the Goldpoint site should take care of it ....  http://www.goldpt.com/mods.html

[gh0st]

It should be the 7236 that is the source of the higher gain (or less loss of gain) compared to 6080 or 6AS7.  I believe the 12BH7 is about the same as the 12AU7 or close.

I dont have much experience with them but I never thought that stepped attenuators would be a great idea for heaphone amp unless they have a lot of steps.
Anyway,  this link from the Goldpoint site should take care of it ....  http://www.goldpt.com/mods.html

good call, thanks

I have a DACT stepper in my balanced beta22 which works a treat with 24 steps, but then the amp is configured for low gain, and the 'phones aren't super sensitive (LCD2). I believe the main advantage is balance and tracking across the 4 channels when using tight-tolerance SMD resistors. Less of an issue in a two-channel amp though, I just liked the look of the TKD when I was looking for a new one, I think the steps are mechanical anyway, someone might correct me, but I think it's a normal pot underneath.

[Laudanum]

You arent the only one using a stepped attenuator in Crack so it must work fine for those using them.   I would think that the pre-attenuation that I linked would take care of your touchy control, or atleast help.   I know that I am always just nudging that Crack volume knob, so I figured a stepped atten. wouldnt be a great choice for me personally.   I guess a lot of it would have to do with how hot your source is, how sensitive your phones are and how tweaky someone is when it comes to the volume.  And probably a handful of other things I missed. :-) 

[Grainger49]

Gh0st,

The problem with the volume control can be helped using a series resistor.  If you had posted before buying the TDK attenuator I would have suggested a 25k attenuator and either a 50k or 70k series resistor.  This allows you to shed the extra input voltage. 

Since this is not really a tube rolling issue you might start a separate thread.

[gh0st]

thanks Grainger, I think I'll just hook up a couple of resistors by way of pre-attenuation (-10db should do the trick). The only inconvenience is the obscure values of the resistors needed to maintain the 100k input impedance, but I should be able to get hold of some.

[Laudanum]

gh0st, the resistor values dont have to be exact, just get them as close as available.

[gh0st]

OCD for resistor values is an obscure condition, but that doesn't mean it's not real

[BNAL]

I wrote a few pages back about the volume of the amp being high then I wanted to allow me to better adjust the volume. I'm not sure if I'm going to change the potentiometer to something with a lower resistance but my have to give it a try and see. I think I my have a 50 ohm alps that I can try and see if I get my flexibility with it.

By the way I'm really enjoying the CBS 5814A with the different power tubes I have. (T-S 5998, RCA 6AS7G, and Mullard 6080). I have a Sylvania 12BH7 on the way. If figured it was cheap enough at $15 to give it a try.

[BNAL]

I through the Alps 50k pot in and it seems the I have more flexibility with the volume. I do have one problem and that is when I turn the volume all the way down I still have some sound coming out of the left channel. I will look at the wiring and make sure that there are no wires touching. Other wise it sounds good once I increase the volume past 10 to 15%.

[Laudanum]

OCD for resistor values is an obscure condition, but that doesn't mean it's not real

Ok then, given that ... a quick browse of Mouser shows plenty of both 68.1K and 46.4K resistors.  So, assuming that both those values as available in  in the same brand and series, your OCD can be satisfied.  Now, if your OCD  requires that they be "boutique" resistors in the exact values, then, I dont know.  :-)

[Grainger49]

OCD for resistor values is an obscure condition, but that doesn't mean it's not real

I was just saying I would keep the total to 100k.  With an active stage following the attenuator it shouldn't have noise problems.

In my first circuits course the teacher (he was the best educator I have ever experienced) pointed out that it didn't matter what resistance you calculated, you had to buy the standard values that were available.

As exacting as he was he wanted us to know that there were limitations beyond out control out there.  I think I said 50k ohms but the value is 51k.  You will find to your ears the differences make little difference.  

But getting a resistor in front of the attenuator will make quite a difference in how well a nice attenuator controls.

[Laudanum]

That "pre-attenuation"  that I linked on the Goldpoint side does just that, keeps the impedance the same as the pot value.  Different resistor values are chosen for different levels of attenuation based on the value of the pot.   The values I stated are those for -10db with a 100K pot or stepped attenuator.

Both from the Goldpoint site : 

http://www.goldpt.com/pre_ckt5.gif

http://www.goldpt.com/pre_rstrs3.gif

[BNAL]

I fixed the problem with the left channel. I like the fact that I have more control over the volume and it does not run out of steam. Not sure if there is a sonic disadvantage to using a 50k pot, if not I will be looking to upgrade it in the future. I still want to try it with the 5998, since that has more gain.

Since this thread is about tubes I feel an obligation to provide input on my observations with respect to combines that I have found work well together.

CIFTE 12AU7 and Mullard 6080 work very well together.
CBS 5814A and RCA 6AS7G is a nice combination.
CBS 5814A and T-S 5998 work very well.

I also have a.Mullard 4003 that is edged out by the CBS. Also, the only tube CIFTE works well with is the Mullard 6080. I feel that is missing something with the other power tubes.

I have a Sylvania 12BH7 coming. It sounds like a winner from what I have read.

[porcupunctis]

I just replace my EH12au7 with the Mullard 40003 and I will have to say that the difference is very noticeable in a very positive way.  This is my first foray into really top notch NOS tubes.  I always thought they were too expensive for whatever improvement they might provide.  I now stand corrected.  I chose the Crack for this experiment since I would only need one tube and that would keep my cost down.  The Crack makes a nice testing ground for rolling tubes.

My crack has the speedball, Solen output caps and Khozmo attenuator installed.  I'm also using an RCA Blackplate 6AS7G. 

Brad, your post makes me want to try the CBS tube and I think I will keep that in mind for a future try.  You have definitely tried more combinations than I have at this point.

[BNAL]

Randell,

The Mullard 4003 is a great sounding tube and is still at the top of my list. It works very well with the 6AS7G. I would say that to my ears the CBS seems to be a little more transparent without losing the warmth in the midrange. I find that the case with the CIFTE when using it with 6AS7G and 5998, but it pairs very well with the Mullard 6080. As a piont of reference the tubes that came with the amp EH 12AU7 and Sylvania 6080 sound very good, but the other tubes take the amp to a different level.

As far as trying different tubes, every few days or so I swap one of the tubes out to see what I think. It is easy for me to do this since the amp is in my bedroom and I like to spend 2 to 3 hours each night listening to music while I read a book. I can't think of any time where I was so disappointed that I had to change tubes.