Crack with modifications

[reglys]

Hi

I have completed my second Crack and I am still amazed by the sound quality of this small amplifier. This time I want to go further in update.

Next step will be adding modifications and the speedball option. I want to do it in this order. Any comments?

1) Replace the stock potentiometer with step attenuator and test the amp with this modification.
2) Replace the stock electrolytic coupling capacitors (100uF) with big Mundorf film capacitors (100uF) and test the amp with this modification.
3) Add the speedball option and test the amp with this modification.
4) Remove the 270 ohms resistor between 15 and 21 terminal and replace it with a Hammond 157M and final test.

Don't tell me that one of these modifications are not worth, I have already all the parts.

Thank you

UPDATE

Finally i have installed all the modification to my Basic Crack. The amp is performing very nicely. Hard to notice most of the upgrade. I do not pretend to be an audiophile.

DACT step attenuator. Very nice upgrade, the sound seems to be more real and detailled. The most significant upgrade for me.
Speedball option: Bass seems to be better.
Film condensator Mundorf: Music seems to be more detailled.
Choke: Hard to notice some difference.

However, my friend and I were doing some tube rolling. We put a RCA 5692 RED base input tube instead of the 12AU7. My friend notices some Hum Noise even when the pot is at minimum and no audio signal is coming from the DAC. I switch back the input tube to 12AU7. The noise was a lot less and appears only when the step attenuator was at 90% and more. So nothing to say about the 12AU7.  By the way, the 5692 needs an adapter 6SN7 to 12AU7 in order to run it. ( I was using an adapter JEMOSA from Amazon)

Checking the datasheet of each input tube.

12AU7 : heaters in parallel : 300 mA at 6.3 Vac

5692 : heaters in series : 600 mA at 6.3 Vac

Is it possible that the bigger amps use on 5692 heaters is affecting the audio signal?

 

[Paul Birkeland]

I would put the Speedball in before you add the caps.  The Speedball doesn't have much flexibility in terms of where you put it, so it will tend to dictate where the caps can go.

 

[reglys]

Thank you for the hint Paul

 

[hmbscott]

The upgrades are definitely worth it for the entertainment value alone, if nothing else!

My hearing isn't perfect, and I apparently suck at critical listening, so I usually cannot hear any obvious effects of upgrades I make to my gear. But, this is something I do notice, some upgrades increase my sense of engagement with the music, drawing me in and extending my listening sessions (I've noticed this with capacitor upgrades in particular, and when comparing very good amps).

I would urge you to look for that "engagement" effect, even if you can't quite puzzle out if some upgrade really was an obvious improvement or not.

 

[Mucker]

I built two Cracks and currently use both. The first Crack was built back in ~2015 and Speedball was immediately installed afterward. Then caps followed shortly after, followed by choke and Cree diodes. Pot and jack followed that. I really didn't give the stock Crack a fair listen, but the result of what I did was obviously very good.

Then curiosity arose, and I ordered a second kit a year and 1/2 ago. Plan was to keep it stock and I did for quite a while. It was a very good test after listening to the modified Crack for so many years. What did I learn?

The bone stock Crack without Speedball was very nearly indistinguishable. Very impressive. Of course some mods were made to it as I went forward because I can't leave well enough alone.

My overall feeling is that Bottlehead already provides premium parts with their kits. Not a stinker in the entire kit and perfectly adequate for the job. I believe any further changes lean heavily towards enhancing electronic cosmetics and provide next to no audible improvement. Tubes will push the sound around more than any part change will. The original design is extraordinarily good. I would carry it further to say the exact same thing about my S.E.X. and Moreplay units. (Hopefully a Stereomour II soon).

Anyway, it has been a fun process tinkering and that's what it has really been all about for me. The one's I did were very reasonable cost wise. I do very much believe in diminishing returns and especially so since these stock kits are so well thought out concerning part selection.

 

[Thermioniclife]

I built my Crack stock at first then installed the small board on the Speedball upgrade and was very happy, after installing the the large board it lost it's charm imo. Of course I  installed a choke and film caps and determined the PS choke did nothing but added expense and weight. The film caps were an improvement but not earth shattering. Just one old mans 2 cents.

 

[reglys]

The upgrades are definitely worth it for the entertainment value alone, if nothing else!

My hearing isn't perfect, and I apparently suck at critical listening, so I usually cannot hear any obvious effects of upgrades I make to my gear. But, this is something I do notice, some upgrades increase my sense of engagement with the music, drawing me in and extending my listening sessions (I've noticed this with capacitor upgrades in particular, when comparing very good amps).

I would urge you to look for that "engagement" effect, even if you can't quite puzzle out if some upgrade really was an obvious improvement or not.

I fully agree with you! ...upgrades are definitely worth it for the entertainment value alone...

 

[2wo]

I would replace the pot early on...John

 

[reglys]

See my update at the beginning of the thread

 

[Mucker]

There are more than a few threads here that discuss issues with 6SN7. Personally, I tried a handful of them with my Crack and they all had varying degrees of hum. It's my understanding that adapters cause the issue and an amp hard wire to a 6SN7 socket to be the solution. Although, it is also discussed about modifying the tube heating to DC from AC.

My solution was to completely abandon 6SN7 and settle on a combination of E80CC (not a direct swap from 12AU7) and 5998 tubes. This combo is the end game for me with Crack and I've been using it for over a year completely satisfied. The amp is an amazing piece of gear with Beyer DT-1990 Pro and vinyl playback. So lush and smooth and just crushes my SS amps. The SS amps are amazing in my other applications but for vinyl playback, I wouldn't think of using anything but a tube amp.

 

[Paul Birkeland]

You can also abandon the top plate and the 6SN7 works nicely with AC heating.  Adapters certainly can be fickle.

 

[hmbscott]

However, my friend and I were doing some tube rolling. We put a RCA 5692 RED base input tube instead of the 12AU7. My friend notices some Hum Noise even when the pot is at minimum and no audio signal is coming from the DAC. I switch back the input tube to 12AU7. The noise was a lot less and appears only when the step attenuator was at 90% and more. So nothing to say about the 12AU7.  By the way, the 5692 needs an adapter 6SN7 to 12AU7 in order to run it. ( I was using an adapter JEMOSA from Amazon)

Checking the datasheet of each input tube.

12AU7 : heaters in parallel : 300 mA at 6.3 Vac

5692 : heaters in series : 600 mA at 6.3 Vac

Is it possible that the bigger amps use on 5692 heaters is affecting the audio signal?

This is speculation, but the higher current draw of the 5692 might not be fully satisfied by the transformer, which is to say the heater may not reach full operating temperature and thus the cathode also may not reach full operating temperature.

Tubes are designed to operate space-charge limited*. operating in this regime allows the cathode to produce the design current/voltage operating characteristics of the tube. However, if the cathode is operated too cold, the tube enters the temperature-limited operating regime, which both changes the current/voltage characteristics, and introduces addition thermal noise in the electron beam. This will probably cause reduced output, increased distortion and increased noise.

This, by the way, is essentially what happens to a tube at end of life. Because its cathode coating that promotes electron emission has become depleted, the tube gradually switches from space-charge limited to temperature-limited operation (commonly called "goes TL"). And so the impact of operating a tube with too low a heater current should produce the same impact as a tube at end-of-life.

This may explain what you are hearing, assuming the tube is being starved for heater current.

*In case you're interested, the space-charge limited condition is where the cathode is hot enough to created a cloud of electrons above the surface of the cathode (the "space charge"), when the cathode is biased to draw current, the current comes from this cloud rather than directly from the cathode surface. As the bias voltage is raised the voltage-current ratio follows a smooth mathematical formula I = P*U^(3/2) (Child–Langmuir law), P is the Perveance, a constant, dependent on the tube geometry. If so much current is drawn from the cathode that the space charge cannot be maintained fully, the some electrons start to be drawn directly from the cathode surface. This transition from space-charge limited to temperature-limited behavior is never uniform across the cathode surface, because there is variation across the surface of it's ability to emit electrons (variation in work function - which is hidden as long as the space charge cloud is fully populated). So as soon last the cathode starts to go TL, the Child–Langmuir law is no longer followed and the tube progressively deviates further and further from design behavior as more and more excess current is drawn. Furthermore, electrons emitted directly from the surface have more, and a broader distribution of, transverse momentum (transverse to the electron beam direction), and that expresses directly as random noise imposed on the audio signal.

 

[Paul Birkeland]

The PT-10 can supply 3.5A on the 6.3V winding.  The 6080 eats up 2.5A, so there's an extra 1A of heater current available for the driver tube.

 

[Thermioniclife]

Hey PB is that clear acrylic on the side of that amp? Look's sweet!

 

[Paul Joppa]

The actual design maximum heater current of the PT-10 is 4.0 amperes, for what it's worth.

 

[Paul Birkeland]

Hey PB is that clear acrylic on the side of that amp? Look's sweet!

Glass!

 

[Paul Birkeland]

The actual design maximum heater current of the PT-10 is 4.0 amperes, for what it's worth.

That's only 2.5 PJ amps...

 

[Mucker]

You can also abandon the top plate and the 6SN7 works nicely with AC heating.  Adapters certainly can be fickle.

Now that is one slick build. Nailed the color.