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April 19, 2019, 09:19:26 am

Author Topic: Crack FAQ  (Read 15236 times)

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Offline Paul Birkeland

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Crack FAQ
« on: May 22, 2013, 01:58:20 pm »
We get a fair number of common inquiries, so I thought I would take some time to cover them here:

1.  Which tubes can I use in the Crack?

Tube Rolling with Crack
(Thanks Angelo)

2.  Can I use a 12AX7, 12AT7, 6922, 12AV7, etc. in my Crack in place of the 12AU7?

Interestingly enough, the original 3-4 Cracks used higher gain tubes like these, but they are far too sensitive for this circuit, and you may only end up being able to use a small portion of the volume control's rotation before you attain maximum output power of the amplifier (referred to as "hair trigger" level control).

In the stock circuit, these tubes will not properly bias the 6080 output stage, which can provide detrimental performance, as well as unforeseen thermal issues which could damage your amplifier.

3.  I can barely turn my Crack up at all before it's far too loud, what do I do?

Disconnect the white wire at the pot that originated at the left RCA jack.  Install a 75K resistor between this wire and where it connected on the pot. 
Repeat this step for the red wire coming from the right RCA jack.
Attach and solder one 33K resistor between each outer pair of lugs on each level of the volume pot and the ground lugs on the pot.
Resistor wattage is unimportant, 1/10 Watt or greater will work nicely.

See the photo at the end of this post for how these resistors are attached to the pot.

4.  I have some channel imbalance in my volume control at lower levels, what should I do?

The solution to #3 will also solve this issue.

5.  When I turn my Crack all the way up, I hear some noise, is this normal?

Generally yes, there will be some noise with the volume control at its maximum level.  The amount of noise will depend greatly on the impedance of the headphones, but the important thing to remember is that this is well beyond a comfortable listening level.

6.  I want to use my Crack with low impedance headphones, why is this bad?

There are several reasons for this.  The first one is that the CR filter formed at the output with the 100uF capacitor and the low impedance headphones will result in a frequency response that is not all that optimal.  For 32 Ohm headphones, you'll be down 3dB at 50Hz.

The second reason is that the output stage of the Crack is a current amplifier, and Ohm's law tells us that Power=(Current2)*Resistance.  When resistance goes down, power also goes down.  While this may not be that big of a deal for a sensitive headphone that is low impedance, it is a big problem for power hungry headphones that are also low impedance.  A very common complaint will be bloated bass from the variations in impedance on a low impedance headphone, as a 32 ohm headphone may be 100 ohms at resonance, and the Crack's output will increase significantly at this spot in the response curve in comparison to a higher impedance headphone.

The last reason that comes to mind is that the output impedance of the Crack is not sufficiently low to drive low impedance headphones with good damping and distortion performance.  This can be circumvented with the implementation of a global feedback loop, but doing so tends to collapse the soundstage of the amplifier. 
7.  Can I just increase the value of the 100uF caps at the output so I can run my Grados?

You can try this if you like, if you go to 220uF caps, replace the 2.49K resistors on the headphone jack with 1k resistors. 

8.  I want to replace the volume control on my Crack, what can I use?

This one comes up a lot.  You need a control that is logarithmic, which will generally be indicated by a reference to "Audio" in the parts description.  Impedance isn't incredibly important, we would recommend 25K-250K.

9.  Why don't you use a nicer pot, knob, tube socket, RCA jack, etc. on the Crack from the factory?

We purchase the parts for the Crack in large-ish volumes to obtain competitive pricing and to maintain a reasonable turnaround time on orders.  Botique parts tend to be carried by fewer vendors and in lower quantities, and we would hate to delay shipment on our products simply for the sake of including these parts. There is the additional caveat that these boutique parts can go out of production randomly (remember the Black Beauty), leading to additional documentation and alterations to our manuals.

We are also very committed to the educational process of both building a kit and also modifying it after construction.  We would be robbing our customers of the opportunity to learn what kind of changes come from swapping out various parts. 

10.  I have lower than expected voltage on terminals 1 and/or 5, does this mean my 12AU7 is dead?

This is quite a common misconception.  As the 12AU7 wears out, its emission will decrease, which lowers the overall current draw of the driver stage.  This will lead to higher than expected voltages at 1 and/or 5.  Having lower voltages at these terminals indicates a strong 12AU7.

11.  I'd like to use a tube rectifier with the Crack, why don't you use one?  How do I add one?

The simple answer is that we don't like the way that they sound.  We have built a few Cracks with tube rectifiers, and they did not have the dynamics or low frequency command that the solid state rectified Cracks do.  Adding a tube rectifier to this circuit requires a different power transformer, different power supply, and different chassis plate.

12. One channel of my Crack isn't working, but when I pull the headphone plug out part way, I get both channels.  Do I have a bad jack?

This is a common misinterpretation regarding the operation of the headphone jack.  When you pull the plug out part way, the tip and the ring are shorted together, which will output a mono signal to your headphones (of just the right channel).  In other words, the these results are incidental and not helpful in repairing whatever issue is causing the left channel to not operate.

(If I missed something, PM me and I'll add it)
« Last Edit: January 14, 2019, 12:30:27 pm by Paul Birkeland »
Paul "PB" Birkeland

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Offline Doc B.

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Re: Crack FAQ
« Reply #1 on: April 30, 2014, 01:00:41 pm »
The second half of this FAQ is dedicated to common assembly errors


1. My Crack worked fine until I installed Speedball. What is wrong? Is it a bad tube?

If the tube worked before the Speedball was installed it's not a bad tube There are several possibilities, and retracing your steps may help you to narrow down where your problem lies -

The first thing we do with any PC board repair is make sure everything that should have been soldered was soldered. Inspect the solder pads on the underside of the PC boards. Don't be proud, a magnifier makes this much easier.

2. I saw a spark when I went to measure voltages

It takes some getting used to using meter probes in tight spots. Did you accidentally touched your meter probe to two terminals or leads at once? This can take out transistors. You can test transistors by unplugging the amp and measuring resistance across each of the three possible pairings of the three transistor leads. If any reading is very low, like under 100 ohms, the transistor is toast and will need to be replaced. It's usually the smaller 2N series transistors that fail.

3. But the transistor looks fine and I don't think I shorted anything with my meter probes, it's an LED that isn't lit

If the transistor is shorted the LED is not getting enough voltage differential to operate. When an LED fails usually there will be no doubt. It makes a pop and a tiny explosion blows the LED into a couple of pieces. If the transistors measure OK but the LED doesn't light it might be that you missed a solder pad when you soldered the board up. Or an LED was installed backwards.

4. I'm not getting any low ohm readings and the LEDs are lit, but I don't get any voltage at the "O" pad on the big PC board.

It is very common for the center leg of the bigger TIP50 transistors that connect to the O pads on the bigger board to end up with too little solder. The heat-sinked transistor dissipates the heat from the soldering iron tip and it may take a little extra hang time on that center pad to get a good solder joint.

5. The voltages on one side are way low

This is usually due to a miswire that is causing a  short. That causes excess current draw and pulls the voltages down. Recheck your wiring from the PC boards to the amp circuit. It can also be due to two separate pads being bridged with solder by accident.

4. It's not any of those things

The forum is here to help. Post your situation with your voltage readings in this forum and we will help you to sort it. But before you do that, go back just one more time and check those connections you have already looked at ten times. It can be hard to spot a miswire once you have missed it the first time, even if you check it several more times. Be methodical, compare your work with the pictures in the manual.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.