[cddc]

A question for you guys regarding differences in shipping fee on eBay:

I find some sellers charge only $3.5 for a 12AU7 tube using the basic Standard Shipping (USPS First Class Package®), while others can charge as much as $6.8 using the same Standard Shipping (USPS First Class Package®) for a similar 12AU7, almost doubled. Why is the difference? Is it the case that sellers set different shipping fees, or it's automatically set by eBay based on different shipping distances?

 

[cddc]

They are an Italian brand, really great stuff, worth a listen I think! If they are intact, they can have beautiful labels too, here are some others in my collection.






Thanks! The Tung Sol 421A is a 5998 more or less, whether or not they are truly different is a hotly debated topic.

I would say if you like Brimar, Fivre is another good one to try. Compared to the Brimar/STC CV1988, I would say the Fivre are a little bit less warm with a slightly more forward midrange, but it is infinitely smooth and what they are most known for. Ever since a gracious Head-Fier gifted me a Fivre 6SN7 with a tube purchase I made, I have been smitten. If you keep your eyes on international eBay, you can usually find a Fivre 6SN7 for $50 USD or less.

Nice tubes.

 

[cddc]

No, the output impedance is identical to the Crack + SB. The main difference between the two amps is a larger power transformer and a more sophisticated power supply in the Crackatwoa. It is shunt-regulated, which from my understanding means there will be greater isolation between the power supply and the output signal. Mine also has the Bottlehead-developed TwoQuiet stepped attenuator.

Bottlehead describes the audible improvement in the Crackatwoa as a "blacker background" and more low end definition.

Cool stuff.

Can anyone explains the difference between shunt-regulated and choke? Will choke cause some loss on the output from power transformer, which leads to less power output from the power tube to headphones?

 

[cddc]

Ok the challenge. Sealed within this PVC pipe is a 6SN7 tube, only the pins stick out. Top, and all sides are well sealed and the pipe is vented. A friend in North Carolina has bet me a steak dinner with a big bottle of wine (he hates GE tubes), that he will pick a GE at the very bottom of whatever tubes I send him, (all 6sn7 equivalents). Each tube is numbered 1-8 but after he does the listening and I tell him what the tubes really are, we can change the numbers, send them on to other friends and they can try to figure out which tube is which. No bets involved with them. I own in excess of 200 6sn7 tubes and will do a lot of listening to make it as hard as possible but I will send him tubes with similar emission so they sound at or near their peak. Tubes may well include RCA, Sylvania, Ken-Rad, Melz, Foton, Marconi, Hytron, Mullard, etc. I will make every effort to insure a tube was not built by another company and re-branded. I have CBS tubes like that, some are obviously GE, others are made by Sylvania I believe. No tubes will be harmed and when all is done I can recover them. I am including many of the tubes my friend knows and loves, 1953 Foton with ribbed plates, tall body 7n7's (a loktal equivalent of a 6sn7), Melz 1578 etc.

If anyone wishes to try this themselves down the road, PM me and I will see about sending the tubes your way. If nothing else, it may be a chance for you to hear some black glass and grey glass tubes from the 40's compared to some more modern tubes.

I should note I am testing this in a $100 headphone amp, just in case the wax melts I am not about to take a chance with another amp lol, no offence to Schiit.

LOL...thought the challenge was to cook something using 6SN7.

 

[New Yorker]

Well, I received my assembled Crack/SB amp a couple of weeks ago and… I am lovin’ it. I didn’t know, at first, how to have it finished, exactly. Then it hit me; why not echo the look of my ancient Victor Victrola? And so that’s the way we went. I think it worked out well!

 

[cddc]

Can 12AT7 and 12AX7 be used in Crack? Anyone tried?

They have 6.3V 300mA heaters, same as 12AU7.

12AX7 is a high gain tube, 12AT7 and 12AU7 are both medium gain tubes.

 

[audiowize]

Can 12AT7 and 12AX7 be used in Crack?

No, absolutely not.

 

[cddc]

Okay, but why?

They do not work in Crack, or sound bad?

From technical perspective they seem okay to me, same pinout and same heater current & voltage.

 

[m17xr2b]

From a technical perspective it's not even close to being OK.
12AU7 needs -8.5 grid voltage, has 10.5mA of current, 7700ohm resistance, transconductance of 2200 and gain of 17.
12AT7 needs -2 grid voltage, has 10mA of current, 11000ohm resistance, transconductance of 5500 and gain of 60
12AX7 needs -2 grid voltage, has 1.2mA of current, 4000ohm resistance, transconductance of 1600 and gain of 100.

These are not simple differences of +-10% but required fundamental circuit differences to operate properly. Some sound will be made but you will throw the operating point of the 6080 into the unknown. A crack it will no longer be.

 

[audiowize]

Okay, but why?

They do not work in Crack, or sound bad?

From technical perspective they seem okay to me, same pinout and same heater current & voltage.

1. Too much gain. The 12AU7 is already pretty hot, but the 12AT7 and 12AX7 have significantly more gain that is absolutely not desired.
2. Available B+ voltage. The 12AU7 is perfectly fine with 80V on the plate, but a 12AX7 with 80V on the plate will either draw grid current or draw ridiculously low plate current. In the stock Crack and the Crack with Speedball, the operating points are all wrong for the 12AX7. The 12AT7 has similar issues. The LED bias and the available plate voltage leave the tube near cutoff.
3. The plate voltage of the 9 pin tube in the Crack biases up the 6080. Too much plate voltage and the devices under the 6080 cathodes get really hot and you run out of plate to cathode voltage for the 6080 to work properly. Too little voltage isn't as bad for the 6080, but you could run into compliance problems with the current sink loads in the Speedball.

If you wanted to use the 12AX7 or 12AT7 in a Crack style amp, you would want 300V of B+ and you would likely need to wrap feedback around the amp to make it usable (30-40dB of feedback for the 12AX7, 20-ish dB for the 12AT7) . Otherwise when you rotate the volume control one degree, you will be very close to full output with most headphones and most sources (also known as a "hair trigger" volume control)

Notice that I didn't say anything about the heater voltage or current...

 

[cddc]

Ahh...I thought they are in the same family and have the same heater current&voltage, hence should work in the socket. Wild thought it is.

Thanks a lot for the correction!

 

[audiowize]

Ahh...I thought they are in the same family and have the same heater current&voltage, hence should work in the socket. Wild thought it is.

Thanks a for the correction!

They are in the same family and they do have the same heater voltage, heater current, and pinout. They are otherwise completely different devices and will need very different component values and operating points to work properly.

 

[cddc]

Tubes are just too complicated to me.

A higher gain 5998 can replace 6080, but we cannot do the same thing to driver tubes.

I guess the key point is that the driver tube sets the operating points for power tube in Crack, so we cannot mess around with the driver tube.

 

[Wes S]

Well, I received my assembled Crack/SB amp a couple of weeks ago and… I am lovin’ it. I didn’t know, at first, how to have it finished, exactly. Then it hit me; why not echo the look of my ancient Victor Victrola? And so that’s the way we went. I think it worked out well!

Love everything about it! Is that the stock wood base? The grain on that looks awesome, and like some type of exoctic wood.

 

[New Yorker]

Thanks! That base is
actually African mahogany, finished with red mahogany dye and denatured alcohol to color it a bit so that it resembles the red mahogany veneer on the Victrola. (It’s not a perfect “match”, nor was it meant to be; more of a subtle “tip o’ the hat” to the older machine.) The top plate is finished with a “gold vein” powder; its textured finish kinda, sorta resembles the texture on the wooden ornamentation around the top of the Victrola.

This masterful work was done by none other than Paul Birkeland of Bottlehead! He will assemble kits if you are unable to do it yourself. Yes, I could have built the thing, as I am patient and good at following well-written instructions. (I’m the king of perfectly assembled IKEA furniture; friends ask me to come over and build theirs, lol.) Alas, I live in a small NYC 1-bdrm apt with no garage, no basement and no ventilation. Worse, I’ve never soldered before. And to complete the Trifecta, I have zero woodworking experience. Suffice to say, I didn’t want to end up with a functional but sloppily assembled Bottlehead Crack that looked like someone’s first soldering job. And I definitely didn’t want the thing to look like the sorry bookshelf I made in tenth grade shop class. So having it built by someone who knows how to build electronics and woodworking was pretty much a no-brainer.

I’m so glad you like it!