Tube Rolling w/Crack

[stupidmop]

 Hi all, had the crack over a year but new to forum(s). I've read this whole thread, very informative, but none of the links work about modding to use a 6dj8 or 12ax7 or whatever. Supposedly can be done and have a toggle switch? sounds yummy as I have a bunch of 6922 and equivalents.  Should I start a new topic? Seems a lot of good modding posts have been lost. Sorry if this is out of place and thanks for any help.

[Chris65]

From the Crack FAQ:

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).

Maybe you were looking for something like this:
http://bottlehead.com/smf/index.php?topic=5989.0

[stupidmop]

 Thanks for the quick response but no I'm wanting to have the one 9 pin socket and use it for the different types. I have a project sunrise and all I have to do is change a jumper and I can use virtually any 6 and 12 volt 9 pin tubes, bias adjustment necessary of course. Nikolay from tubemaze told me his crack is modded in this fashion but I can't get anything on the search. Just don't want to start a new thread if I shouldn't. Like I said I'm new to this forum stuff.
 Edit: should prob note that my crack is stock and I plan to keep it that way, no speedball for me. I have alot of tubes and enjoy rolling. I've heard speedball makes these changes less apparent. I'll keep looking....

[Paul Birkeland]

As is mentioned in the FAQ, you'll need to switch a lot more to use higher gain tubes.  With something like a 12AX7, assuming you have the Speedball and have modified it for the 12AX7, you would want 30dB of padding at the input of the Crack. 

Without the Speedball, you can't get enough loading resistance to use something like a 12AX7 or 12AT7.  You might be able to switch the heaters around to use a 6922, which should bias up around 70V on the plate with the stock circuit, and I believe the 6CG7 would work at that point too. 

[stupidmop]

 Yes, the 6922 is my main concern. I have some 68 teles that I wanna put in the crack, among others.

[Paul Joppa]

If you re-wire the heater for 6922 and leave pin 9 unused, you can sub a 6AU7 with no changes. Nobody wants a 6AU7, they all want the 12AU7, but it's the same tube except for heater voltage. Just a thought.  :^)

[Paul Birkeland]

Yes, the 6922 is my main concern. I have some 68 teles that I wanna put in the crack, among others.

If you do this, be aware that they have roughly twice the voltage gain of the 12AU7.  You will certainly have the hair trigger volume control issue with the 6922 unless a lot of padding is used.

[stupidmop]

 Bummer. Don't think me skills are up to that. Thanks for the info. Just have to keep the sunrise around

[danw]

Hi.
Is this the same tube as Tung-sol 5998? If not how does it sound compared to TS5998? Thank you.

h ttp://www.ebay.co.uk/itm/1-Tung-Sol-6520-Tube-USA-5998-type-/141533110716?pt=Vintage_Electronics_R2&hash=item20f407e1bc

[JamieMcC]

Unfortunately it is not the same tube the 5998 types have a domino pattern on their plates.

It looks like a regular Tung-sol 6as7g which are often found branded as Chatham they are nice sounding tubes they normally go for $20 for a strong testing. I would pass on this one due to the low test results.

[mcandmar]

I looked into 6520's before and couldn't find any datasheets, seems to be an obscurity like the 2399.  I've read suggestions they are just another tightly matched/tested 6AS7G but i have found what looks like 5998's labelled as 6520's so that contradicts that theory.

[borism]

I bought a TS 5998 from vacuumtubes.net in Florida about a month ago and received a Chatham branded 2399. The plates have the domino pattern and while I don't have a TS labeled 5998 for comparison, I prefer the sound of this tube to the Russian military 6AS7 winged "C".
Clearly, the seller seems to think that the 2399 is equivalent to a 5998. According to a report on tube maze the difference is that the 2399 has copper grid rods but the sound is identical.

[Paul Birkeland]

I looked into 6520's before and couldn't find any datasheets, seems to be an obscurity like the 2399.  I've read suggestions they are just another tightly matched/tested 6AS7G

This is exactly what the datasheet specifies.  They also have much higher heater to cathode insulation ratings, which are already much higher than the Crack requires.

-PB

[mcandmar]

A ha so it does.  Don't know why i couldn't find that datasheet before.   Attached is a pic of the 6520 that got my curiosity, is it an uber spec 5998, or a 6AS7G using the 5998 plate structure?

[JamieMcC]

I wonder if it might be similar to a WE422a looking at the structure it doesn't look so?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.che.ncku.edu.tw%2FFacultyWeb%2FChengCY%2FZatu%2FWE274422.jpg&hash=a5b085aba5f7ec1d652dc3300489698302ea8ec8)


http://www.the-planet.org/pdf/WE422A.pdf