Crack modified for 6SN7 voltage amplifier

[levlhed]

That sounds like it sounds cool.  Looks neat-o too!
Maybe I'll try and get one of those adapters to try out a 6NS7 and see how it fairs against my Tele smooth plate...

[levlhed]

bah...gotta wait for the slow boat from China for the adapter!  Are there any reasonable USA sources for an appropriate adapter to stick a 6SN7 in the 12AU7 socket?

Also, since I did not build my own Crack (I actually own the factory prototype!) I'm not too familiar with the belly of the beast.  I know I saw a recommendation to install the octal socket before final assembly/Speedball/etc, but how difficult would it be to retro fit?

Thank you!

[Grainger49]

You can cobble one together till the slow boat arrives.  Attach wires to the octal tube socket and jam them into the existing socket.  It lacks any sophistication or grace but amazingly works.

It has been done before.  Just keep small children and pets away from the voltages on the wires. 

[levlhed]

yes, I suppose there's no reason that wouldn't work as a temp solution.  Maybe I'll try it...though I have to get an octal socket then hah
Is there a guide handy regarding the pin out/line up?  I'm sure you don't literally mean jam it in any old way:)

[Paul Joppa]

Tube data sheets are available from several sources on the web. I think the 1973 GE "Essential Characteristics" handbook is still available as a reprint, from Antique Electronics for instance - nice to have around.

I just measured the pins on a 9-pin tube. They are 1.0mm (40 mils) diameter, and AWG 18 gauge wire is very close to that size.

[levlhed]

any idea where to get a small amount of 18g wire?  Seems silly to buy a whole roll of it just to do something like this!

[Grainger49]

Radio Shack sells small gauge wire.  Probably more like 24 AWG or the metric (Chinese) equivalent. 

[Jim R.]

Daniel,

Shoot me an email with your adddress and how much you need and I can send some to you if you like.

-- Jim

[levlhed]

I appreciate the offer, however I touched base with a buddy of mine that hand builds (really sweet) tube guitar amps and told him my plan.  He's going to send me something he has lying around that should work pretty slick for a temp solution.  Heck, if I hadn't already ordered an adapter I'd probably just make something nice with this and roll w/it.
(https://lh3.googleusercontent.com/_sHsKkHRJzJs/TcyUAPiWpOI/AAAAAAAAB44/eQ6k03cjsfc/s640/DSC01569.JPG)
(https://lh5.googleusercontent.com/_sHsKkHRJzJs/TcyUAq66NOI/AAAAAAAAB48/MKJjLcnk66U/s640/DSC01570.JPG)

[Jim R.]

Ok, sounds good.

Let us know what you hear.

-- Jim

[levlhed]

I got the item pictured above yesterday.  Haven't had time to fire up the soldering iron yet though.

My friend also shared the following:

"A few things off the top of my head on the 6sn7...

I really like them, they're an older tube than the noval you're replacing.  They pull twice the amount of filament current (600mA)  than a 12au7.   Make sure your power transformer can handle it.  Also, you might notice a 60hz hum if your filaments are AC.  Regulated DC on the filaments is the way to go with these tubes, IMO.  These tubes can also be quite microphonic.  Heat-shink over the tube can help a lot.  Have fun!"

[dubiousmike]

Interesting!  Your friend's comments would seem to explain some of the posts from users who have reported minor audible hum when using the 9 -> 8pin adapter to run 6sn7's.

Per the crack schematic (in the PDF manual), the heaters on both sockets are drawing AC from the transformer.

[Doc B.]

I wouldn't worry about it too much. The gain of the 6SN7 is the same as the 12AU7 and I haven't noticed that a 6SN7 is any more prone to hum.

[Jim R.]

Another thing you can consider is just rewiring the existing socket and trying a 6cg7 or 6fq7.  They won't be a real 6s7 but they'll have a lot of similar sonic characteristics.

-- Jim

[Billyk]

In my brief experience, I like the 6SN7 sound. I have two RCA cookie base with the "long legs" and they are microphonic and exhibit some hum, the recent Russian Tungsol is very quiet as is the Sylvainia chrome top. ATM I prefer the Sylvainia. Tube rolling with this amp is a lot of fun if a bit hard on the wallet!!