Tubes - 6SN7

[Laudanum]

I know this tube has been mentioned in the big tube rolling thread but I have a specific question related to it.  I realize it requires an octal socket and some rewiring or an adapter/converter.  But, doesnt the 6SN7 draw double the heater current compared to 12AU7?  I see quite a few trying this tube and I'd like to give it a listen myself but does the PT have the headroom to provide the additional current?  And can the 6SN7 be used ok without Speedball?

Thanks

[Grainger49]

The 6SN7 is an Octal version of the 12AU7.  I culled the information from PJ's post (on the old site) for converting the FP III to the 6SN7 here:

http://www.bottlehead.com/smf/index.php/topic,567.0.html

That should apply to the Crack as well.  A much cheaper option is the 7N7 which is Loctal.  It needs you to defeat the lock on the socket or you can't change tubes.  That means bending the center socket flanges out.  That takes less than 20 seconds.

[Laudanum]

Well, if I tried it I would probably just use a socket converter/adapter first.  They have them on Ebay from china for 14 bucks shipped.  It converts from the the 9 pin socket to an octal socket.  The converter itself is rewired specifically for using a 6SN7 in place of a 12AU7 (with parallel filaments).   I wouldnt punch the chassis for the larger octal socket unless I was positive that I wanted to permanently run the 6SN7 instead.   Unfortunately, I didnt understand everything provided in your link.  It mentions changing the resistor for the heater supply but that was for the foreplay II in regards to the DC heater power supply according to PJ's edit.  The tube filament supply in crack is AC directly off the tranformer 6.3V secondary.  So I guess Im confused (as usual).

Thanks

[Grainger49]

For $14 I might buy a pair and try some old 6SN7s I have in my FP 2.

The FP III has more current than necessary for a pair of 12AU7s.  The Extended upgrade adds one tube with a heater, the OD3 doesn't have a heater.  The FP III is also designed so a large number of other tubes can be used.  That is why there is a "trimming" resistor in the heater circuit.  It might not be necessary in Crack.  Paul Joppa, or Paul Birkeland, might chime in to let you know about the need for that trimming resistor.  

I just wanted to tell you that they are equivalent and needs no changes in the plate or cathode circuit.

[Jim R.]

Ummm, I'm not sure you want to use an adaptor that parallels the sections -- that, under the best circumstances would give you a mono Crack.

Also, the 6cg7/6fq7 is more like a 6sn7 than the 12au7, especially in terms of the warmer sonic signature.  That would only take rewiring the heater connections on the socket.  And the 6cg7s are typically a whole lot less expensive than the premium 6sn7s.

-- Jim

[Grainger49]

A second to Jim's comment.  The Crack is wired for a 6.3V heater with the 12AU7 being "folded."  That means a standard adapter will not work.  If memory serves you can contact the seller and specify the pin out conversion you need.  If in doubt we can post it here.

[Laudanum]

I'll have to look into the 6CG7's and 6FQ7's too. 
In reference to the converter/adapter I think there was a link to or mention of an adapter that addressed the heater voltage.  Specifcally a 9 pin to octal converter for  12AU7 (6.3V heaters) to 6SN7.   I think it's linked in the Tube Rolling thread and one member did his testing with it.  I think Doc was discussing his preference for 12AU7 over 6SN7 sonically, with him.  I'll have to read through again.
But all this is just food for thought at this point.  Nothing I was set to move forward with.

Thanks guys, more really good info. I appreciate it.

[Jim R.]

Oops, my bad -- thought it said parallel sections, not parallel filaments.  I really shouldn't be answering technical questions until I'm fully awake :-),.

Laudenum, I'm actually building my crack with a dpdt locking switch that will allow me to switch between the 6cg7/6fq7 and the 12 volt tubes 12au7/12 bh7/e80cc, etc.

-- Jim

[Paul Joppa]

There is plenty of current to supply 6SN7 heater power. You could even run a 5687 (0.9 amps) if you increase the plate current appropriately.

[Laudanum]

Jim, that sounds cool,  tube rolling on the fly, I like it.

Thanks Paul, the current was what I was mainly wondering about with my initial question.

I'll have to do my research and see which of the tubes of the different compatible types provide the best sound for the dollar.

[Doc B.]

Try the 5687, it's very linear and sounds great. We will be using it in the upcoming Smack headphone amp for low impedance headphones.

[Paul Joppa]

To get around 70v at the plate, either run the 5687 with more bias (two LEDs in series) or increase the plate current (looks like about 15mA to me) or a combination of both (e.g. 2.5v bias from a 431 chip, and maybe 8mA current).

If you have the Speedball you won't need to change anything except the wiring to the socket. You'll get 40v at the plate, more or less, but the C4S load will keep the 6080 cooking.

[Laudanum]

Thanks Doc and Paul.  Boomarking this for possibe future reference.  Looks like Speedball may be the best option down the road for a lot of good reasons.

[Billyk]

This adapter from ebay has been successfully used by myself and others...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320659633166&ssPageName=STRK:MEWAX:IT

[Laudanum]

Thanks Billy, that's the one I was thinking of.