Tube Rolling w/Crack

[Jim R.]

Steve, thanks very much for that information -- that helps a lot, but also tells me that a dpdt won't exactly work to switch between 12au7 and 5670 configurations.  Oh well, at least now I have the info for future reference.

-- Jim

[Jim R.]

Hi Paul,

Now, that sounds tempting.  I still plan to try the stock config first, but I'm thinking the additional current drive would be nice with the 650s for a bit more detail...  perhaps.

Thanks again,

Jim

[Paul Birkeland]

Yeah, I think it really comes down to taste.  Doc loves his 12AU7's, and I totally understand that when we cue up tape and listen.  PJ is pretty into 6SN7's (as is Ed Fallon), and in PJ's case I think it's reflective of his preference for musicality.  In my case, I might go for a 6414 or something like that, except that gain is an issue, so wiring for 5687's or ECC99's seems like a good choice. 

Putting in a switch for the filaments seems easy enough and like a good plan.  With  the Speedball installed, you should be able to switch between 6CG7/6FQ7, 12AU7, and 12BH7 without tremendous voltage mismatch between the driver tube and the output tube.

[Jim R.]

Yes, it really does all come down to personal taste, and that's why I at least originally wanted to have the ability to use the 6cg7 (and because I have a few very nice examples of those on hand), but also because I too really like the musicality of the 6sn7 even though I do find that the ability to render dynamic contrasts to be high on my preferences list and the 6sn7 is really not the ideal tube for this.  On the other hand, (and I see you anticipated my next questions) is that with the direct coupling and no cap between driver and output, dynamic contrasts are much better than in cap coupled amps anyway.

So, all these unknowns and experiments to be done, but at least with my original plan to have a switchable filament arrangement, I can play with enough variations to keep me happy for a while and not worry about putting undue stress on the amp.

This is the kind of thing that makes BH kits so much fun!

-- Jim

[Dyna Saur]

Hey Jim,
The pin assignments for the 5670 are;

1-heater
2-cathode (2)
3-grid (2)
4-plate (2)
5-shield
6-plate (1)
7-grid (1)
8-cathode (1)
9-heater

Heater-6.3V, 350mA

Class A
plate voltage 150V
cathode bias resistor 240 ohm
amplification factor 35
plate resistance 6400 ohm
transconductance 5500 micromho
plate current 8.2 milliamperes
grid current -8V (Ib=10 milliamperes)

If you are re-wiring the driver 9 pin socket for 5670, then it will also be compatible with 2C51 and the ever popular and lovely WE 396A.

If 12BH7s are to your liking, then the 6GU7 is the 6V filament near equivalent, just wire the filament power to pins 4 and 5 and leave 9 open. Same pinout as 6CG7 / 6FQ7.

[levlhed]

With  the Speedball installed, you should be able to switch between 6CG7/6FQ7, 12AU7, and 12BH7 without tremendous voltage mismatch between the driver tube and the output tube.

12BH7's come highly recommended to me as suitable 12AU7 subs.  With Speedball you're saying there'll be no concerns?  Would a particular model of output tube be better suited for use w/ a 12BH7?

[Paul Birkeland]

With the Speedball a 5-10v difference in driver plate voltage won't really affect the output tube.

[Natural Sound]

Yeah, I think it really comes down to taste.  Doc loves his 12AU7's, and I totally understand that when we cue up tape and listen.  PJ is pretty into 6SN7's (as is Ed Fallon), and in PJ's case I think it's reflective of his preference for musicality.  In my case, I might go for a 6414 or something like that, except that gain is an issue, so wiring for 5687's or ECC99's seems like a good choice. 

Putting in a switch for the filaments seems easy enough and like a good plan.  With  the Speedball installed, you should be able to switch between 6CG7/6FQ7, 12AU7, and 12BH7 without tremendous voltage mismatch between the driver tube and the output tube.

Sorry if this has been asked before. Isn't the 12SN7 the same as a 6SN7 with different heater voltages (12V vs 6V)? Wouldn't this make a more suitable replacement for the 12AU7 since the filament voltage is the same? 12SN7's tend to be a lot cheaper too.

[Jim R.]

The actual filament voltage in the Crack, as in the s.e.x. amp (same power transformer) is 6.3v and you can run some of the 12v tubes on this by running the heaters in a parallel, rather than serial configuration.

Hope this helps,

Jim

[Dyna Saur]

12SN7 has a 12.6 volt (only) filament, it is not tapped like the 12AU7 or 12BH7 which allows those tubes to run on either 6.3 or 12.6V.      Crack would need a 6SN7 (and an enlarged chassis hole to accomodate the octal socket.  "Hole Shrinkers" in reverse, anybody ?  ;^)   Yah, they're called Greenlee Punches ;-)

FWIW, though it doesn't really mean anything useful  for "The Crack", there is also an 8SN7.    Like my erstwhile experiment with the 8CG7, it will work, but gain will be low, and sound "subdued" as the operating voltages will be skewed from the 8CG7's reduced emission.

/ed B

[Grainger49]

   .  .  .    Crack would need a 6SN7 (and an enlarged chassis hole to accomodate the octal socket.  "Hole Shrinkers" in reerse, anybody ?  ;^)

/ed B

Knockout, see this link:

Project Tools

See the first post under More Advanced Tools, and the second post for a link to Harbor Freight where you can buy a set cheap!  I have used my set often.

[Natural Sound]

Hmmm... now you got me thinking. I still haven't started building my kit yet. Maybe I'll punch the hole out and call up BH for some hole shrinkers. That way I can build the amp stock, get familiar with the way it sounds and then maybe drop in an octal socket and try the 6SN7 later. Are there any other component changes if I go this route?

[Paul Birkeland]

Go with the octal socket right off the bat and buy some 6SN7 to 12AU7 adapters online.  Getting in there to enlarge the hole later will be pretty hard.  When you enlarge the 9 pin hole, be sure the socket mounting hole closest to the cooling vents is still a socket mounting hole (so position your punch appropriately), otherwise the speedball upgrade will give you problems.

[Natural Sound]

Go with the octal socket right off the bat and buy some 6SN7 to 12AU7 adapters online.  Getting in there to enlarge the hole later will be pretty hard.  When you enlarge the 9 pin hole, be sure the socket mounting hole closest to the cooling vents is still a socket mounting hole (so position your punch appropriately), otherwise the speedball upgrade will give you problems.

Thanks! Are there any other component changes when going from the 12AU7 to the 6SN7?

[Paul Birkeland]

Nope, no component changes, just the pinout/socket change.