Different Driver Tube

[eeyore]

If we divorce ourselves from the socket, B+ and heater requirements, what do you guys think would be the best driver tube for the Crack for general 90% listening?

Enjoying the Crack now, but feel the need to tweak and upgrade...

[Natural Sound]

If we divorce ourselves from the socket, B+ and heater requirements, what do you guys think would be the best driver tube for the Crack for general 90% listening?

Enjoying the Crack now, but feel the need to tweak and upgrade...

I took my amp in a different direction and used a 6SN7. You need to punch the top plate out for an octal socket. The rest of the components are the same. You just need to follow the different pinout of the octal. So far I haven't added the speedball upgrade. I like the sound just the way it is.
http://www.bottlehead.com/smf/index.php/topic,1222.0.html

[mrarroyo]

Or you could use the 6CG7 the same as the 6SN7 and no need to punch a larger hole.

[GLF]

I wired mine to take 6DJ8/6922. The only change was to the heater wires.

Andrew

[Natural Sound]

Or you could use the 6CG7 the same as the 6SN7 and no need to punch a larger hole.

The 6CG7 has a very good following. If someone decides to use it in their amp please let us know how it sounds. I used the 6SN7 because I had a bunch of them lying around.

[Doc B.]

I'll suggest giving the 12BH7 a shot. It's sort of a beefier, more linear 12AU7.

[eeyore]

12BH7 sounds like a good option. I have a good stash of 12SN7, and sounds like that would also be a good alternative. Any other tubes. Doesn't have to be triode or dual triode, and it could be a triode strapped pentode, two single triodes, etc?

[ToolGuyFred]

I'll suggest giving the 12BH7 a shot. It's sort of a beefier, more linear 12AU7.

I'm using a 12BH7 in mine and it gets a firm thumbs-up, but I didn't try it until I'd installed the speedball upgrade.

[Jim R.]

Eeyeore,

You can't use the 12sl7 as it doesn't have a center tapped filament and the Crack only has a 6.3v heater winding.

-- Jim

[eeyore]

Yeah, it's a new heater transformer to deal up 12sn7. I am actually thinking about either a 5687 family or 6n6p as well, so it's one of thtose three. How is the current set with the Speedball, cause all three tubes have a different current and grid bias. The 5687 looks like it may require higher B+ voltage to run well. The 6n6p seems to be a good balance with the current, B+ and grid bias.

[eeyore]

6n6p looks like -4V @ 120V at 12.5 mA
5687 looks like -4V @ 125V at 21 mA
12sn7 looks like -3V @ 115V at 5 mA

I figured the above operating points based on a maximum of 170V on the B+. sound right? How to set the Speedball to get these figures. I guess I also have to change the bias LED at the cathode as well?

[eeyore]

Hi, have decided use the 5687 family. Looking at the curves, a bias at -3.2V (that way I can use two HLMP-6000) and setting the current at 20mA, looks like the quiscient voltage is 120V. I suppose there'd be enough headroom with the B+ to swing from a -3.2V grid to -5.5V, meaning the other end is at -0.9V at 65V around the plate. Does that mean I nee d to make sure I can sink up to 2.1W at the C4S ((170V - 65V) * 0.020)?

How would I go about determining the resistors at the C4S to source 20mA? I assume R1 (237R) has to change, and so does the 150K as well?

Any suggestions or assistance appreciated!

[Paul Joppa]

I believe the Crack is designed to have the driver plate at 75v. That value is important because it is direct coupled.