Use of ECC804 in Crack

[Loon]

I am toying with the idea of experimentally using an ECC804 as the input tube in the Crack.  It seems quite similar to the ECC82 and is dirt cheap.

One question I have is the assignment of the ninth pin.  The 12AU7 has pin 9 as centre heater tap and the ECC804 has pin nine as a shield.  A quick look at the Crack circuit shows grounding of pin 9. Is it fine to ground the shield?  What is the role of this shield?

Cheers

http://www.r-type.org/pdfs/6-30l2.pdf

 

[Loon]

Next Question:

A quick look at the Va Ia curves shows a desirable plate current of about 9mA in the circuit (staying with a -1.5V bias) for the ECC804. How do I calculate the value of R1 in the Speedball to achieve this current?

Thanks again in advance!

 

[Paul Birkeland]

It looks to have the same pinout as the 6DJ8.  The 6.3V twisted pair of wires would go to A4/A5, and A9 should be grounded, preferably to the chassis at T3. 

I don't see a great set of plate curves available.

 

[Loon]

Please follow link in post 1 for curves.

How do I calculate R1 on speedball of a 9mA constant current output?

Thanks!

 

[Paul Birkeland]

R1 is about 0.855/current.  I'd bump R2 to a 100K/2W part.

 

[Loon]

Awesome!  Thanks Paul.  If I can get into the ballpark with resistor values I can always fine tune values.

This pinout should also work with an E288CC.  Thinking of playing with this as an input tube to get an OP for the 5998 and 7802 with a lower voltage (around -50V grid voltage), which I expect to be able to achieve running with a -1.5v bias on E288CC and plate current of about 8mA.

Fingers crossed.  Hope nothing explodes!