Rolling 12AT7 on Crack

[ipetruk]

Hi,

Someone I know has tried rolling 12AT7 and seems to be happy with the results.  The difference is in gain, so as soon as you put your volume pot in the right position it should be ok, as I understand. Is there some reason you don't recommend rolling them?

Thanks,
Igor.

[Paul Joppa]

Yes there is. Because the 12AT7 has 3.5 times the gain, it needs 3.5 times the voltage on the plate to avoid grid-current distortion.

[Paul Birkeland]

PJ covered the AC side.  On the DC side with the Speedball, the plate voltage on the 12AT7 will rise to maybe 125V, which doesn't leave a lot of operating voltage for the 6080.  This will also increase the voltage across the Speedball large PC board and will increase heating of those components.  The plate voltage will be a bit lower in the stock circuit, but you'll be loading a tube with an Rp of around 15K at that operating point with a 22.1K resistor, so THD will be pretty high and you get back to PJ's point about needing more voltage.

Why do you want more gain in your Crack?

[ipetruk]

I don't want anything in particular except more tubes. I just heard that 12AT7 was successfully used, which just means more rolling options. So I asked about this.

In fact the person I am talking about made his own thread here: https://forum.bottlehead.com/index.php?topic=14228.0

Thank you for your clarifications.

[Paul Joppa]

I replied to the referenced thread in more detail; the corresponding conditions for 12AT7 are 0.9mA at R1=1000 ohms as the starting value. Still not recommended due to the low current.

I'll add that there may be significant differences in the required  current due to variations in the metallurgy used by the manufacturer.