Cathode follower and heater to cathode voltage? 12AX7

[CoffeeTones]

Hello, how should the heater to cathode voltage be obtained / tested and the information used to insure acceptable operation of the 12AX7 cathode follower?

[MWJB]

By "acceptable operation" do you mean in terms of adhering to the data sheet, or a good sounding & reliable amp?

Nobody who builds the latter, employs the former ;-)

Just avoid using the Russian spiral filament 12AX7 LPS in the cathode follower position, they appear to be the only tube with a weakness in this respect.

[CoffeeTones]

No doubt, a good sounding & reliable amp.

My supply is 320V and cathode can be between 185V and 200V depending on the tube and the preceding gain stage' cathode resistor value of 820r to 1.2k

I'm using 100k anode resistor here and don't really want to go lower than 100k for the load resistor off pin 8.

12AX7 tubes I have include new production Tung Sol, Chinese 9th gen, JJ 83S, EH, Sovtek LPS, and other less desirable ones.

[guitician]

Raising the filament with DC will help lessen the potential between it and the cathode.

[CoffeeTones]

Ok, I tried these divider / cap values from the PI node and got a 10V drop at that node. Is there a way to avoid the drop? It may be no big deal but I'd rather not have the drop there.

http://i1140.photobucket.com/albums/...ps081f253e.jpg

Edit: How about 1M / 100k for the divider and tap the screen node instead? Less PT load and drop?

[MWJB]

No doubt, a good sounding & reliable amp.

My supply is 320V and cathode can be between 185V and 200V depending on the tube and the preceding gain stage' cathode resistor value of 820r to 1.2k

I'm using 100k anode resistor here and don't really want to go lower than 100k for the load resistor off pin 8.

12AX7 tubes I have include new production Tung Sol, Chinese 9th gen, JJ 83S, EH, Sovtek LPS, and other less desirable ones.

The LPS is the only one I'd consider mechanically "less desirable" here (but could be great in V1 or PI).

400vdc supplies and ~220+vdc on the cathode are not unusual, nor a cause for particular concern. Even a low voltage amp like a Maz 38 might have 180vdc on the cathode.

[CoffeeTones]

Ok, I tried a 330K / 100k bypassed with a 10uF 150V cap to ground, heater center tap to the 330k / 100k junction. This was at the PI node. Before adding the center tap, there was about 80V at the 330k / 100k junction. There was a 10V drop at the PI node. How about a 1M / 100k divider at the screen node instead? Less drop and load on the PT? Can the drop be avoided?

[guitician]

I would think that a very high resistance can be used since your not powering anything, just charging up a capacitor. The less capacitor leakage the better as far as keeping the drop low.