12AT7 H-K voltage question

[mrn1ngstr]

Maybe a dumb question... According to the data sheet, heater-cathode +/-90V is the limit. Is this 90VDC, 90VAC, does it matter? And is that before or after signal is added? The reason I ask is I have a direct coupled PI that I want to change from 12AU7 to 12AT7 and Im trying to figure out if I need to increase the plate resistor on the preceding stage, elevate the heater or just leave it as is. Right now the cathode is sitting around 93.5V(voltages are just slightly higher than written on the schematic).

[tubeswell]

90V peak

[VacuumVoodoo]

More exactly Vdc+Vacpeak less than 90V

[mrn1ngstr]

Thanks guys.

Voodoo you say VAC in addition to VDC? Not 90V being the difference between the two?

So would that mean the best option for what I want to do is elevating the heater?

[VacuumVoodoo]

Yepp, the PI cathode can swing between 50V and 130V so applying around +50V DC to heaters should be sufficient.

[TUBEDUDE]

the schematic doesn't show your heater supply, but if you're using an AC heater supply voltage, ,you get a benefit by elevating the heaters of eliminating noise. .

[mrn1ngstr]

Ok, sounds good. And it looks like the best place to get those 50V would be off the power supply. Thanks.

[gingertube]

Yes - get the heater "lift" voltage using a voltage divider from a "quiet" power supply node.
Many use the output tube screen supply node as this is, or at least should be, one of the best filtered power supply nodes in any amp.
Cheers,
Ian

[mrn1ngstr]

Ill take your advice- your reputation precedes you. Thanks Ian.