12AT7 substitutes?

[dgta]

I'm building an input stage very similar to SY's RedLight (basically a voltage amp and concertina). But the concertina cathode is going to see more than 90V and I can't jack up the filament supply. The 12AT7 only takes 90V h to k.

Are the 12AZ7 and 12DT8 the same tube as the 12AT7 electrically and sonically? They sure look identical on paper but I've never used either. Are there other tubes that are identical to 12AT7 but with higher Vh-k?

[barrymagrec]

Mullard quote 150 volts max heater - cathode for the ECC81 - direct equivalent to the 12AT7

[cogsncogs]

Yes they are identical, but with these differences: The 12AZ7 has higher heater current (225/450mA) and a higher heater to cathode rating of 200V negative, 100V positive with respect to cathode as opposed to 90V (100V 12AT7WA) for the 12AT7 and the 12DT8. The 12DT8 has an internal shield (pin 9) between the two plates and 12.6V heater only. The 12AZ7 is the one you want for higher h-k.

[SY]

If you bias up the heater like I did in the RLD, you can keep Vhk within spec. If you use (say) 70V as the bias, that lets you get the cathodyne Vhk as high as 160V. Will that work for you?

[dgta]

Thank you for the responses.

SY, I already I said I CAN'T bias up the heater, for reasons too lengthy to go into...

Cogs, both the RCA and GE datasheets for 12DT8 show Vhk=200V for cathode positive. Not sure where you got 90V.

Barry, yes the Mullard is the only brand of ECC81 with higher Vhk, thanks for the heads-up.

[SY]

Interesting- so the heaters must be a load for something?

Maybe you should use a different tube. Some of the Compactron multi-section tubes have high Vhk ratings- for example, a 6AF11 will let you run the cathode 200V above the heater in the triode sections, and you have a nice power pentode in there as a bonus, if you choose to use it. There's probably better fits in that class and it's worth a search through the tube manual, I just happened to know the rating for that particular tube.

[dgta]

SY, no the heaters are not a load. The regulated heater supply is also used for something else. Easiest solution is a 12AT7 equivalent with higher Vhk.

[v4lve lover]

EC92 is one section of a 12at7 whit 100 volts of heater -cathode insulation . UC92 is a higher voltage type wich is made for mains aplication so it will take way more . 160 volts H-K

EDIT PC92 will take 250 volts like ALL P TUBES

[dgta]

Does anyone have the data sheet for the Mullard CV4024? That's an ECC81 equivalent but I don't know the h-k voltage.

[barrymagrec]

Originally Posted by dgta

Does anyone have the data sheet for the Mullard CV4024? That's an ECC81 equivalent but I don't know the h-k voltage.

CV4024 is a goverment number and can be Mullard, Brimar or any other manufacturer that meets the CV4024 spec - must be on the web somewhere.