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?
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?
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.
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?
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, 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.
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.
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.
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.
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
EDIT PC92 will take 250 volts like ALL P TUBES
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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.
No luck on CV4024 specific data sheet, but it should be generically equivalent to the 12AT7WA, and probably the E81CC as well..
The RCA 6201 has a cathode to filament rating of 100V which I guess is still not going to be enough for your application.. Other brands are rated 90V typically. I have found that 6201 seem to consistently sound better than most 12AT7, particularly the RCA and GE GL6201(five star).
Unfortunate that the filament supply is not dedicated so that it could be elevated.
The RCA 6201 has a cathode to filament rating of 100V which I guess is still not going to be enough for your application.. Other brands are rated 90V typically. I have found that 6201 seem to consistently sound better than most 12AT7, particularly the RCA and GE GL6201(five star).
Unfortunate that the filament supply is not dedicated so that it could be elevated.
Found the CV4024 after much searching:
http://frank.pocnet.net/other/ServiceTypes/cv4-4/CV4024.pdf
Vhk only 100V.
I guess I'll use the 12AZ7.
http://frank.pocnet.net/other/ServiceTypes/cv4-4/CV4024.pdf
Vhk only 100V.
I guess I'll use the 12AZ7.
Vhk is not an absolute specification (for non military/life support applications!). Chances are you can exceed it without ever noticing a problem, except maybe a slight shortening of useful (quiet) life. 12AT7s are so cheap that that hardly seems a problem.
Incidentally, pretty much all valves had a 90V Vhk rating for years, and then in the 60s it started to creep up on the popular valve data sheets, even thought there were no particular changes to their manafacture...
Incidentally, pretty much all valves had a 90V Vhk rating for years, and then in the 60s it started to creep up on the popular valve data sheets, even thought there were no particular changes to their manafacture...
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Yup, I'm working on that as well. Plenty of them around, just a matter of finding some that aren't severely overpriced.
Merlin, that's a good point, but on the other hand, I haven't heard anyone (yet) say that there's anything wrong with 12AZ7 or 12DT8. Those appear to be identical to 12AT7 and are both rated at 200V Vhk. And they're cheap.
The 12AZ7 is definitely OK sounding, I used one in a phono pre-amp prototype back when dinosaurs walked the earth..
That unit is still doing its job somewhere on the north shore of MA. 😀
would not simply Floating the Heaters up to +70 Volts as previously suggested be the best Idea in the Long run? Then Future tube replacement even from now manufacture Tubes would be possible. I see allot of Amp's use something like a 12AX7 with the Cathode at +100 Volts so as to Direct couple the Grid to the Previous stage Plate and eliminate the Coupling capacitor. you may have to use a dedicated heater winding for only this tube depending upon the Voltage as well as the Vhk of other Tubes in the system as there could arise a conflict.
No, the OP is using the voltage to power other parts of the circuitry. I'd still look for compactron equivalents...
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