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Old 17th Sep 2017, 10:32 pm   #1
MrBungle
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Default Substituting 12AT7/ECC81 in place of 12AU7

Back on my VTVM project for an evening and I've just blown up the 12AU7, fortunately a Chinese cheapy. They don't like 170V DC across the heater. That'll teach me to use yellow wire for everything. Oops! I also can't find the replacement one I bought just in case I blew that one up. Double oops!

Now on with the show, I've got a large box of valves here I picked up at a boot sale a month or so ago which contains a few Mullard and Philips ECC81s that the heater tests good on. That's a 12AT7 if I'm not mistaken. As I am devoid of anything that looks similar, is this is a suitable substitute for the 12AU7 in a VTVM role i.e. simple differential amplifier?
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Old 17th Sep 2017, 10:43 pm   #2
Boater Sam
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Default Re: Substituting 12AT7/ECC81 in place of 12AU7

Should do, ECC81 is the lowest gain of the ECC series.
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Old 17th Sep 2017, 10:43 pm   #3
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Default Re: Substituting 12AT7/ECC81 in place of 12AU7

The ECC81 is an RF valve with higher mutual conductance and amplication factor but it shouldn't cause any damage, perhaps may need some work to keep it stable.

Edit. Sam I must have read the specs wrong, it is late.
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Old 17th Sep 2017, 11:13 pm   #4
barretter
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Default Re: Substituting 12AT7/ECC81 in place of 12AU7

The ECC82 has an amplification factor of 20 ; the ECC81's is 60. ECC81s do have higher transconductance but it's not massive so you shouldn't have any oscillation problems. They were often used as phase splitters in audio amplifiers .
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Old 17th Sep 2017, 11:36 pm   #5
MrBungle
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Default Re: Substituting 12AT7/ECC81 in place of 12AU7

Thanks all. Will chuck it in and see what happens!
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Old 18th Sep 2017, 12:27 am   #6
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Default Re: Substituting 12AT7/ECC81 in place of 12AU7

Well it works with it in and it's quite jumpy now. Will order another 12au7 and see how that goes too. Thanks again
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