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12AT7 for 12AX7 in preamp
12AT7 for 12AX7 in preamp
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Old 8th February 2019, 08:33 PM   #1
awkwardbydesign is offline awkwardbydesign  England
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Default 12AT7 for 12AX7 in preamp

I found and resurrected an old Concordant pre I cloned about 30 years ago. But I have loads of 12AT7s, but only two mismatched 12AX7s. And less gain would be nice too, but my question is, can I simply sub the 12AT7s in place of the 12AX7s without causing damage? I realise I will eventually have to recalculate the resistor values for the 12AT7s, and change the output caps to keep the bass response, but would there be any initial risk in swapping them?
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Old 8th February 2019, 08:39 PM   #2
rongon is offline rongon  United States
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12AT7 for 12AX7 in preamp
A 300k ohm plate load on a 12AT7 would probably result in that type of tube being run very cold-biased, too close to cutoff all the time. It would probably make sound, but it would not be an improvement. It's a simple enough circuit to re-design to fit 12AT7/ECC81, though, if that's what you want to do.
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Old 8th February 2019, 08:40 PM   #3
audiopro is offline audiopro  Canada
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If it works, you won't do any damage. You're definitely going to have to make adjustments to get it working right though.
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Old 8th February 2019, 08:41 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by awkwardbydesign View Post
would there be any initial risk in swapping them?[/IMG]
Probably no risk, but check the DC voltage at the input plate to see if it's at
a reasonable point. The plate resistor is rather high in value for a 12AT7.
You'll have to recalculate all the resistors.

Last edited by rayma; 8th February 2019 at 08:46 PM.
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Old 8th February 2019, 08:59 PM   #5
awkwardbydesign is offline awkwardbydesign  England
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Thanks for the replies.
Rongon and rayma; I'll recalculate (if I can) the anode resistor to compensate. When I cloned this I knew even less than I do now, if that's possible!
The circuit as it stands has the volume pot after the line stage, which is fine for noise reduction, but not output impedance, so I'll probably try it before the line stage later on. Thus resurrection has cost me nothing so far; I had to build a psu for it, but I had everything already, so not buying more valves appeals to me.
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Old 8th February 2019, 09:13 PM   #6
Eli Duttman is offline Eli Duttman  United States
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Surprisingly little change in net gain will be observed, after things are squared away. Remember, the gain of a common cathode triode is function of both μ and RP. The 12AT7's lower μ is compensated for by its much lower RP.

I suggest the substitution not be made. The 'X7 triode is more linear than the 'T7 triode. Nice current production 'X7s are not especially expensive. For instance, Jim McShane charges $33.50 for a matched pair of "reissue" TungSol.
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Old 8th February 2019, 09:25 PM   #7
Ketje is offline Ketje  Belgium
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The second tubehalf has it's cathode on 130V whereas the Vfk is 90V max for the ECC81, you have to lift the heater circuit for say 50volts.
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Old 8th February 2019, 09:26 PM   #8
awkwardbydesign is offline awkwardbydesign  England
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Thanks Eli, I guess I'm just mean! But I've created a loft (workshop) system for about 45. Free Heybrook HB3s, just 22 to refoam, less than 20 to recap the power amp, and 5 for a DVD player. So not spending appealed to me. But I guess I may have to go mad!
And Ketje, now you're spoiling all the fun! But thanks for pointing that out.
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Last edited by awkwardbydesign; 8th February 2019 at 09:30 PM.
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Old 8th February 2019, 10:13 PM   #9
Eli Duttman is offline Eli Duttman  United States
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Thanks Eli, I guess I'm just mean!
I too am cheap. Money does not grow on trees. However, Devil take parsimony.
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Old 8th February 2019, 10:23 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by awkwardbydesign View Post
I'll recalculate (if I can) the anode resistor to compensate.
Probably all the resistors will have to change. Remember that it's a direct-coupled circuit.
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