Can I substitute a 12AX7 tube for a 12AT7?

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50jess

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Can I substitute a 12AX7 tube for a 12AT7?
« on: 4 Sep 2011, 05:02 pm »
I have a Dared SL-2000A preamp with 2 12AT7 tubes.  Could I substitute 12AX7 tubes for the 12AT7?
What would be the effect sonically? 

FullRangeMan

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Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #1 on: 4 Sep 2011, 05:07 pm »
I feel no, as the gain is different, other wise they are brothers tubes.

12AX7 = 100 x gain
12AT7 =  70 x gain
12AU7 = 20 x gain
7025 = 100 x gain

Pez

Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #2 on: 4 Sep 2011, 05:13 pm »
You can try and you won't blow anything up if that's what you are asking. But as fullrange had pointed out it might not sound good.

ltr317

Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #3 on: 4 Sep 2011, 05:20 pm »
It really depends on the circuit.  If the owner's manual doesn't provide tube substitutions, I would check with the manufacturer/delaer/distributor. 

FullRangeMan

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Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #4 on: 4 Sep 2011, 05:27 pm »
It really depends on the circuit.  If the owner's manual doesn't provide tube substitutions, I would check with the manufacturer/delaer/distributor.
Very good advice... caution never hurts.

Bill Baker

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Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #5 on: 4 Sep 2011, 05:30 pm »
The Dared preamps already have quite a bit of gain. Going with a 12AX7 may create a situation where you will have no usuable volume range and the sound would be "hard". If anything, I would go with a tube with slightly less gain. If you are happy with your volume range now, stay with the 12AT7.
First thing to do before changing tubes in any product is check with the manufacturer or distributor to make sure it won't create any problems.

50jess

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Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #6 on: 4 Sep 2011, 11:55 pm »
Yes, I'm happy with the volume range right now.  What tubes provide less gain?

Ericus Rex

Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #7 on: 5 Sep 2011, 01:02 am »
What tubes provide less gain?

As fullrangeman said above, the 12AU7 is lower gain than the 12AT7.

Steve

Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #8 on: 5 Sep 2011, 01:37 am »
Good question. A general comment. The 12AT7 family has approximately 3.5 times the transconductance of a 12AX7 family. Subbing a 12AX7 for a 12AT7 will mean only 1/3-1/4 the plate current, so idle plate voltage will be much higher than with a 12AT7. Two questions emerge.

1) In an amplifier, is there enough signal voltage swing capability to drive the output tube to full power before maximum plate voltage swing reaches B+, and "flat tops".

2) As the plate current drops closer to zero, the curves become more nonlinear. More distortion. 

So no matter what the circuit, conditions won't be optimal.

Cheers.
« Last Edit: 6 Sep 2011, 11:59 am by Steve »

avahifi

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Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #9 on: 5 Sep 2011, 10:11 am »
Note that in a RIAA phono preamp circuit using a single dual triode, a 12AT7 tube will not have adequate open loop gain to follow the equalization feedback loop at lower frequencies.  Thus you will end up with very muddy and inadequate midrange and bass response.

In general, 12AX7 tubes are selected where high open loop gain is necessary, 12AT7 tubes where higher output current drive is more important than high voltage gain.

Probably don't second guess the designer unless you understand the circuit operation.

In your case substituting a 12AX7 tube will result in more feedback, a brighter and crisper sound (which you may or may not like) but leave the unit more sensitive to the output load.

Regards,

Frank Van Alstine

decal

Re: Can I substitute a 12AX7 tube for a 12AT7?
« Reply #10 on: 5 Sep 2011, 01:50 pm »
Quote
Probably don't second guess the designer unless you understand the circuit operation.

 Heed these words of wisdom.