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Old 10th January 2003, 07:38 PM   #1
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Default can i substitute 12ax7 for 12ax7a

Hello, I am building an EL519 based amp that uses 12ax7a The base trebel and mid control that im adding uses 12ax7 's. I wanted to know if it made a difference which i used. Im probably getting all the tubes from ebay so if would be better cost wise if i could use as many of the same tubes as possible.

--Tom
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Old 10th January 2003, 07:42 PM   #2
dhaen is offline dhaen  Europe
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Our Valve afficionado has just come on line. I'm sure he'll answer you in due course.

Cheers,
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Old 10th January 2003, 08:17 PM   #3
Joel is offline Joel  United States
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You can substitute a 12AX7A, a 7025, or if you only need a single triode a 6AV6. A 6EU7 is the electrical equivalent with a non-paralleled heater. Different pin-out too.
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Old 10th January 2003, 08:21 PM   #4
dhaen is offline dhaen  Europe
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So is a 12AX7 different then?
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Old 10th January 2003, 08:31 PM   #5
Joel is offline Joel  United States
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I believe the original 12AX7 did not have a spiraled heater.
An "A" was just a later "improved" version, as was a 7025 - supposedly a low noise version. Any 12AX__ made in the 60's and onwards will have all of the improvements. You'll often see a single tube labeled "12AX7 12AX7A 7025" on the envelope.

A 6EU7 is a low noise version. They give noise specs in the RCA manual. Whether there is any audible improvement or not is up for debate. It's a cheaper alternative though, and is essentially identical.
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Old 10th January 2003, 08:36 PM   #6
dhaen is offline dhaen  Europe
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So the answer to the original post:
Quote:
can i substitute 12ax7 for 12ax7a
Is yes. I believe.
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Old 10th January 2003, 09:12 PM   #7
Colt45 is offline Colt45  Serbia
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I thought 'A' was just controlled heater warmup? no difference anyways.

theres also 5751, CV4004, 12AX7WA, on and on.. so many .
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Old 10th January 2003, 11:44 PM   #8
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Default A

Hi,

Quote:
I thought 'A' was just controlled heater warmup? no difference anyways.
Correct.

The 5751 has a lower mu than the 12AX7A so these are NOT equivalent.

NEVER replace a 12AX7(A) and its' equivalents numbers in a phono preamp unless you're sure RIAA correction does not depend on its' characteritic values.

Often low noise was obtained by...lowering mu and selection for noise figures.

In practice you'll see the heater light up (flash) on a non A suffixed small signal tube when power is applied.

It does not make any significant difference in real life though.

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