ASL MG Head DT/OTL MKIII Tube Rolling Experience

[jclyle]

I put a Mullard 12AU7 in my ASL today. Sounds good to me!

 

[manofmathematics]

Do you notice a large gain difference between a 12AU7 and a 12AX7?

 

[jclyle]

very little. tomorrow I might try a 12AT7

 

[manofmathematics]

Post your findings with the 12AT7 here, if you don't mind.

 

[Skylab]

I don't think the 12AT7 will work - I'd stay away from that.

 

[manofmathematics]

Is there a chance of harming the amp?

 

[Skylab]

I'm not sure, but there is always a chance. The 12AT7 really isn't a drop in replacement for the 12AX7, whereas in most circuits, the 12AU7 will be.

If you want an upgrade, though, go for a 5751, as mentioned above.

 

[manofmathematics]

Thanks for the advice Skylab. I've used 5751's. I was originally curious about 12AU7's because I've got a few of those coming that were included in a trade. I figure that if I can't use them, then I'll just throw them my father's way. I'll stay away from the 12AT7's though. Thanks again.

 

[manofmathematics]

Decided to throw in a pair of Valvo power tubes for now.

Going to start rolling with the recommended Raytheon 5751 windmill getter.

 

[sacd lover]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif I'm not sure, but there is always a chance. The 12AT7 really isn't a drop in replacement for the 12AX7, whereas in most circuits, the 12AU7 will be. If you want an upgrade, though, go for a 5751, as mentioned above.

The 12at7 is closer electrically to the 12ax7/5751 than the 12au7; especially the mu. The 12at7 has the same pinout as the other two so the tube will work just the same = light up. I have used 12at7/12az7's in place of 5751's in the Mapletree Ear+ with acceptable results. However, I would not use a 12au7 in the Ear+ .... not enough gain.

Remember, just because a tube has the same pinout does not make them compatible. The 6sn7gt and 6sl7gt, for example, have the same pinout and will work. But in most circuits, if you switch them, you are going to have problems with noise, clipping or distortion.


12AX7 Substitutes
Close or identical 12AX7R, CV10319, CV492, ECC83
Different rating or performance 12AX7A, 12AX7S, 12AX7WA, 5751, 5751WA, 6057, 6681, 6L13, 7025, 7382, 7494, 7729, B339, CK5751, CV4004, CV4017, CV8156, CV8222, CV8312, E2164, E83CC, ECC803, ECC803S, ECC863, M8137, QB339
Different filament voltage 20EZ7, 6AX7

12AU7 Substitutes
Close or identical CV10323, CV10666, CV4003, CV491, CV8155, CV8221, ECC82
Different rating or performance 12AU7A, 12AU7WA, 5814, 5814A, 5963, 6067, 6189, 6680, 6CC40, 7489, 7730, B329, B749, CK5814, CV3900, CV4016, CV9092, E2163, E82CC, ECC802, ECC802S, M8136

Look how far off the plate resistance and current are off for the 12AX7 vs the other two. The 12at7 and 12au7 are the closer matches; except for the mu/ gain.

12AX7 ....

Characteristics and Typical Operation
Class A AmplifierPlate Voltage ................................. 250V ........
Grid No. 1 Voltage ............................ -2 V
Amplification Factor .......................... 100
Plate Resistance (approx) ..................... 62.5K --- Current ................................. 1.2 mA

12AT7 ....

Characteristics and Typical Operation
Class A AmplifierPlate Voltage ................................. 250 V ........
Grid No. 1 Voltage Derived from Cathode Bias Resistor ....................... 200 Ω
Amplification Factor .......................... 60
Plate Resistance (approx) ..................... 10/9K --- Current ................................. 10 mA

12AU7 ....

Characteristics and Typical Operation
Class A AmplifierPlate Voltage ................................. 250 V ........
Grid No. 1 Voltage ............................ -8.5
Amplification Factor .......................... 17
Plate Resistance (approx) ..................... 7K --- Current ................................. 10.5 mA

 

[manofmathematics]

Outstanding! Thanks for that sacd lover. Good stuff to know.

 

[Skylab]

Interesting. I based my comments on the "Tube Substitution Handbook", 2nd Ed., which lists the 12AU7 as an acceptable substitute for the 12AX7, but does not list the 12AT7. I willingly admit I am not an engineer, and have only a rudimentary understanding of tube circuits. But the Tube Substitution Handbook has never steered me wrong.

 

[manofmathematics]

I'm going to try a couple 12AU7's and see how they perform. I hope well because I have four good ones.

Anyways, the Valvo/Raytheon combo has very good and a ton of bass so far. I'm kind of surprised the amount that I'm hearing.

 

[sacd lover]

Quote:

Originally Posted by manofmathematics /img/forum/go_quote.gif I'm going to try a couple 12AU7's and see how they perform. I hope well because I have four good ones. Anyways, the Valvo/Raytheon combo has very good and a ton of bass so far. I'm kind of surprised the amount that I'm hearing.

The 12au7 will work if you have enough overall gain in the circuit path.

 

[sacd lover]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif Interesting. I based my comments on the "Tube Substitution Handbook", 2nd Ed., which lists the 12AU7 as an acceptable substitute for the 12AX7, but does not list the 12AT7. I willingly admit I am not an engineer, and have only a rudimentary understanding of tube circuits. But the Tube Substitution Handbook has never steered me wrong.

You perked my curiosity and I found this ....


NJ7P Tube Database Search
12AU7
Medium-Mu Twin Triode
Base & Bulb ( RCA RC-15 - 1947)

Substitute Data
Preferred Substitutes ......................... 12AT7

Substitutes ................................... 12AZ7 12BH7

So, this would also seem to indicate the preferred sub for the 12at7 is the 12au7. The 12az7 is identical to the 12at7 except for a slightly larger heater current draw of 0.45 / 12az7 vs 0.30 / 12at7.


The 12ax7 goes in a different direction being looped in with higher mu, low current tubes ....


NJ7P Tube Database Search
12AX7
High-Mu Twin Triode
Base & Bulb ( GE - 1973)

Substitute Data
Preferred Substitutes ......................... 12DF7 12DM7

Substitutes ................................... 12AD7


Plugging in 5751 brings the subs full circle back to the 12ax7a ....


NJ7P Tube Database Search
5751
High-Mu Dual Triode
Base & Bulb ( RCA RC-29 - 1975)

Application
Miniature type used as a phase inverter and as a high gain amplifier in industrial control devices. Requires miniature 9-contact socket.

Mechanical Data
Outline ....................................... 6-2EIA

Base ...................................... 9A

Electrical Data
Heater Voltage ................................ 12.6/6.3 V

Heater Current ................................ 0.3/0.6 A

For other characteristics and typical operation, see 12AX7A