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Old 14th January 2011, 05:12 PM   #1
Anchan is offline Anchan  United States
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Default Tube rolling input tubes Simple SE

I've seen a few threads and random comments on this topic, but it has been difficult to nail down.

The JJ 12AT7 sounds really good no doubt, but the gain is a tad much.
As I understand it, the 12AY7 is a bit lower gain. Can this be directly subsituted directly in its place?
Here are the data sheets for the 2 tubes:
http://www.drtube.com/datasheets/12ay7-rca1953.pdf
http://www.drtube.com/datasheets/ecc81-jj2003.pdf

If I am reading correctly, they both are biased around 10ma, which is the standard config for the Simple SE anyway.

When deciding whether something can be a direct replacement, can someone give me a few pointers on analyzing a tube data sheet? A lot of times on these forums, a lot of anecdotal information is exchanged, but getting down to basics and understanding the datasheet better would go a long way.

There was a posting on someone using a 12BH7, and they mentioned that they adjusted the CCS for 20mA. But the I believe the datasheet states 11.5mA.
http://www.drtube.com/datasheets/12bh7a-rca1955.pdf
What am I missing? Is the 12BH7 a direct replacement for the 12AT7 in the simple SE circuit? From my understanding of the datasheet it would appear so, but please set me straight if not.

Still learning. Thanks for any responses.
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Old 14th January 2011, 05:56 PM   #2
rknize is online now rknize  United States
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It appears you are looking at the "characteristics" section of the datasheet. These are just there as an example of how the tube behaves at a certain operating point. This gives you a rough idea of the tube's transconductance, mu, and so forth. Some data sheets have more than one of these. Some have none.

To find the "best" operating point of a particular tube in an established circuit, you need to look at the plate curves and draw the appropriate load line. The explanation is rather long winded and you would be better served reading a book on the subject. I recommend that you pick up a copy of Valve Amplifiers by Morgan Jones.

The 12AY7 does have lower mu, but it also has rather high plate resistance. I don't think it makes the best driver tube. The 12BH7 has even lower mu and much lower plate resistance. From the plate curves, you can see that the tube is more linear with higher plate current.
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Old 14th January 2011, 06:50 PM   #3
Anchan is offline Anchan  United States
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Quote:
Originally Posted by rknize View Post

To find the "best" operating point of a particular tube in an established circuit, you need to look at the plate curves and draw the appropriate load line. The explanation is rather long winded and you would be better served reading a book on the subject. I recommend that you pick up a copy of Valve Amplifiers by Morgan Jones.

The 12AY7 does have lower mu, but it also has rather high plate resistance. I don't think it makes the best driver tube. The 12BH7 has even lower mu and much lower plate resistance. From the plate curves, you can see that the tube is more linear with higher plate current.

Load lines. I guess that is the next step in understanding all of this. So its not as simple as just comparing variables on a datasheet. I'll pick up that book you mentioned. Thanks a lot- I appreciate it.
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Old 14th January 2011, 07:05 PM   #4
rknize is online now rknize  United States
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No problem. I was in the same boat as you when I started out about a decade ago. The closer I looked the less sense it all made. MJ's book guided me through it.

Last edited by rknize; 14th January 2011 at 07:11 PM.
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