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Old 30th April 2011, 08:13 PM   #1
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Default 6AM4 driving 45???

I was given a schematics for using 6AM4 driving 45. Enclosed is the schematics - can the design be improved for better sound. Thanks.
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Last edited by xecluded; 30th April 2011 at 08:19 PM.
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Old 30th April 2011, 09:01 PM   #2
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Hi!

The 6AM4 is great for phono preamps but it is not very suitable for a driver tube in a power amp IMHO.

Check the plate curves:

http://frank.pocnet.net/sheets/093/6/6AM4.pdf

The cathode resistor chosen and B+ will not yield the voltages as indicated in your schematic. It would run in a very non linear region and thus dominate the sound of the amp.

Best regards ... Thomas
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Old 1st May 2011, 03:51 PM   #3
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Quote:
Originally Posted by Vinylsavor View Post
It would run in a very non linear region and thus dominate the sound of the amp.

Best regards ... Thomas
Yes, but this can be useful for harmonic cancellation with certain output tubes. Not sure about the 45, but it works with the 300B.
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Old 1st May 2011, 06:43 PM   #4
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I thought the amp sounds excellent at my friend's place. I want to duplicate it but just want 2nd opinion and see anything can be improved before I do it.
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Old 1st May 2011, 09:07 PM   #5
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Hi!

If you heard the exact same circuit already and you like it then go ahead and build it. You will always get different opinions and preferences if you ask on a forum.

Best regards

Thomas
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Old 1st May 2011, 09:33 PM   #6
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Just want to see if it can be improved... Hope that's not too much to ask
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Old 1st May 2011, 09:37 PM   #7
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Hi!

No that is not too much to ask, But as I meantioned: different people have different opinions what would be better. As I mentioned I think the circuit is inferior. I would use a different driver which will not produce as much distortion and which would have more headroom. But this would lead to a completely different circuit which you have not heard yet. At the end you might not like it.

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Old 2nd May 2011, 05:10 AM   #8
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Originally Posted by xecluded View Post
Just want to see if it can be improved... Hope that's not too much to ask
Not too much to ask. The 6AM4 isn't ideal for that application. There are some things you can do:

*) Ditch that 22K passive plate load, and replace with an active (CCS) plate load. Linearity and output swing both improve. 22K is really too much load for a high-u type triode.

*) Ditch that 100uF cathode bypass capacitor so that you can set it up for gNFB. If you have a good open loop design, you can always improve it with gNFB. If you don't want to do that, replace the cathode bias resistor with one or more LEDs to set the bias voltage.
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Old 2nd May 2011, 09:37 PM   #9
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Thanks Miles for the tip. I will try to figure how to do the 2 items that you've suggested. I could only follow the already design to put something together but when it comes to design etc..., i am really clueless. Will ask around to see how that can be done. Thanks again.


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*) Ditch that 22K passive plate load, and replace with an active (CCS) plate load. Linearity and output swing both improve. 22K is really too much load for a high-u type triode.

*) Ditch that 100uF cathode bypass capacitor so that you can set it up for gNFB. If you have a good open loop design, you can always improve it with gNFB. If you don't want to do that, replace the cathode bias resistor with one or more LEDs to set the bias voltage.
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