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Old 21st April 2011, 04:12 AM   #1
yero is offline yero  United States
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Wink to much gain

Hi : I would like to thank two members for assisting me regarding my pas-3 120 hz hum. Connecting the return on the filament with the B+ return fixed the problem. I am using an alps dual ganged 100K pot. when i turn the pot to 9 o'clock my system is way to loud. Being that it is a linear pot I tried ESP log taper trick but i still reach full volume at 10 o'clock. Do I need to go to a 250K pot or is it practical to lower the gain of the pas high level stage. the high level circiit is modified as per David Vorhis 2/76 article from the Audio Amateur. Thanks much!!
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Old 21st April 2011, 10:23 AM   #2
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Originally Posted by yero View Post
Hi : I would like to thank two members for assisting me regarding my pas-3 120 hz hum. Connecting the return on the filament with the B+ return fixed the problem. I am using an alps dual ganged 100K pot. when i turn the pot to 9 o'clock my system is way to loud. Being that it is a linear pot I tried ESP log taper trick but i still reach full volume at 10 o'clock. Do I need to go to a 250K pot or is it practical to lower the gain of the pas high level stage. the high level circiit is modified as per David Vorhis 2/76 article from the Audio Amateur. Thanks much!!
Hi,

Going to a 250k pot wouldn't change anything; you'd still have the same level at 10 o'clock.
One option would be a series resistor between your signal input and the top of the pot. Using 100k there halves the signal at the top of the pot and thus at all pot settings. It may not be enough though and then you could, in addition to the 100k series resistor, put a parallel resistor in parallel to the pot (from pot top to ground) of say 33k, that will make your signal 1/4 of the original. If it is still not enough, experiment with the 33k parallel resistor, make it even smaller.

Have fun!

jan didden
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Old 21st April 2011, 11:09 AM   #3
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You could use a tube with less gain (12AT7/ECC81 instead of 12AX7/ECC83).
Another option would be to increase or add (local) feedback.
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Old 21st April 2011, 11:17 AM   #4
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Sorry, but I'm little bit not agree with Jan.
100 kohm in series connecting will be big resistance for small HF signal current and will decrease it. IMO, don't touch and listen how it is.
What kind of tube /1-st tube/ is in prestage?
Good listening!
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Old 21st April 2011, 11:21 AM   #5
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Originally Posted by hidnplayr View Post
You could use a tube with less gain (12AT7/ECC81 instead of 12AX7/ECC83).
Another option would be to increase or add (local) feedback.
But You can adjust plate current, changing Plate and cathode resistors.
Current of 12AX is about 1-2 ma. but for good sound 12AT needs current about 6-8 ma.
/Plate res. will be about 20-30 kohm, and cathode res - about 300-500 ohm/.

Last edited by azazello; 21st April 2011 at 11:23 AM.
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Old 21st April 2011, 12:05 PM   #6
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Yes, you can change the amp itself. Technically that is a good solution.
But it is very much more complex than just adding a few resistors.
I'm not sure the original poster wants to go the route to change the amp.

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Old 21st April 2011, 12:06 PM   #7
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Originally Posted by janneman View Post
Yes, you can change the amp itself. Technically that is a good solution.
But it is very much more complex than just adding a few resistors.
I'm not sure the original poster wants to go the route to change the amp.

jan didden
Yes, but reducing the input signal is the worst possible sollution i can immagine.
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Old 21st April 2011, 06:06 PM   #8
yero is offline yero  United States
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I have a large stock of 12ax7 NOS from al over the world including Germany, Great britain and holland so I would perfer to stay with 12ax7. I am able to change the plate and cathode resistance. I do have two 12at7 but would perfer to save them. Local feedback sounds like a good option.However this would require specific instructions to make these changes because I do not know what values to use and how to connect the local feedback. Woul any one be able to help me. many thanks to you all.
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Old 21st April 2011, 06:31 PM   #9
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IMO,....feedback will change the sound..../I'd like advice You use 12AT7 or ECC81, or ECC82/.
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Old 21st April 2011, 06:49 PM   #10
SY is offline SY  United States
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Originally Posted by yero View Post
Local feedback sounds like a good option.However this would require specific instructions to make these changes because I do not know what values to use and how to connect the local feedback. Woul any one be able to help me. many thanks to you all.
It's a complete redesign. Problem is, 12AX7 is not a great tube for modern line amps (you don't really want much gain out of them) and has lousy output impedance. You can only get the gain down by increasing feedback and that will make keeping things stable something of an exercise. The tube swap suggested won't help much- the gain is mostly set by the feedback loop, not by the open loop gain. Going from a 12AX7 to a 12AT7 mught drop the gain a dB, hardly worth doing.

If it were me, I'd just pull out the whole line stage board and replace it with a circuit that's happy at between zero and 10dB gain and with a low source impedance.
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