What's the Best Way to Reduce Gain in a Preamp - diyAudio
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Old 18th April 2014, 03:46 PM   #1
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Default What's the Best Way to Reduce Gain in a Preamp

Hi all,

This forum is so helpful...Thank you all!

Here's another question I hope will help me and others.

I have too much gain in a preamp. I'm using 6J5gt's or 6C5gt's in a simple SE circuit.

What is the best way to reduce gain?

I am using a 50k linear pot along with 7.5k resistor after the pot.
I am using a load resistance of 12.5k

Lower load resistor value?
Use cathode resistor and bypass cap?
Higher pot value?
Increase input resistance?
Find lower Mu Tubes? Triode connected pentodes? I have EL34's, 6V6's and 6DJ6's.

Any comments are welcome.
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Old 18th April 2014, 04:23 PM   #2
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There are tons of ways to lower the gain, but a schematic would help to determine which would best suite you preamp.
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Old 18th April 2014, 04:25 PM   #3
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Do you actually need any amplification at all ?
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Old 18th April 2014, 05:38 PM   #4
tomchr is offline tomchr  Canada
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Without a schematic, it's nearly impossible to give advice for your specific circuit. I'm guessing the better options will be to use a tube with a lower mu or use an un-bypassed cathode resistor. But without a schematic, we're shooting in the dark here...

Messing with the volume pot will have little to no effect.

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Old 18th April 2014, 06:10 PM   #5
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If you just want to throw away some gain without affecting tone then I suggest splitting a plate load resistor in two, making a voltage divider. Done early in the amp it may give 'more headroom' but worse s/n ratio. Without a schematic...
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Old 18th April 2014, 07:13 PM   #6
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Thank you all,

I completely understand about the need to see the schematic.
I quickly drew it up.

Thanks again
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Old 18th April 2014, 08:56 PM   #7
Ketje is offline Ketje  Belgium
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Wrong way
Try it like this
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Old 18th April 2014, 10:25 PM   #8
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you need to define what the source voltage is, and what the required drive to the amp is. Without that, everyone is giving well-intentioned best guesses.

I suspect that a simple cathode follower is what you need. But without the info above, I'm guessing too.
"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Old 18th April 2014, 10:28 PM   #9
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Ketje has it right on the volume pot being wrong. I notice you are basically trying to use it at the generic operating point from the datasheets, plate voltage 90V, grid 0V, 10mA plate current. What they don't mention is that this is with a B+ voltage in excess of 300V, so the load resistor can be a sizable value. If you want to add distortion for that "tube" sound, this setup is fine.
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Old 19th April 2014, 04:46 AM   #10
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place a 150k in series with a 15k to ground. Take the output across the 15k
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