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Help Needed: Lower Gain on a Tube Amp?
Help Needed: Lower Gain on a Tube Amp?
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Old 23rd February 2013, 04:31 AM   #1
twix* is offline twix*  Canada
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Join Date: Jan 2005
Location: Regina, Sask.
Default Help Needed: Lower Gain on a Tube Amp?

Hi everyone, Can someone with some Smarts please help me figure out how, if possible to lower the gain on a very simple tube headphone amplifier circuit. I have very limited(basically zero) knowledge when it comes to eletronics and circuits on how they work etc. All I am good at is following instructions with great soldering skills.
The amplifier is a old Antique Sound Lab MG-HEAD OTL Headphone amp that I woke up from the dead. It was collecting dust for many years and now I want to try use it again. Back in the day, I did mod the crap out of if. It still works great except it would be real nice if I can lower the gain somehow because as of right now, when I turn volume very slight it gets too loud to quick. It always had this issue. The only way I knew years ago to change gain was to install a lower gain tube which works but is limited. I am sure (hope) that it is as simple as changing a ground resistor value, but which one? Please help.
Here is schematics. This is a DT version and mine is OLT version . Exact same circuit but not shown is the added OTL output added which should not matter to change gain.

mg head otl - Google Search
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Old 23rd February 2013, 06:15 AM   #2
azazello is offline azazello  Bulgaria
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Join Date: Aug 2008
Location: Bulgaria
To change input tube, it's need more skill how the tube works.
Simple is to put R divider on the input: Connect serial two resistor /for ex./ 220 kohm each. One end connect to middle point of potentiometer, second end - to ground and middle point of 220 & 220 kohm - to grid of input tube.
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Old 23rd February 2013, 06:24 AM   #3
Eli Duttman is offline Eli Duttman  United States
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Join Date: Apr 2004
Location: Monroe Township, NJ
Install 100 KOhm 1% tolerance resistors in the lines between the I/P jacks and the "tops" of the volume controls. The additional resistance attenuates the I/P signal by 6 dB. No more "hair trigger" volume control!

While the unit is on the bench, consider replacing D1 through D5 with UF4007s. The OEM parts generate a good deal of switching noise. UFnnnn parts are drop in replacements for the corresponding 1Nnnnn parts that generate much less switching noise.
Eli D.
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