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twix* 23rd February 2013 04:31 AM

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

azazello 23rd February 2013 06:15 AM

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.

Eli Duttman 23rd February 2013 06:24 AM

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! :D

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.

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