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FDP Forum / Amp Mods, Repairs, and Projects / DRRI... removed preamp tubes and did not know the bias also goes up....

Webfoot

US

May 27th, 2012 10:33 PM Edit Profile Print Topic Search Topic
So about a month ago I started pulling 12ax7s out of my DRRI. I heard that amps become more lively because there is more current available. I got to the point where I was plugging into the normal channel and had pulled 4 tubes (including reverb and vibrato).

Fast forward to today... was going to experiment with 6V6s but before I swapped them out wanted to measure the bias they were currently at... it measured 30ma! What a surprise as I had biased this set around 17 to 19ma. My conclusion is that the bias circuit is not self regulating and every time I pulled a 12ax7 tube out, the bias would increase a couple mAs.

Also coincidentally, it seemed the amp was getting darker in tonality (lacking treble). I thought the tubes were going bad. So readjusted the bias back to 18 ma and more treble came back. So higher bias means less treble.

So I think this is the real reason people think the amp sounds better... it is actually making a hotter bias for the power tubes.

RussB

Connecticut

a little out of tune
May 27th, 2012 10:38 PM Edit Profile Print Topic Search Topic
There's a lot of fail in your post.

First and formost, who ever said,

""I heard that amps become more lively because there is more current available.

I've been around a while, and never heard that. Some folks like the gain boost they get when removing V1 (if they play the Vibrato channel) This is due to the shared cathode resistor, removing 1 tube changes the bias of the other 12AX7 tube.

This is simple to check. Install all the pre-amp tubes, then check bias. Remove them, and re-check bias. I say you won't see much, if any difference

FDP Forum / Amp Mods, Repairs, and Projects / DRRI... removed preamp tubes and did not know the bias also goes up....

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