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Old 20th May 2007, 08:25 PM   #1
nostar is offline nostar  United States
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Default gain curve questions

I'm using and modifying a small tube amp which uses the 2 stages of of a 12ax7 as it's voltage amp stage. Using this link as an example, since its pretty much identical in design (values vary):

http://sound.westhost.com/amp-basics1.htm#valve

I did some analyzing of this amp by running a sine wave into it from 20-20Khz and scoping the different stages. The cathode bypass cap Ck on the first stage of my amp is 25uF (with an Rk of 3K), which creates an expected relatively flat gain curve on the output of the first stage throughout the audio range of roughly 20-18Khz.

The 2nd stage though, has a .01uF cap at Ck (with the same Rk of 3K), which causes a high-pass effect at about 5Khz and above, so in effect it seems like a treble boost, but in effect it is really squashing the gain from 5Khz and below (low-cut). I added a switch which I am sort of calling a 'bright' switch that switches between a 25uF and the stock .01uF cap at that point, but it acts more like a power boost than a bright switch since it doesn't really boost, in fact it either cuts gain from 5Khz and below, or passes the whole range.

I guess my question is what would be the manufacturers reason for using that cap? These amps were originally in an old tape deck with really crappy on board speakers, so I thought perhaps it was an attempt to brighten up the speakers maybe? Thoughts?
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Old 21st May 2007, 11:46 PM   #2
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Could be part of the NAB equalization? Was this amplifier for tape head playback?

jh
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Old 25th May 2007, 08:00 AM   #3
nostar is offline nostar  United States
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They were used for both tape playback and recording to tape
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