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Paia 9407 Tube MicPreAmp, Quick Start Manual, Page: 6 / 6

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Paia 9407 Tube MicPreAmp Quick Start Manual Download Page 6

This capacitor filters/smooths the voltage to the tube plates from the voltage multiplier output and at 47uF or 
more, hum particular to the tube path (blend cw) and due to ripple originating at Vcc to IC1 is minimized.

Then, if not enough 'Drive' is obtainable to get the tube drive LED lighting,

3) Substitute 100k,1% resistors for the 33k, 1% ones at resistors Rs 20 and 39.

This increases the gain of the MIC input stage, presenting more signal to the tube.

Then, if hum is apparent in the audio output when a condensor mic is connected with blend ccw for solid state,

4) Substitute a greater value capacitor with a 50V or more rating for capacitor C12.

This improves the filtering/smoothing of the voltage used as the phantom supply to the mic (again, from the 
voltage multiplier/Vcc ripple) and I've heard reports of users going as great as 1000uF here.

I'd say try just 1 and 2 first, then do 3 and/or four if needed.  With the Sovteks, the dc-filament mod isn't needed 
and can make hum apparent where it wasn't before due to the increased ripple currents associated with the 
added resistor/capacitor filter.  This mod came to be back when we stocked tubes labeled, Made In China.  

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The setting of the Symmetry trim can be 'calibrated' as follows to optimize the s/n / headroom through the tube.

A signal generator is connected to the input and the output is viewed on a scope (or in a pinch, monitored by 
ear) with the blend set to 100% tube signal (blend fully post/cw) and the Drive control and the Symmetry trim 
alternately adjusted.

Start by setting the Bend fully post and the Drive and Output at about 12:00 to 1:00.  Connect the input signal 
source and vary it's level externally from minimum up until the tube begins to clip due to overdrive and stop at 
the onset.  Most likely, it will be either on the top or the bottom of the waveform extreme (oh yeah, use a sine or 
triangle here so you can detect the flattening due to clipping).  Adjusting the Symmetry trim one way will increase 
this effect and the other will remove it.  Adjust to remove it and go back and increase the input to the tube by 
either increasing the external signal input level or the Drive control to the onset of overdrive again, and, readjust 
the SymTrim to remove it.  Go through this process again and again and eventually, you'll notice that no more 
input signal or drive can be applied to cause overdrive that is removeable with adjustment of the SymTrim.  This 
is the 'calibrated' setting for the trim.

related,

http://www.prosoundweb.com/recording/tapeop/paia.php

Summary of Contents for 9407 Tube MicPreAmp

Page 1: ... A common trouble is the tube not being fully seated in the socket because it takes more force than you might expect to get it in the first time This coupled with there being a spring in the cover can make it seem like it s held in place with the cover spring but this isn t the case It is possible to peek through an opening near the mounting ears in the base of the socket to confirm that the tube ...

Page 2: ...h the chassis If the hot wire going to the power switch is on the added power connector terminal in contact with the case it can short and burn the transformer If the neutral transformer wire going to G contacts the case via an added connector it makes a ground loop which an introduce a hum in the audio output A connector with two circuits isolated from the case prevents trouble in either instance...

Page 3: ...60kHz on off switching and in a 4 12 vdc range a near zero or Vcc 16 to 18 vdc reading would point to no switching action that could be due to a failed IC or soldering trouble in the area of sections IC1 A through IC1 D E looking at the schematic and on the board 0 0 0 It is best to measure the dc power supplies right at IC pins The power supply voltages should be getting into the eight pin dual o...

Page 4: ...r As it goes one way the tube gain will be higher and more unsymmetrical The other direction will provide less tube gain with symmetry more drive can be applied before the tube clips overdrives The 9407 Tube MicPreAmp sounds good with a guitar connected to the xlr input via a TRS jack to xlr plug adapter and it would accommodate the bass range So does the 9305 TubeHead It is two channel and they c...

Page 5: ...2 V tube6 R16 22 to 24 V R16 C17 21 to 22 V C17 R22 60 to 90 mV tube2 R22 60 to 90mV R22 R29 12 to 16 mV tube3 R33 115 to 145 mV tube 8 R32 90 to 110 mV tube 7 C20 R24 110 to 160 mV C20 R28 0 3 to 0 8 mV R28 H 0 3 to 0 8 mV R11 R12 35 to 40 V Scott s Tube MicPreAmp mods 1 modify the wire length for wire A so instead of running from the switch to the board along between the circuit board and the fr...

Page 6: ...ade In China The setting of the Symmetry trim can be calibrated as follows to optimize the s n headroom through the tube A signal generator is connected to the input and the output is viewed on a scope or in a pinch monitored by ear with the blend set to 100 tube signal blend fully post cw and the Drive control and the Symmetry trim alternately adjusted Start by setting the Bend fully post and the...

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