Enhanced Apache TX-1 Audio

[Robert & Linda McGraw K4TAX]

Borrowing from the excellent work others have previously done, I have taken the audio system one step further forward toward "better audio".  I used 0.056 mfd 400 volt paper or Mylar capacitors because they were available.  Any reasonable close value works, however 0.1 mf units may be overkill and increase unwanted low frequency flicker noise.

For preservation, I left all original components in place.

Starting with the input 12AX7 and the .005 to pin 2, add a 0.01 mf across that component.  This capacitor is basically in "mid air" between the component terminal board on the edge of the chassis and the tube socket.  Don't worry about the "spring" shield around the lead of the other cap.  Just hang the new one in mid air between the two points.

*To the 510 pf capacitor electrically between pin 1 and the top end of the GAIN CONTROL, add a 0.056 mf.  This is located on the component terminal board.

*To the .005 between the arm of the GAIN CONTROL an pin 7, add a 0.056 mf.  This is located on the component terminal board.

*To the 510 pf between pin 6 of the 12AX7 and pin 2 of the 12AU7, add a 0.056 mf.  This is located on the component terminal board.

*Lift the end of the 100K ohm resistor B+ that goes to pin 6 of the 12AU7.  This is located on the component terminal board.

*Connect a 0.056 mf capacitor between pin 1 of the 12AU7 and the top end of the LEVEL CONTROL.  Blue wire.  This added capacitor can be located between two points on the component terminal board.

*Add a 47 mfd 35 V electrolytic across the 2 mf capacitor in the cathode circuit of the 12BY7.  A wee bit hard to get to but worth it to improve the low frequency response.  

*Pull the 6AL5 from the socket.  It is no longer needed.

*Adjust the LEVEL CONTROL for about 50% rotation and leave it there.  Modulation is controlled by the front panel GAIN CONTROL. 

These changes nicely improve the low frequency response down to about 150 Hz and open the top end to over 5 KHz.  You may desire to run some external EQ or processing to further enhance overall modulation level and control of such.

One noted comment is that the OB2 VR tubes that regulate screen voltage were observed to fire on positive peaks thus limiting the amount of positive modulation as screen voltage is clamped at 210 volts.  Change them to 0A2 {150 volt} VR tubes.  This will require resetting bias and clamp controls for CW and SSB operation.  It should be noted that the changing of the screen regulators will allow the screen voltage to go to 300 volts and this is 50 volts above the recommended value for class C plate modulated conditions.  At the same time, I have not experienced any failures of 6146 or 6146A tubes.

If you have any questions, ask.  I think you'll be pleasantly surprised how well and how effective and how easy these changes are to implement.

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