Spot the clone...

[SSLtech]

Once it's set it's never needed to be trimmed as often as say, the meter trim, or the stereo couple adjustment (which needs to be re-set every time you hook it to a different partner) likewise R25. They should be changed and re-set if you swap out T4's.

I set them by using a trimmer pot, then substituting the correct value fixed resistor once the value is known.

R25 is slightly more tricky, since you have to re-set the zero db position of the front panel trimpot every time you shift the value of R25.... they interact and make it a little more tiresome, but it should still only take about 20 minutes or so to make the meter track fairly well... Setting the value of R7 should take 10 minutes or so.

Keith

 

[SSLtech]

Oh yes... the rest of the skinny on the R25 set method:

set "peak reduction" to off (minimum).

feed a constatnt tone to the input, adjust the 'gain' control until the meter reads zero VU in the 'output +4' switch position. Now, switch the meter to "Gain reduction" and check that it also reads zero. adjust the front panel trimmer if any correction is required.

Now, switching between 'GR' and '+4' should produce no change in meter reading... 0VU in both positions. -With the meter switched to '+4' slowly advance the 'peak reduction' control until the signal has dropped about 1dB. Switch between '+4' and 'GR' and check for any difference, and switch back to '+4', increase the peak reduction' until the signal has dropped 2dB and repeat the process....

What you're checking for is that the indicated gain reduction in 'GR' position agrees with the actual output signal reduction. If there is never any difference, youtr R25 value is correctly set. (unlikely!) Most of the time some adjustment is required, and even then the 'GR' position might read about ½dB too high from 0VU to -3VU, then perhaps ½dB too lw from -6VU to -10VU... as long as it's a reasonably accurate approximation, you're good. These are not usually precisely-matchable, nor does it really matter... the bouncing meter still gives a great indication of how hard the unit is working, and that's all you'll ever get from a correctly set-up real unit. -If the GR indication is -10VU and the signal is only dropping by 7dV, you've got some changes to make.

Every time youchange the value of R25 you have to re-set the front panel trimmer for the zero VU gain reducition needle position. For this reason it's a good idea to use a pot. The two will interact rather anoyingly, but it still shouldn't take long. Once the correct value is established, measure it and replace it with a fixed resistor.

Keith

 

[CJ]

You guess would laugh your ass off if I told you how I determine the correct curves!