183 master vol cap

[angelodp]

What is the function of the .02 cap on the board that goes to the MV wiper?
How will taking that value up or down effect tone.

[martin manning]

That would be the PI input cap. It's a high-pass arrangement, and I believe at it's present value the knee is subsonic. Accordingly, if you double its value you probably won't hear much difference. If you cut it in half you'll move the knee up by one octave where it will begin to cut out some lows. If you want to try that, wire another 0.02u across a 1/4" plug and insert it in the loop return jack. The second 0.02u will be in series with the one in the amp and give you 0.01u.

[angelodp]

Thanks Martin, I had swapped out to .01 a while back ( H's suggestion ) and was not sure as to what that change was producing. Probably because I also changed other components as well. One at a time is the lesson. Now I am migrating to 183 and can hear those lows change as I put the .02 back in. I might just go back to the .01 and cut some of those lows. BTW, this is on my Dlite which is morphing into 183.

[Structo]

Hey Ange,
The .02 is the coupling cap from the preamp to the phase inverter.
That cap feeds the grid of the PI.

Randell Aiken has a great piece on long tail phase inverters.

http://www.aikenamps.com/LongTailPair.htm

[angelodp]

It would seem that lowering C1 to .01 will work as long as C2 is 10x to improve low frequency response, as that would be exactly what the arrangement would be. C1- .01 C2 - .1
What constitutes an improved frequency response... is that entirely subjective.

From Aiken Below.

These capacitors (C1 and C2) are used to block DC levels from previous stages, in order to keep from upsetting the DC bias voltage on the grids of the phase inverter tubes.
These capacitors also determine the lower -3dB point of the frequency response of the phase inverter. If the input impedance is two times the grid resistor value, for instance, or 2Meg, and a -3dB point of 53Hz is desired, a capacitor of C = 1/(2*pi*53Hz*2Meg) = 1500pF, or .0015uF, would be required. Too large a coupling capacitor will increase the tendency for the phase inverter input to generate "blocking" distortion. If C1 is made small (less than .01uF or so, with 1Meg grid resistors), it will improve the low frequency response balance between the two output phases if the second coupling cap, C2, is made at least ten times larger than the first cap, C1.

[martin manning]

Ange, I think you've got it right... RA is talking about frequency response balance between the two triodes in the PI. C1 will control the bandwith of the signal admitted from the previous stage, and then to help balance the PI's frequency response he's suggesting that C2 should be 10x C1. But he says this is only necessary if C1 is small, i.e. less than 0.01, so you should be good to experiment with C1 between 0.01 and 0.02 as you like, without worrying about the value of C2.