coupling cap value help for tubelabSE

[sbelyo]

Hi Guys..

Here's the link to Tubelab's schematic

http://www.tubelab.com/AssemblyManua...matics_TSE.htm

The primary on the OPT is 5K ohm and the output tube will be a 45

The OPT's go as low as 28 Hz so I want to know what value of should the coupling cap (C9, C11) be to get me that low?

If I use 1 / ( 2 * pi * C * R ) and R is the value of the primary I should need 1.1 uf to get to 28.9 Hz

But I'm not shure what the value of R is based on the schematic?

[tubelab.com]

The "R" value that is seen by the coupling cap is the 121K ohm resistor (R15) plus the equivalent resistance seen by the signal looking into the bias pot, which is dependent on the actual pot setting. You will always have at least 140K ohms (bias pot at minimum) and in practice about 160k ohms.

Given this, just about any cap value between .1 uF and 1 uF works fine. .1uF works out to 11Hz, and values over 1uF cause the bias adjustment to react too slowly. I use .22uF in most of my amps, and .68uF Auricaps in the good ones. I still have some of the old Auricaps that were rated at 450 volts. The new ones are rated at 400 volts which is marginal in some TubelabSE designs.

[sbelyo]

thanks George

[sbelyo]

Would there be any problems with the circuit if I went down to 0.047uf ?

That would give me around 24 Hz

[tubelab.com]

It will work, but I tend to prefer a larger value. The formula gives 24 Hz, but roll off (and phase shift) starts quite a bit higher.

[sbelyo]

I'll go with the 0.1uf then. I was only concerned about size, but it looks like it will fit