LTPI capacitor value impact

[jordan86]

I built this attached LTP scenario into an amp (taken from a Vox inspired Dr Z clone). Just curious if any theoretically minded guys could tell me what raising or lowering the value of C8 would do? Most LTPIs I've seen use a much larger cap there (.1uf), many times larger than the respective C9. More often than not it seems it is at least 4X larger than C9, respectively.

Aiken has this comment and image from his LTP page (with his image), that I don't fully understand.

"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."

[sluckey]

I don't think Aiken's comment applies to your circuit. In your circuit, C8 is simply holding the lower grid at AC ground. A .1µF cap will do a better job than a .01µF cap.

[HotBluePlates]

... tell me what ... C8 would do? ...

Depending on your chosen point of view, C8 (from your attached schematic)/C2 (from Aiken's diagram) could be seen as coupling the feedback signal into the "other side" of the long-tail.

Or, that part could be seen as holding that side's grid at about-ground potential, while the same section is drive at its cathode (which is a "grounded-grid" circuit arrangement).

Aiken says to make that cap large, because we want its impedance as low as possible/feasible for frequencies of interest.  And you will recall that the capactior's reactance (at 80Hz for a guitar's Low E) is:

     X_c = 1 / (2 x π x 80Hz x C) = about 1 / (503 x C), where "C" is in Farads (so many decimal places for a sub-µF cap)

[HotBluePlates]

... tell me what ... C8 would do? ...

Depending on your chosen point of view, C8 (from your attached schematic)/C2 (from Aiken's diagram) could be seen as coupling the feedback signal into the "other side" of the long-tail.

Or, that part could be seen as holding that side's grid at about-ground potential, while the same section is drive at its cathode (which is a "grounded-grid" circuit arrangement).

Aiken says to make that cap large, because we want its impedance as low as possible/feasible for frequencies of interest.  And you will recall that the capactior's reactance (at 80Hz for a guitar's Low E) is:

     X_c = 1 / (2 x π x 80Hz x C) = about 1 / (503 x C), where "C" is in Farads (so many decimal places for a sub-µF cap)

I don't think Aiken's comment applies to your circuit. ...

I think Aiken's comment still applies.  If the cap were a smaller value, perhaps its voltage fluctuates more and reduces the gain of that side (but only for very low frequency, depending on the cap value).