Tube FX loop before or after tone stack

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Apr 15, 2019
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You’ll have to figure out a way to knock down the level when using pedals and then bring it back up before going back into the amp. It’s a well known issue, which I’ve experienced myself.
The official SLO 100 schematic has a note on it that says to parallel the 2.2K resistor with a 1K resistor for a 0dBm send level and to bypass the cathode on the recovery stage. This seems to be something similar to what you are talking about. Maybe you could put that on a DPDT switch to toggle between line level and instrument level.

0dBm? That's one milliwatt. An odd denomination for specifying levels in an audio circuit. Nonetheless, I began poking a calculator just to see what we are talking about here. 1K, 2.2K and 1M in parallel is 687. 1mW with 687 Ohms works out to 0.8289Vrms = 1.172Vp. That seems a little hot for instrument level to me.

Is 0dBm even possible here? Take that 1.172Vp = 2.344Vpp and multiply it by the 100K/687 voltage divider and you get 343.54Vpp. What are the chances that this CF can swing 343.54Vpp with a supply of +378V? I drew a load line and came up with 255Vpp and then I added 20V for going beyond the 0V grid curve for a total of 275Vpp. So 0dBm is nowhere near possible under his stated conditions.

As long as I had the calculator out, I took that 275Vpp and ran it through the voltage divider for 0.94Vp = 0.6659Vrms. That works out to 0.6454mW with 687 Ohms. Take the log of that and multiply it by 10 and it's -1.9dBm.

Wouldn't it be better to work with -10dBV and +4dBu since these are standard line levels?

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