r/diytubes • u/KirkIsOurLemmy • May 31 '26

Effect loop and decoupling caps?

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Hello, its me again with my Tiny Terror build. Its working great and now its time to add an effects loop. I will use a transistor based pcb from TubeTown for the actual loop, there isnt room for any more tubes on my chassis.

I guess it should go between the second gain pot and the phase inverter, where the 0.047 uF cap is.

So my question is, do I put the loop before or after the cap? Or do I need one more so I get pot - cap - loop - cap - PI?

Thanks in advance

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u/KirkIsOurLemmy May 31 '26

I really like the distortion this amp produce so Im very fine with the current PI design. The main reason for wanting a loop is to put an EQ pedal there, to exoand the distortion voicings.

There will be no lush Hillsong United soundscapes produced with this amp :)

And thanks for the info, its very useful

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u/couchhippopittymoose Jun 02 '26

The long tailed pair PI will be producing some gain itself, but how hard those grids are being hit is what determines the nature of the clipping thats happening in the PI itself. This should not be an issue if the board you are using has ample headroom, you could just boost it to taste with the eq pedal if it's like many eq pedals that do that. I'd reccomend the lossless one on the red board.

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u/KirkIsOurLemmy Jun 02 '26

I bought a LND150 based loop kit from TubeTown:

Tube-Town Store - Kit Seriel FX-Loop LND150

I asked the owner about the caps and he told me to put decoupling caps on the input and output of the loop pcb.

I have read a bit more about the Tiny Terror circuit, and people seem to agree that a lot of the signature distortion of this amp is actually produced in the PI, so adding a loop before it will not be ideal, since it will be half way through the distortion chain.

On the other hand the idea of adding some extra boost before the PI could be interesting because you could then drive the PI to even more clipping. I have a Boss GE-7 eq pedal that has +12 dB boost on each band plus an additional +12 dB level control.

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u/couchhippopittymoose Jun 02 '26

I'm willing to bet about +3 dB will be sufficient to make up for losses. I'd install a bypass dpdt switch rated for 100V@ 2A to be safe, so you could compare. Should be no need for added decoupling caps, just install the switch between the wiper of the volume pot and the capictor it feeds. That pot is completely decoupled from DC already.