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I just built an 18w Plexi and it's working well. I had erroneously believed that the HI vs. LO inputs were controlled by the 1M resistor that is added to the HI jack but not on the LO jack. However, now that I'm looking at adding a VRM, I see that I need to move that resistor to the other end of the input cable (input of V1) along with adding a DC blocking cap. Once that 1M resistor is moved, it is clear that it impacts both HI and LO inputs.



I exchanged a quick email with Stephen, and he confirmed that HI/LO is controlled by the 68K input resistors. After some Google searches, I'm struggling to find an accurate description of how that HI/LO switching works. Is it the wiring of the Cliff jacks? Does the LO input actually engage both the HI and LO input paths in parallel, while HI is isolated to just the HI input path?

Here is the input jack theory we include in most of our manuals now.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks for that. I see that the 1M resistor is bypassed by the LO input in the standard wiring config. Does adding that 1M resistance to both HI and LO with the VRM config change anything with the behavior of the LO input?

The 1M needs to be on the grid to act as a grid-leak resistor or the tube may no function properly. The blocking cap used with the VRM blocks DC not the AC signal. So from an AC signal perspective, it doesn't matter where the 1M is connected and performs as above. From a DC perspective, it has to be on the grid for good design.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

That finally makes sense to me. Thank you.