RK I having "fluxuating" FX Loop Send level issues.

Hello,

I just noticed this yesterday; I have a Lexicon PCM81 which is connected via a DMC System Mix to the series FX Loop of my Road King Series I head. While playing, I noticed that the input level LEDs on PCM81 had dropped "way" down compared to where they normally register(below red/clip).

I immediately put the amp in "Standby" and then back to "On". The LED's on the PCM81 were now registering normally.

My question is; is this an indication of a preamp tube going bad or something else related to the loop circuit?

I look forward to reading your responses!

I would start with the FX Loop driver 12AX7, V4.

Your symptoms resemble a tube failing in a cathode follower circuit, and the Loop (V4) and Tone Stack Driver (V3) are cathode follower spots.

Read here: http://forum.grailtone.com/viewtopic.php?f=4&t=25102&start=19

Avoid New Sensor brand tubes (Sovtek/Mesa "Russian-1", Electro-Harmonix/Mesa "Russian-2", Tung-Sol reissue & Mullard reissue) as those brands are known to give volume (signal) fluctuations in cathode follower use. If the tubes are original, I'll bet there is one of the tubes I mentioned in V4.

The current Mesa 12AX7 is a JJ which is fine in those spots. Just about any Chinese or NOS would be fine as well.

HTH,
Dom