Diagnosing a spitting/popping channel- need some help!


Did a search here and I think I am on right track, but might need some help here. Getting a sporadic spitting/popping sound sometimes in left front monitor speaker in a 5.1 setup.
I have the following signal chain:
  • Pioneer HT receiver
  • VAC Standard Preamp
  • Odyssey Audio Monoblock Extreme amps
  • Monitor Speakers are ACI Jaguars and there are dedicated ACI Subwoofers for each channel.


I use the Pioneer HT receiver L/R preamp outputs into the VAC preamp HT bypass inputs for when I use the TV so the Pioneer HT receiver amp sections are out of the chain for the L/R fronts.

Have done the following troubleshooting:



  1. I switched the preamp out L/R cables and the spitting/popping continued out of the same left front speaker (rather than switch sides).
  2. I turned off the Pioneer receiver completely and still get the popping.
  3. I assume that means it is not the Odyssey amp; and not the Pioneer so it must be in the VAC preamp?

Am I right in assuming it is likely a bad preamp tube in the VAC?

The VAC manual says it has the following two pairs of tubes in the Line stage:
2 x 12AU7 & 2 x 12AX7 (line stage)

Where/how do I start checking tubes?

Am I on right track here?
Thanks.
lkdog
Tap on each tube with theĀ  end of a pencil....if you get the pop, clean the pins of the tube
Swap the left and right channel tubes. Start with the AU7. If no popping then swap the AX7.
IKdog, you have not localized the problem yet. Swap cables on the output of the VAC. If the problem changes sides then you know it is the VAC. If it does not change sides then it is the Lt amp or beyond.
Had a similar problem with a Vac Standard Preamp. Problem was bad 100K Ohm plate resistor for the 12au7. Measured good but was breaking down with high voltage.