Hey Guys,
We are aware of the “hissing” issue that some people are having with the effects loop and we have been actively digging into it.
Let me see if I’m able to explain how the effects loop works:
The signal level comes into the amp at a fairly high level, the level is then dropped before it is sent to the pedals (this is done to keep the signal from being too hot, which might fry some pedals). On the return side (when the signal passes back into the amp), the amp boosts the signal up substantially to drive the phase inverter and power tubes. The level we set for the pedal phase is so low (again to prevent any pedals from being fried) that any noise from the pedal will be magnified when the signal is sent from the pedals back into the amp. If the pedal doesn't add much noise, the amp doesn't have much noise to magnify when sending the signal back to the amp. If the pedal introduces a great deal of noise, then the amp will more significantly magnify that noise in the return line level. This issue has been tested with several pedals; it will not happen with all makes and models of pedals. Actually, this is only happening with a handful of pedals. Please keep in mind that the loop circuitry is always active, even when it is bypassed via the footswitch.
So, if this is making sense at all, we do have a simple modification that basically adjusts the send/return levels so that the amp sends more signal to the pedals, and boosts less on the return side. This will greatly reduce any pedal noise while retaining overall signal level if your particular pedal is causing this issue. The modification is a simple resistor value change, and only requires the replacement of one small component. If you feel comfortable making this change on your own, or if you have a trusted amp tech, we’ll be happy to send you the replacement parts and wiring diagram for free. If you’re uncomfortable making these mods, please contact your dealer so we can make arrangements to have the amp sent back to the factory so the work can be performed (for free). Please keep in mind, this is only necessary if your particular pedal setup is causing an unusable amount of background noise. Please also bear in mind that a small amount of hiss coming from pedals is normal (just as hiss coming from cascading tube gain stages is normal).
Cheers,
Shawn