4CM with Tube Amp, Is +4 db safe?

[Daevasmodeus]

I've been using my FM3 in a 4CM setup with a Mesa Boogie Mark IV at -10db for a while and it's been working out fine although I have to put my amp output volume at like noonish. I recently picked up another amp that doesn't quite get a good amount of volume at -10db without maxing the output volume on the amp, which seems like it might be stressing the power tubes pretty significantly. I attempted switching to +4db on the output and it worked out much better but I'm curious if this is safe on a normal guitar tube amp.

The wiki said "Set to -10dB when connecting the input to a pedal, a guitar amplifier input or when the output is clipping the input section of another device. Use +4dB when connecting to the output of a pro level device or to a louder output signal." This made me think that +4db is not safe for a guitar amp potentially, because "pro level device" seems like it's implying a mixing board or FOH. I'm not triggering any output clipping lights on the FM3 and everything sounds fine but wanted to check if anyone knew for sure if this is ok to do or not.

 

[Rex]

I think you're unnecessarily worried about the volume knob "stressing the power tubes." How loud an amp is depends entirely on how hard the power tubes are working. In other words, for a given amp at a given volume, the power tubes are putting out the same amount of work no matter what the knobs are set to.

Either your new amp is capable of giving you the volume you need, or it isn't. Setting your FM3's output level to +4 dBu isn't likely to hurt anything. But know that you amp’s volume knob comes before the power amp. It sets the volume into your FM3.

 

[Severed]

I use +4 with an EVH 5150III 50 watt in 4CM
have always considered amp loops to be in the pro audio category, but I may be wrong /shrug

 

[Daevasmodeus]

I probably should have specified that I'm also trying to keep the Input 2 -- > output 2 block from clipping too much. To do this I have to lower the preamp volume from being too high and I make up for the lack of volume by turning up the Master Output Volume on the amp. The Mark IV deals with this fine and I'm able to balance not sending too hot of a signal into the FM3 with raising the master volume high enough where it is as loud as I want it. As I understand it, the master output volume on both of my amps are completely independent of the fx loop and are only raising the power amp volume going into the speakers.

My new amp, a Mesa Roadster, has preamp volumes for each channel, a master output volume, and an FX loop send signal volume. Balancing all these together to not clip Input 2 -- > output 2 block causes the overall volume to barely reach stage volumes even when maxing the master output volume when using -10db. Using +4db fixes everything and the amp is closer in volume to what it is normally with nothing in the FX loop.

Also to clarify, I'm using max volume unity gain on output 2, and I am not running any amp sims, just drives in the front and time effects post preamp. +4db works fine with the Mark IV too, I just had to lower the master output volume. Even on that amp the master output volume had to be much much higher than with nothing in the FX loop on -10db. And if I just throw a delay pedal in the FX loop on either amp, nothing changes with the volume, so it seems like this is specific to 4CM and multiple competing levels adjustments.

 

[Rex]

Looking at schematics, it looks like Boogie does put the effects loop before the master volume, and preamp-out is a separate thing after the master volume. I was conflating the two.

I probably should have specified that I'm also trying to keep the Input 2 -- > output 2 block from clipping too much.

There's not an Input 2 --> output 2 block. Which block is clipping: Input 2 or Output 2?

Using +4db fixes everything and the amp is closer in volume to what it is normally with nothing in the FX loop.

Go with what works.