I'm a bit lost with this issue. Please help.

I have a Fender Mustang iV v2 as a master amp, and I am sending the effects out to the "Power Amp In" on a Fender M-80 head with two cabinets. I can only adjust the volume on the slave amp by adjusting the non-master volume knob on the master amp. Adjusting the volume on the master amp doesn't increase the volume on the slave amp. The slave amp has no "master volume" knob for the power amp, just a clean volume and a crunch volume.

Is my slave amp crap without a power amp volume knob? Or am I sending the wrong signal to it from my master amp?

first question is why? what do you need two cabs for (other than for fun, which they are then do it), if you are mic'ing up you wouldn't need to worry about it. it won't be a ton louder.

if there is no volume after the power amp in the slave amp, there is nothing to adjust. the effects loop must be after the poweramp volume.
There is generally nothing to adjust on the slave amp. Its all controlled by the master.
You could use headphones out into poweramp in right? That might allow you to use your master volume on the 'stang. Or that might be a bad idea
Last edited by geo-rage at May 22, 2016,
Quote by fly135
It appears that your slave amp is working exactly as you'd expect.

Yup, that's what it does.
Let me add this - the stack is really quiet as a slave amp, which is why I'm asking about volume.
Not all FX loops operate at the same nominal signal level. It appears that your slave amp and the Fender are different in that way. You could try the suggestion above of using the headphone output. But the headphone out should have some cab simulation filter that may impact the tone coming though another amp. If you are really serious about making it work, then something like this will boost the signal.

Last edited by fly135 at May 23, 2016,
The master amp would have the master volume pot after the effects loop circuit. Generally on most amps the channel volume is before the effects loop.
Most amps intended for arena use would have a dedicated Preamp Out/Ext. Amp Out intended for connecting external amplifiers so the effects loop can still be utilized for the master amp.
..I was watching my death.
I called Fender. They told me the Mustang IV amp effects out is line level and is not meant to be used to drive a slave amp. The Mustang series of amps is meant to be stand alone. The solution tech support recommended was to buy a power amp with volume control that accepts line in and use that power amp to drive my cabinets and sell my stack's head.
We could of told you to go buy a power amp. But usually people asking a question like yours are wondering how to make it work without buying expensive stuff. You could also find a cheap mixer to put between the two amps that will make it work.
"Slave" amps have come up here a couple of times recently, and while this might have made some sense in the '80's, it doesn't now. If you want more power out of your amp, a secondary power amp will work (you can get 1500W for under $300 and around 9 lbs of added weight). Then you have to find speakers that will handle enough of that power to make a difference, and at that point, you're nearly to a point where you should consider running a direct line out to a PA or just owning a bigger amp.
So I can't find a 1500W amp for $300. I can find one for $1000. I can find a 2x75 @ ohm amp for $440. Where should I be looking?