Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Cinefactus22]

I'm having problems with my Randall RH300 G3 guitar head.

When I turn the amp past 5 (because we play that loud...) the amp eventually (from what I can ascertain) drops after about 30mins to half my volume...and I'm left turning way way up and getting mushy...

I was running my Zoom G7.1ut (at the -10db setting) into the FX loop and into a oversized Mesa 4x12 recto cab in the 4ohms input (still drops out at 8ohms, I think quicker than 4ohms cuz I'm cranking it louder). I read somewhere that once you run two cabs it engages the full 300watts and was previously only pumping 200 watts, not sure if that's the case or not but I did add a Peavey XXL 4x12 (which I think is 16ohms) and started running my pedal through the front (+4 db switch engaged) which has made my sound greatly improve. The added volume of the other cab (or the extra wattage) has allowed me to keep my master volume below 5 without issue.

I would still like to figure out the problem since other bands will be using my amp onstage and it would seriously suck to see it happen to them.

Other information that might be relevant:

-It has the same symptom when I use the amp distortion and the Zoom FX is not connected.
-I have kept watch on the temperature, the internal fan works fine and have cleaned it of dust, have placed a house fan focused on it; air conditioned room.
-The sound is great but has an abrupt volume drop, still loud and the same quality of tone but not as impressively loud as when I first turned it on.

I've read that this series of amps have been problematic...

[Roly]

"30 minutes" strongly suggest that this is basically a thermal problem.  A quick Google of this amp produced several people apparently having thermal/cooling problems with this one, including a post here back in 2010;

http://www.ssguitar.com/index.php?topic=1513.0

...and one on MTS where his amp caught fire!   

a host of problems related to basically sloppy engineering
...
oh and also turning the f**k off when you turn it up to a certain point. That was the real kicker for me when I had mine. Get loud? NOPE, turns the f**k off.
-----
a friend of mine bought the plus model due to the bad reputation of the non-plus models. it sounds great but it has the same problem of turning off once the volume goes past 4.
-----
a couple weeks ago and I played two G3 in a row, factory fresh, with the "turn off" problem,

The output stage appears to be MSOFET-based.  From the above it seems that this model amp, at least, has some sort of generic problem, most likely related to the cooling of the MOSFET output stage.  Unless somebody turns up a known fix (such as added cooling) I don't think there is much that can be done with what seems to be basically bad design problems - sorry.

[J M Fahey]

Can't find that schematic now, but those modern Randalls often have a thermal mute built in.
Not mechanical (bimetallic) but a PTC type thermistor which when hot cuts power to the power amp input stage.
You *might* bypass it, but it would be a dangerous move.
Cooling is never bad, so I'd add a little fan there.
Post a couple chassis pictures, inside and outside, maybe you can add two.
Check this RG75B schematic.
Thermistor RT1 (PTC), through Q4/Q5/Q3 cuts current to differential pair Q1/Q2 , muting the amp.