randall rh200 head cuts out at 4ohms

[ray-p]

im using a 200 watt randall head with a 300 randall watt cab. when i output the amp at 8ohms and connect to the cabs 16 ohm input, i have a wonderful, sufficient, metal amp. when i output the amp at 4 ohms and input it into the cabs 4 ohm input the amp is about twice as loud but cuts out when i play low chugging palm muting. its not a speaker pop, the speakers can take it, the cab stops making sound. if i run the 4ohm output into the 16ohm input its identical to the 8ohm output in sound and volume.
when i was first looking for a cab, i thought my head was broken. it does this on other cabs too. its too bad because it is a loud amp but it seems to be hiding its other testicle in the 4 ohm output.
thanks to anyone who will educate me. i dont have the cab specifics but i can find them.

[J M Fahey]

Hi raypist.
Please at least post the exact head and cab models.
Links to their manuals or spec sheets would be even better.
Doubts: is your head tube or SS? Does it have an output transformer?
When it stops: only audio or power also goes?
I don't get your 4 ohm output/8 ohm output bit.

[teemuk]

I believe it's a solid-state amp, or actually a series of them.

Maybe it's a protection circuit kicking in? At least some Randall power amps have a circuit where the power amp's current limiter also enables a muting circuit so that he power amp can't output voltage to a hazardous load.

[ray-p]

cab is
http://www.amazon.com/gp/product/B0002GLE9S
(its only a very breif run down)

and i cant find a head description.

yes its a solid state. yes it sounds like what teemuk is describing, like a relay is switching something off. why does the amp thing its outputting to a hazardous load?

[J M Fahey]

Hi raypist.
Actually the "mystery guy" is the head itself.
It sure *must* have a model name printed on the front panel somewhere.
What does it say on the back panel?
Please describe the output jacks and what's written around them, specially the 8 ohm / 4 ohm stuff, any power ratings given, etc.
A couple pictures would help.
Thanks.

[ray-p]

oh, the name of the amp is in the subject line. its a randall rh200. the front looks like this
http://img1.classistatic.com/cps/kj/090625/686r1/919736j_20.jpeg

i cant find a back pic, ill have to get to the studio and take one.

the back has a lift ground and an effects loop. the 4 ohm output is 220w and the 8 is 200w.
ive heard about an overheating problem with these amps, ive never experience it just this cutout problem.

[Brymus]

  :trouble

[J M Fahey]

Hi raypist. have a good long look at: http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Randall/RH200+Head/50/1
Basically they say it all.
¿Does your amp have a 4/8 ohm switch?
Many talk about a heat problem, and "a big heatsink hidden behind a panel" and also about adding a fan.
I guess that pretty sums it up.

[ray-p]

no it doesnt have a 4/8ohm switch.

the amp doesnt have an overheating problem. its also the case that most of the reviews on that link are positive. i studied it prior to buying.

this problem occurs hot or cold. i can use the amp for hours as long as its plugged into the 16ohm input. i really think teemuk is onto something with the muting circuit. i cant seem to find anything out about that though.

[ray-p]

what the hell are "chip points?"

[ray-p]

well im hoping you half-assed electricians aren't chalking this one up as a win.

[phatt]

well im hoping you half-assed electricians aren't chalking this one up as a win.

Ha Ha With comments like that you won't recieve many chip points.
I don't know your gear but as a general rule 4 Ohms load on a SS power stage will dramatically up the *Current* the output devices have to deliver. (the lower the load the higher the current)

Hence you are triping the safety relay circuit.

Although a lot of gear quotes 4 Ohms as safe, some don't cope to well.
If the Amp has an EFX loop you could insert a simple volume control at that point which would enable you to just dial back the big signal that would be going to the power stage.
I had a similar problem with a DJ setup years back which was solved by lowering the final preamp output signal just enough to stop the silly thing triping out.

I would be annoying Randall if you want better answers.
Cheers Phil.

[Joe]

Can you measure the DC resistance of the cabinet at the different ohm settings (4/8/16)? Maybe the cabinet is miswired?

[ray-p]

less chip points but more information, thanks.

phatt, this is an interesting solution. ill look into it. what kind of volume control? a limiter? compressor? volume pedal?

joe, the problem happens on multiple cabs.

[phatt]

Hi,,
If you have a volume pedal handy that will save time.
Just back off the volume a tad and see if the problem goes away,,,, if so then consider a small jiffy box with a 100k Volume pot hard wired inside and 2 short length guitar plugs. Not that hard to wire up.
Cheers Phil.