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.
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.
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.
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?
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.
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.
:trouble
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.
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.
what the hell are "chip points?"
well im hoping you half-assed electricians aren't chalking this one up as a win.
Quote from: raypist on September 25, 2009, 03:18:02 PM
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.
Can you measure the DC resistance of the cabinet at the different ohm settings (4/8/16)? Maybe the cabinet is miswired?
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.
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.
cool, ill let you know if it works out.
why will this work differently than the volume controls on the guitar or preamp?
Hi raypist,
It's effectively a master volume that is left alone once set.
Ideally this would be a *Fixed* Voltage divider *inside the Amp* but that's
going to make a lot of work for you.
When an Amp is designed there is a limiting value before the main power Amp which has nothing to do with the master volume on the front.
IF a master volume was the only limiting R across the signal path before the main power stage then the guitar player would not know how much volume was safe.
Much like a popoff valve on a turbo engine,,, without it the driver has no way of knowing when to back off the throttle (master volume) and the engine would certainly self distruct.
In your case my guess is the Amp is not designed well enough to cope with 4 ohm loads. A fan won't fix it,,, nor a bigger heat sink.
Output transistors with a bigger *Current Capacity* would likely fix it but then you would have to rebuild the whole output section. :duh
Makers want to make their gear as loud as they possibly can as the competition is fierce,, so in the end they push things to far and this is the result.
I'm just showing you the simplest way round the issue if you are prepaired to loose some of that extra power.
You may find that 8Ohms is all round a better solution.
IMExperience 8 Ohms is giving the power amp a better chance of coping with hot enviroments and gives a better margin of error,,,, keeps it away from the danger zone of * to much current dissapation *
Phil.
I also have an rh200 and it used to cut out when it overheated (consistently at 4ohms) so I removed the front panel and cut the cloth out then I removed the rear panel and cut a large \________/ pattern out of it to allow safe heat dissipation and that has seemed to cure it so far however I'm going to add Marshall style vents to the top as well as an extra measure. Typically overheating is the problem with most randalls in general and if you can modify your head box to allow for better heat dissipation it helps dramatically.
Amen to it.
Try to mount some kind of fan, iy will always help.