marshall el84 20/20 confusion

ediz · 15-03-2005 09:17PM #1

Hi, i'm something of a novice when it comes to rack gear and i've been looking about for a valve power amp to add to my rack (which currently consists of a single pre-amp, ada mp1). I spotted a marshall el84 20/20 for a decent price second hand and was about to swoop as i'd no need for more power and it was a decent price when I checked the marshall specs on it (http://www.marshallamps.com/images/handbooks/pdf/rack.pdf).

Bear in mind i'm somehwat clueless about rack gear but as I read it if I use it is there no way out of having to fork out for a second cab? I've poked about with other peoples stereo power amps and they all seem to allow single channel usage through one cab but in the manual for the 20/20 it repeatedly warns: "when 20/20 is in use, speakers must be connected to both sides at all times" .

Is there anyway of circumventing this or do I have to factor the price of a new cab into the bargain I was excited about

? Greatly apreciate any advice before I do something unpleasant to another innocent amp (i've already borked a rather nice trace elliot velocette combo I need to get fixed)

feylya · 15-03-2005 09:25PM

Or you could just rewire your current cab assuming it's a 4x12...

Eoin Madsen · 16-03-2005 02:11AM

Running an amp unloaded (no speaker) or with the wrong load (lower impedance than the amp) is bad for it. Valve amps are particularly volatile in this respect. If your present cab has 2 or 4 speakers you could rewire it for separate inputs. It's also possible to bridge the amp channels as well, but unless they have an easy option for it or you really know what you're doing, I wouldn't recommend trying it.

What is the wattage and impedance of the cab that you want to use?

ediz · 16-03-2005 02:51PM

At present I have the sum total of zero cabs as i've previously been using combos. I'd been looking at getting a 2x12 so the problem with the power amp is that i'd need to buy a second one or else get a stereo 4x12.

This may seem a pointless question as I know zilch about rack gear but is it feasible to create a dummy speaker load on the seond channel by putting a matching resistor (4/8 ohms etc) hooked up to the output? If that's in any way manageable it'd be preferable to bridging the two to 40watt (unlike the higher output marshalls the 20/20 doesn't have the facility inbuilt unfortunately )

feylya · 16-03-2005 05:13PM

Well, if you still have to buy a cab, why not just buy a stereo 2x12?

Eoin Madsen · 17-03-2005 11:58AM

ediz wrote:

At present I have the sum total of zero cabs as i've previously been using combos. I'd been looking at getting a 2x12 so the problem with the power amp is that i'd need to buy a second one or else get a stereo 4x12.

This may seem a pointless question as I know zilch about rack gear but is it feasible to create a dummy speaker load on the seond channel by putting a matching resistor (4/8 ohms etc) hooked up to the output? If that's in any way manageable it'd be preferable to bridging the two to 40watt (unlike the higher output marshalls the 20/20 doesn't have the facility inbuilt unfortunately )

It's not really feasible, no. A proper dummy load is more complicated than that, especially for a valve amp.

Get a stereo 2x12. It needs to contain two 8 or 16ohm speakers rated 20-30 watts each. Any cab containing two such speakers can be rewired for stereo. It's a simple modification.

ediz · 18-03-2005 03:36AM

Cheers for the answers, I think you're both right, i've rather sheepishly since discovered the existence of stereo 2x12's (for some reason i'd come to the assumption that stereo speaker cabs were confined solely to the province of 4x12's). My beady little eyes are now fixed on a stereo marshall 1936 (until I inevitably discover that what I intend for it will probably burn my house down)

marshall el84 20/20 confusion

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