concert bass minimum impedance

[po-mo preschool]

Hey all,

It seems like people on this forum are working with the assumption that the Concert Bass has a minimum impedance load of 4 ohms. I thought so too, until just the other day I was looking at scans from the 1979 Sunn catalog, where it lists the impedance as 2 ohms. Have a look:
http://sunn.ampage.org/site/catalogs/1979/1979catp8.jpg

As you can see, this is the later model Concert with the red logo. Is there any reason to believe the specifications changed during the time it was produced? Has anyone played theirs with a 2-ohm load? I actually own the amp but I only have a single 4-ohm cab and can't test this out.

[JoeArthur]

There may have been an upgrade sometime in the late 70s.  I can't tell you specifically as the only schematics for these models that I have are dated 1975.  At the time, the output stage looked pretty much identical to the earlier models.  Of course schematics don't tell you things like changes in heat sink designs as an example.

If there was an upgrade, to a similar but higher power handling device like some of the later motorola devices then it would probably work fine... but I wouldn't push two pairs of 2N3055's down as low as 2 ohms. 

[po-mo preschool]

Thanks for the reply...interesting point about the heat sink!

[Robbie Nuke]

I have a first generation Concert Bass amp, the ones with the brushed aluminum front panel and Distortion knob. I usually run it with a single 8 Ohm cabinetand the back panel heat sink gets warm to hot to the touch. Once in a while I run it with two 8 Ohm cabinets (total of 4 Ohms) and it can get quite hot to the touch!

I saw a first generation Concert Slave amp on eBay showing a rear panel photo, the nomenclature on it stated that "running a load less than 4 Ohms may cause overheating". I would tend to agree with that statement.

So unless there was a way to increase the heat sink area and/or increase air flow (i.e. forced air fan), I would avoid a 2 Ohm load.

[Mike B.]

Based on what you've just stated, I, too, would avoid a 2 ohm load on an old Concert Bass amp.

Mike