Ohms question.

Reinhard

I am picking up my new amp today. It is a 30W head I am going to run into a 16ohm Marshall cab. The amp also has a half power option that knocks it down to 15W. When I half the power, do I need to half the ohms going to the cab as well e.g. 30W om 16 ohms, 15W on 8 ohms?

MikeM

TECHNICALLY, no you don't. Tube amps can handle 1 mismatch either up or down (It affects the plate load so you will get either more or less overdrive, long story, easiest way to test if you prefer one up or down really is to try it), but using only one speaker (read your Alnico blue ?) will get you lower volume as well.

FenderBender

Usually amps are designed to present the same speaker impedance to the output stage whether set on full or reduced power settings. Is the head designed to run on a 16 ohm load? Let me have the make and model no and I'll take a look for you.

MikeM

Graeme, it's a Ceriatone Dizzy30. A quad of El84s. I am pretty sure it just cuts 2 of them.

AlanRatcliffe

MikeM wrote: Graeme, it's a Ceriatone Dizzy30. A quad of El84s. I am pretty sure it just cuts 2 of them.

Just pulling two valves, the impedance doubles, so you would ideally need to double the speaker load. But Graeme is right - there is a common and simple way used to lift the valves while keeping the impedance the same. Most amps that have a switchable output power use this method.

Reinhard

Hi Guys
I mailed Nik from ceriatone. He said I can half the output ohms when using half power, and should I use it a lot, just swop the inner and outer tubes to split the wear.
Wonder if this amp is fixed bias.

Reinhard

Graeme Parfett wrote: Usually amps are designed to present the same speaker impedance to the output stage whether set on full or reduced power settings. Is the head designed to run on a 16 ohm load? Let me have the make and model no and I'll take a look for you.