Can a rogue audio Cronus be used just as a preamp?

Can I buy a 200 Watt plus outboard amp and use the Cronus as a preamp only?

No, you can't! At least without connecting load resistors to the speaker outputs of the amp. And the resistors would have to be rated to handle high power levels, and should also be a non-inductive type.

The reason is that operating a tube amp that has output transformers without a speaker load, or equivalent, can and probably will ultimately damage the output transformers and/or the power tubes, due to a phenomenon called "inductive kickback." Google that term for more information, if you wish.

The risk is particularly severe if music signals are going through the output stage of the amp while no load is present, which would be the case here since the design does not provide removable jumpers between the preamp and power amp sections of the unit. But it would be an unhealthy idea even with no signals present, because inductive kickback could still occur as a result of turn-on and turn-off transients.

Regards,
-- Al

A follow-up to my previous post. It is conceivable that with some designs a suitable alternative to load resistors would be to simply remove the power tubes. That would avoid the inductive kickback problem, but would create the risk that internal voltages could rise to levels that over the long term would be unhealthy for other components, such as capacitors. So I would not use that approach without obtaining confirmation from the manufacturer that it would be ok.

Regards,
-- Al