I recently came into possession with a wonderful 1970 Dual Showman reverb. I had run a simple instrument cable between the head and the cab with no issue. Today, based on someone's advice, I picked up a speaker cable (live wire 12 gauge). I plugged it in correctly, but no sound after maybe 20 seconds. The amp normally was audible before that time period with the instrument cable. I turned it off in fear of damaging the head. Is that speaker cable not intended to connect a head to the cab? Sorry for the newbie question, but please sling some advice my way! Thanks!!