Your ohm selector appears to be set on 8 ohms. You should check the speaker to make sure it's an 8 ohm speaker. I thought they put 16 ohm speakers in those amps.
There should be a sticker on the back of your speaker. If not, then if you have a multi meter, you can check it by setting to ohm, then pulling that speaker plug out and touching the tip and sleeve. It won't be exact but will tell you if it's around 8 or around 16.
Also just a guess here. At the top it says minimum of 4 ohm's. When you have two speaker jacks, seems likely that the internal speaker might be 8 and when run with another 8, you'd have a 4 ohm load. But definitely check it out so you can make sure it matches.
redslim, im not trying to be disrespectful but did you care to read the manual which you can download? it will take you some 5-10 minutes to read. as it seems you dont have a clue how to use such an old amp and things can be very dangerous to your own health, besides killing tha amp by abusing it.
and maybe youll spend 15-30 to google some settings for your amp which are already plentyful in the internets... or maybe just try and find out? we even dont know what kind of music youre playing! should i suggest eric clapton or zakkkk wylde?
just some suggestions...