700b and a Newbe here.
- Thread starter Jarbutt
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I have one and its a great amp with the power it can push out. just general warnings are that it can become a fire hazard if it gets hot or too much power. 350+ watts per channel is great and will shake a room if turned up to high but just have fun. make sure you hook it up to some cheap speakers before anything expensive so you dont blow them and another warning is fuse your speaker wires because they tend to blow speakers also. im still a little new to the 700B also so i havent learned everything
This amp has a wide following. Some people love them some hate them. The 700B was one of the early high power amplifiers and is pushing 40 years old. Although never intended for sound reinforcement a lot of them ended up in PA's. If it is driven hard, especially into 4 ohms, it will over heat and possibly fail. After 40 years a lot of them have been repaired and many times the work was less than satisfactory.
To get a rough idea of it's history, look at the back of it. There are 4 vertical columns with transistors in them. They are hidden by a cover. To remove the cover push on it in the middle. Once all of the covers are off look at the 5 transistors in each column. They likely say XPL909 or Xpl910 except for the bottom one in each column. the important thing is all of them should read the same. Also there shouldn't be any white grease smeared around them. If there is a mix of different types, then the amplifiers has been repaired and the tech didn't follow the mfr guidelines.
With the amplifier disconnected, you can remove the top cover and the 4 bolts on the panel. put a piece of cardboard where the top panel was and fold the panel back of top of the cardboard. Look around circuit board to see if it has been worked on. Hopefully everything is original. The are several capacitors on the board which should be changed out due to their age.
Depending on your knowledge level of amplifiers you may want to use a voltmeter to check a couple settings. They are detailed in the service manual. I posted the manual at the AK data base. 700B Service Manual
There has been a lot of activity with this amp recently because of an update to the circuit board provided by White Oak Audio. If you search around you'll find a lot of info. One of the criticisms of the amp is if it does fail it has no protection circuit for the speakers. I have designed and sell a relay circuit that drops into the amp to protect against amplifier/speaker failure. I'm not going to comment further on that, someone else may offer something.
If you push the amp hard you need fans blowing on it. Two 4", twelve volt fans wired in series will do the trick. There are some neat fan controllers on EBAY for computers that sense temperature and adjust the fan speed accordingly.
To get a rough idea of it's history, look at the back of it. There are 4 vertical columns with transistors in them. They are hidden by a cover. To remove the cover push on it in the middle. Once all of the covers are off look at the 5 transistors in each column. They likely say XPL909 or Xpl910 except for the bottom one in each column. the important thing is all of them should read the same. Also there shouldn't be any white grease smeared around them. If there is a mix of different types, then the amplifiers has been repaired and the tech didn't follow the mfr guidelines.
With the amplifier disconnected, you can remove the top cover and the 4 bolts on the panel. put a piece of cardboard where the top panel was and fold the panel back of top of the cardboard. Look around circuit board to see if it has been worked on. Hopefully everything is original. The are several capacitors on the board which should be changed out due to their age.
Depending on your knowledge level of amplifiers you may want to use a voltmeter to check a couple settings. They are detailed in the service manual. I posted the manual at the AK data base. 700B Service Manual
There has been a lot of activity with this amp recently because of an update to the circuit board provided by White Oak Audio. If you search around you'll find a lot of info. One of the criticisms of the amp is if it does fail it has no protection circuit for the speakers. I have designed and sell a relay circuit that drops into the amp to protect against amplifier/speaker failure. I'm not going to comment further on that, someone else may offer something.
If you push the amp hard you need fans blowing on it. Two 4", twelve volt fans wired in series will do the trick. There are some neat fan controllers on EBAY for computers that sense temperature and adjust the fan speed accordingly.
