Mike Dubay Posted January 16, 2010 Share Posted January 16, 2010 Here is a couple of quick pictures of my most recent projects. I picked up two Bogen M60A PA amplifiers on the cheap. First one was at an antique fest, got the seller down from $35 to $12. Surprisingly, another one came up on ebay soon after wards, had a good experience, $70 delivered from Texas. So $82 for the pair was not bad. One of the two had a pair of 8417's that have some life in them, tested at 135% and 134%. Both 7247 tested good for ma and transconductance. Spent the fall and early winter modifying them for Hifi use. I removed all of the microphone inputs, tone controls, and volume controls. Stripped it down to just the 7247 tube for gain and splitter and the two 8417's. I rebiased the 7247, intially is was biased too close to 0v for 1 volt inputs. Completely recapped it, scrapped the can capictor and got 330 uf of electrolytics under the chassis. Replaced the coupling caps with polyproplyene, nothing special run of the mill from Cornell Dubilier. I added an adjustable Bias network, span and balance, and a ma meter for the 8417's. Replaced the power bulb with an LED lamp to illumiate the cat's eye lens. Added a stand-by network to keep the 8417's reigned in during warm up. Replaced the one unit that did not have a grounded power cord with a 3 wire plug and grounded the chassis. Yikes! 605 volts to the output transformers and an ungrounded chassis? Each unit was converted to a star ground system. They have not been cleaned up for looks just yet. I'm running them with out the covers to promote cooling. Quote Link to comment Share on other sites More sharing options...
Mike Dubay Posted January 16, 2010 Author Share Posted January 16, 2010 Here is a closer look at number #1 which is the ugly duckling. Active corrosion on the chassis was sanded off, and had to evacuate a mouse nest from this one. Luckily the wiring was not chewed on what's so ever. Boy, do these "twins" play loud and clear. I listen to these while working out every evening through my KG4's. They can take all of the output from my Sony CD player and my PC with no audible distortion. The output transformers are rated from only 60 Hz to 20kHz. With a little bit of equilization at the Microsoft media player I get nice solid bass. I would have to say that the 60 Watts these are rated for is very conservative. Quote Link to comment Share on other sites More sharing options...
The Dude Posted January 16, 2010 Share Posted January 16, 2010 Nice thanks for sharing duder Quote Link to comment Share on other sites More sharing options...
thefluffy Posted January 17, 2010 Share Posted January 17, 2010 We had a couple of Bogen Power Amps when I was in Iraq this summer. It is a name I am unfamiliar with... but those things sure could pump some power out. Of course these were all modern SS stuff. Nice read! Daniel Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 17, 2010 Share Posted January 17, 2010 We had a couple of Bogen Power Amps when I was in Iraq this summer. It is a name I am unfamiliar with... but those things sure could pump some power out. Of course these were all modern SS stuff. Nice read! Daniel I would bet that over 90% of kids who were in school in the '50-'60s have listened to a Bogen amp. They sat in school auditoriums and school offices around the country, allowing everyone to hear the morning announcements. I'm sure that was what was used at my high school when they came on in '63 to announce JFK had been shot. Those amps just worked, day after day after day. Bruce Quote Link to comment Share on other sites More sharing options...
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