I have a rare opportunity and have acquired 2 Bogen MX30A PA amps. The output transformers are huge on these puppies and the amp only produces 30 watts. I know part of the size is from the different speaker voltages but still it is twice the size of the power transformer All the tubes were good in both amps except one 7868 tube. I have a few more good 7868 tubes on hand so that is not a problem. Most of the small tubes are Mullard's.
The plan is to completely rebuild the two amps for HiFi use and I am looking for ideas. It needs to have a MM phono stage and a line stage. My Mind is muddled from looking at different designs with different tubes. I am pretty sure the output transformers are 6.6k and I will check that for sure later today. There are four 9 pin preamp tubes, One 12ax7 for the phono amp and three 7247's. The 7247 tubes are used for the line stage, tone controls and driver/phase inverter. I am not opposed to changing the design or tubes for that matter. Maybe this design is the best for this amp and looking at options.
I started to strip one amp and there will be plenty of room to build what ever is needed on the chassis. I removed the XLR inputs, mic transformers and sockets and the back two 12ax7 sockets along with all the junk that went with them. I was thinking about moving all the power supply caps, diodes and resistors to this area far away from the rest of the amp. I am also working on the power supply schematic and thinking about adding a 2.5H 43 Ohm choke after the first power supply capacitors to smooth this beast a bit. Any thoughts will be appreciated.
MX30A schematic and specs attached
The plan is to completely rebuild the two amps for HiFi use and I am looking for ideas. It needs to have a MM phono stage and a line stage. My Mind is muddled from looking at different designs with different tubes. I am pretty sure the output transformers are 6.6k and I will check that for sure later today. There are four 9 pin preamp tubes, One 12ax7 for the phono amp and three 7247's. The 7247 tubes are used for the line stage, tone controls and driver/phase inverter. I am not opposed to changing the design or tubes for that matter. Maybe this design is the best for this amp and looking at options.
I started to strip one amp and there will be plenty of room to build what ever is needed on the chassis. I removed the XLR inputs, mic transformers and sockets and the back two 12ax7 sockets along with all the junk that went with them. I was thinking about moving all the power supply caps, diodes and resistors to this area far away from the rest of the amp. I am also working on the power supply schematic and thinking about adding a 2.5H 43 Ohm choke after the first power supply capacitors to smooth this beast a bit. Any thoughts will be appreciated.
MX30A schematic and specs attached
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> It needs to have a MM phono stage
It *has* a MM phono stage. Unless you already chiseled it out?
"Passive EQ", good hi-fi vibe.
Bogen's power supplies were good except they picked caps too small. Change C56 C57a C57b to 100uFd each. Other than that, I'd avoid changes. Bogen's boys were pretty sharp.
It *has* a MM phono stage. Unless you already chiseled it out?
"Passive EQ", good hi-fi vibe.
Bogen's power supplies were good except they picked caps too small. Change C56 C57a C57b to 100uFd each. Other than that, I'd avoid changes. Bogen's boys were pretty sharp.
Thanks PRR, No, I left that MC 12ax7 socket in place and the three 7247 tubes also. I will be replacing everything in both amps so all the resistors and capacitors are gone. I thought there might be something better or newer I could build. Stock it is then and 12ax7 to mm phono stage.
Here are a few pictures of the amps and one of a modified power supply schematic. One is stock and the other stripped for rebuild. The capacitors and choke on top are just for show right now and I have purchased a CE-MFG a 525V 40/40/20/20uf , a 475V 30/30/30/30uf and a 2.5H choke for each amp.
I think one sitting on top of each Altec Model 19 is going look bomb.
Here are a few pictures of the amps and one of a modified power supply schematic. One is stock and the other stripped for rebuild. The capacitors and choke on top are just for show right now and I have purchased a CE-MFG a 525V 40/40/20/20uf , a 475V 30/30/30/30uf and a 2.5H choke for each amp.
I think one sitting on top of each Altec Model 19 is going look bomb.
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I am also adding a house ground to these amps. What would be the best ground system for this type power supply, a single point ground or a lifted ground off the chassis? There is no center tap and I am not sure if the power transformer is case grounded to the frame. Thanks for any suggestions.
Because they are going to be my main stereo amplifiers, now I have plenty of room to do it and I think it would be better to have a house ground to the frame with a lifted Single Point ground to the important parts.
Willy-Nilly frame grounding works but that does not make it right. It is cheaper, just like the voltage doubler and if there are space constraints, easier. Wait till you see the power supplies with the Metalized Polypropylene Motor Run Capacitors in them.
PRR, You seem to be a Bogen man, How do those push pull potentiometers work, are they 500k Push and pull? Thanks
Willy-Nilly frame grounding works but that does not make it right. It is cheaper, just like the voltage doubler and if there are space constraints, easier. Wait till you see the power supplies with the Metalized Polypropylene Motor Run Capacitors in them.
PRR, You seem to be a Bogen man, How do those push pull potentiometers work, are they 500k Push and pull? Thanks
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