Recapped and Upgraded for Hi-Fi use by Al Forbes at Alpha Sound. Includes Tubes + Original Tube Cages and Schematics.
Tube Compliment: 6550 X2 - GZ34 X2 - EZ81 - 6Z4 - 12AU7 or 12AX7. Pair of Bogen MO-100 mono block tube amplifiers. These are 100% plug and play and sound stunning. They have been serviced and upgraded for Hi-Fi use by Al Forbes at Alpha Sound. Only 4 of these were made and this pair was Al Forbes personal set for his home audio system.All audio tubes are vintage made in the USA and have been tested strong. Originally these amplifiers sounded pretty good as-is, and were designed for industrial use. The recapping and upgrades elevate these to another level making them true Audiophile Hi-Fi tube amplifiers on par with vintage McIntosh. You would have to spend thousand of dollars each, to acquire a modern day equivalent. 1: Main filter capacitors were increased in size. Originally they were 30uF (wired in series giving 15uF total) They were replaced with 50uF (giving 25uF in total).
2: All filter capacitors now have small-value Orange Drop capacitors added as bypass caps. 3: The trouble prone Selenium bias rectifier was replaced with a silicon diode.
4: The wax paper main coupling capacitors were replaced with Orange Drop capacitors of the same value. 5: A Standby switch with Green indicator was added to allow you to preheat the tubes before applying high voltage. 6: The volume pot was re-located from the back to the front panel.An RCA Hi impedance input was added to the back panel. The 2 wire AC line cords were replaced with 3 wire grounded cord for safety. 7: A filter choke was to the screen supply making the amps much quieter. 8: The 12ax7 phase splitter/driver tubes were replaced with 12au7's, which is much more linear in this application. The 12ax7's will still work, but the gain is excessive. 9: The bias test point and adjustment pots were moved to the front of the chassis. Each 6550 power tube has its own independent bias adjustment pot and test point, which was originally on the top side of the chassis. Testing and adjusting used to required removing the tube cage, opening links, and taking readings. To set bias now, connect a meter between the Black test point and the Red test point (there are two) Hold the test switch down and set for 0.5VDC (1/2 Volt). This corresponds to 50mA tube current per tube. 10: Output impedance is selectable for 70V line, 25V line, 16 Ohms, and 6 Ohms.
I recommend the 6 Ohm tap when using 8 Ohm speakers, they will clip at about 70W into 8 Ohms. I prefer padding the tubes with bubble wrap and leaving them in place. Keep in mind the the tube cage will need to be removed and bubble.
Removed before powering on, I will place reminders on the cage. If you have any questions please feel free to ask, my name is Mark. The item "Pair Bogen MO-100 Mono Block Tube Amplifiers.
Recapped Upgraded For Hi-Fi Use" is in sale since Monday, February 1, 2021. This item is in the category "Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps". The seller is "hperullo13" and is located in Charlotte, North Carolina. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Japan, Sweden, South Korea, Indonesia, Taiwan, South africa, Belgium, France, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Israel, New Zealand, Singapore, Switzerland, Norway, Saudi arabia, Ukraine, United arab emirates, Qatar, Kuwait, Bahrain, Croatia, Malaysia, Barbados, Bangladesh, Bermuda, Brunei darussalam, Bolivia, Egypt, French guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Jordan, Cambodia, Cayman islands, Liechtenstein, Sri lanka, Luxembourg, Monaco, Macao, Martinique, Maldives, Nicaragua, Oman, Pakistan, Paraguay, Reunion, Uruguay.