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Bogen / NTS-1125 look like a 50-60 Watter...

Posted by Interstage Tranny (A) on March 13, 2007 at 12:17:52

In Reply to: Re: Hoping for a History Leason #1: Bogen / NTS-1125 posted by m8o on March 10, 2007 at 22:59:30:

Hello there,
These have some beefy iron, don't they ? Some of the Bogen amps like these only have Line outputs, no Voice Coil taps. Worst case, you can use another step-down Line to VC tranny.
I have no experience with these, but their circuits were sort of common. There were J50, CH50, CH 60 and some more versions. But, I think this unit might have very high voltages to make 100-125 Watters. The 50-60 Watt versions ran over 600 Volts on the plates, 300+ on the screens of the 807s. I think the 100 Watters used closer to 700 Volts on the plates. Be careful with that HV. One nasty shock can stop your heart, i.e. kill. When probing underneath, use one hand only, keeping the other hand in your pocket, for safety. Attach the tester's negative lead securely to chassis ground, to assure one hand measuring. Also, remember to short the power supply caps, when actually working on these, as residual capacitance voltages do get stored in the cans.
Most of these used 6SN7 as cathode follower drivers, with CT chokes loading the output tube grids. While the R-C coupling devotees will like that, I prefer the versions with a 6V6 driver tube, splitting the phase, thru an interstage.
While I am here, thank you for the kind words on the Duke info. I do try to be helpful. It breeds goodwill and good karma. I never had a problem with sharing the knowledge.
If there is a 600-700V supply, augment that with some higher capacitance power supply caps in series. They didn't use much capacitance in the '50s. Energy storage is critical to bass response and speed and speaker damping/control.
While I was a legitimate, electronics service field engineer, I have become much more practical in these later decades. While theory still guides me, the practical engineering is more important to me. With sound equipment, it is the sound that counts.
If these amps have voice coil taps, they might be fine bass amps.
The short listening, in mono, I have done with some Bogen 807 amps, has me remembering them as sweet and punchy. 807s are very, very rugged tubes, much more rugged than any 6L6Gs that they resemble. Do not change to the 5933 industrial #, which does not sound as good.
Also, while it might be tempting to change output tube types, the 807s are consistently a sweet sound. I have used Tung-Sol 6550s at 600 Volts B+, but they do not sound sweet that way. 807s at 600 Volts can be very sweet. This amp's bias might have to be adjusted to a little higher negative number(more bias), for stability.
Good Luck with these High Voltage Brutes !

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