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RE: MP Engineering OTL

Posted by Banat on June 6, 2016 at 13:43:51:
Hi Kasanay
Nice vintage OTL amps ! , I like it ,
-referenced to yours schematics I would say that yours amps are conceptually very close to Futterman H1 amp , basically they are very simple.. ,
same 6AU6 tube on input voltage gain stage further DC coupled to grid of 6S4 phase inverter /driver stage tube ,and the same but many more numbers than in H1 12B4A triode power tubes in SEPP OPS ,
- one of differences is that 6S4 cathode resistor (R7-33K) is by your amps connected directly to ground line, but by Futterman OTL amp that resistor is connected on positive or `hot` speaker line , which is one characteristic original solution for Futterman OTL amps ,
- as Lew says there is no coupling cap to LS ,
- SEPP OPS have bipolar PSU same as is by H1 , but
negative bias network is designed in different way than from H1 , and so on..,
- any way , I think that your vintage OTL amps need and deserves very very careful cross check examination of every active and passive element before further and normal every day use ,after all they are made in 1962 !
-it is great that you have original schematics and specification list ,that will help you a lot for your future repairing & servicing job.

Best Regards !