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In Reply to: RE: MP Engineering OTL posted by Kasanay on June 02, 2016 at 14:16:47
Take a look at the schematic. This appears to very similar to the Futterman H1. These use the same 12B4A power tube (although a lot more of them), the same 6AU6 driver and the same 6S4 inverter.
I am, by no means, an expert in this field but I am willing to learn all I can. I look forward to any comments, thoughts, advice and/or recommendations any of you my have as I go through the process of bringing these gems back to life.
Let me know your thoughts.
Follow Ups:
Hi KasanayNice 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 !
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 06/06/16
Banat,
Thanks, for the reply and info. As I stated in another post, I am no expert so i hope you don't mind me asking for information:
"SEPP OPS" = Single Ended Push Pull ?opposing sides?
When you say "which is one characteristic original solution for Futterman OTL amps" are you saying the Futterman design is superior in area?
Thanks,
- SEPP OPS mean Single Ended Push Pull-Output Power Stage ,- No I did not say that Futterman OTL amps are superior,
I just say something like this , `by connecting phase inverter tube cathode resistor to loudspeaker `hot` line and not directly on the ground line is one characteristic solution or invention detail of J.Futterman`, that characteristic solution was implemented in many different models of J.F. OTL amps ,
and that specific solution allows to both side of SEPP OPS to dynamically behave in the same manner,
- but the question is , why MP Engineering did not implemented that solution in your OTL amps ? ,
maybe J.Futterman patent pending rights was the reason which did not allows that implementation .
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 06/06/16
You use the plural, "gems". Do you have a stereo pair? Nice.I don't see an output coupling capacitor in the schematic.
Edits: 06/06/16 06/06/16
Yes I have a pair of these gems. Actually, I have 3 - a stereo pair and a spare (not really sure how that happened, but I'm glad it did).
So I may be looking at the amp incorrectly - I was referring to C4 and C5 as "output coupling capacitors" as they are between the inverter and the output tubes. Is this incorrect? Are these caps considered "interstage coupling caps"?
Thanks,
Without referencing the schematic a second time, a capacitor between the input stage and the driver stage can correctly be called a "coupling capacitor", but when I said there was no output coupling capacitor, I was referring to the interface between the output tubes and the speaker. Thus you need to carefully adjust DC offset before hooking these to a speaker, which I think is why there is a meter placed into the circuit by activating that switch shown above the 3.3R resistor. Counterpoint SA4 and KSS both I think were Futterman designs with no coupling capacitor too. I think Counterpoint used some sort of servo mechanism to keep DC offset low or absent. But both of those amplifiers could occasionally cause problems.
Lew,
You are correct about the meter. I set to bias to -20V and then press the offset test button and adjust it to 0.
I was fortunate to receive a message from Matthew Polk of Polk Audio giving me clear instructions on how to do this. Matthew in familiar with the process as he also owns a pair of these amps.
I can get the left channel spot-on but the right channel - not so much.
I hoping that replacing the coupling and other caps will correct this.
Thanks,
There does not appear to be a separate bias adjustment for each power tube. So you do have to get the tubes to work together. For example if two power tubes are weak, you may not want them to be in the same bank of power tubes. That may be why the one channel is a little off.
This could very well be the issue. I think I will replace the coupling caps first. That will narrow down the number of possible causes.
I did come across this post:
http://www.audioasylum.com/forums/otl/messages/1/18823.html
Although it references a different amp, I believe the same applies.
Tahnks,
Here is the power supply:
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