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MP Engineering OTL

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Posted on June 2, 2016 at 14:16:47
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016



Hello,
I just scored a pair or very rare OTL amps that were build by MP Engineering in about 1960. I am no expert by any means so I am looking any help this group can offer.
I am in the process of bringing them back to life as they have not been fired up in decades (maybe 5).
So far I have warmed up the amps and checked voltages. All test points are surprisingly close to spec. I have the bias dial in to -20v.
The amps have a built-in offset meter with a balance pot. I need to get these dialed in and am looking for advice/ tips.
One side will pin the needle and reads 3.4VDC at the speaker terminal. The other side reads .01VDC and the needle is about 1/2 way between 0 and pinned. Turning the balance on either channel seems to have little to no effect on the readings. I have not done any testing of the tubes.

 

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RE: MP Engineering OTL, posted on June 3, 2016 at 07:58:42
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Without a schematic, diagnosis would be very difficult, but I bet this is a Futterman-type OTL. What are those output tubes? Are they 6LF6s or related? They are really close together, too close. One clue would be the presence of an output coupling capacitance; are they cap-coupled?

They look home-made or else you've taken off a cover to make the photo.

 

RE: MP Engineering OTL, posted on June 3, 2016 at 17:06:52
airheadair
Audiophile

Posts: 393
Location: California
Joined: October 18, 2010
 They don't look like 6LF6's since there is no anode cap. I found a thread on the web which says they are 12B4A tubes, and (further down) that this is a "best kept secret" tube. Someone on the thread claims to have a set of schematics for them.

Here is the link:

http://forum.polkaudio.com/discussion/158368/an-otl-output-transformer-less-amplifier

 

12B4A, posted on June 3, 2016 at 19:42:10
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Didn't Julius use those in one of his earliest efforts, before he went to the 6LF6?

 

RE: 12B4A, posted on June 5, 2016 at 08:22:31
airheadair
Audiophile

Posts: 393
Location: California
Joined: October 18, 2010
 I have no idea. Rhino, do you know?

 

RE: 12B4A, posted on June 5, 2016 at 21:09:56
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 It is my understanding the early Futterman H1 used 12B4As.
These amps are very similar.

 

RE: MP Engineering OTL, posted on June 5, 2016 at 21:45:08
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 These are very similar to the Futterman H1.
Tubes are 12B4A (same as H1)
The amp uses capacitor coupling. Currently they are big 4uf oil caps. I am planning to replace these along with the electrolytics in the very near future.
They are not quite home made. They were made by MP Engineering of Fairfield Conn. - a small company that was in business for 30 years. I did remove the access panels.

 

RE: 12B4A, posted on June 6, 2016 at 07:13:37
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 He might have used them also, later, as output tubes in his headphone amplifier. During one of my two visits to his shop/factory in NYC, he was building a headphone amplifier that he proudly showed off to me. It had smallish output tubes with no cap lead. I know someone who owns one, and we speculated whether his sample was the one I looked at in 1979 or thereabouts.

 

RE: MP Engineering OTL, posted on June 6, 2016 at 08:11:38
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016



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.

 

RE: MP Engineering OTL, posted on June 6, 2016 at 08:14:02
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016



Here is the power supply:

 

RE: MP Engineering OTL, posted on June 6, 2016 at 10:04:56
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 Here link to my Dropbox folder containing an ever-growing collection of photos and information:
https://www.dropbox.com/sh/fiq8sclbo3sosl4/AACXvUuK_PmM2u1iTiRkxuzca?dl=0

 

RE: MP Engineering OTL, posted on June 6, 2016 at 12:40:55
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 You use the plural, "gems". Do you have a stereo pair? Nice.

I don't see an output coupling capacitor in the schematic.

 

RE: MP Engineering OTL, posted on June 6, 2016 at 13:43:51
Banat
 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 !


 

RE: MP Engineering OTL, posted on June 6, 2016 at 14:01:03
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 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,

 

RE: MP Engineering OTL, posted on June 6, 2016 at 14:16:09
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 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,

 

RE: MP Engineering OTL, posted on June 6, 2016 at 14:38:45
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 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.

 

RE: MP Engineering OTL, posted on June 6, 2016 at 14:57:18
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 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,

 

RE: MP Engineering OTL, posted on June 6, 2016 at 15:19:15
Banat
 - 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 .

 

You might have to move some tubes around to get the DC balance, posted on June 7, 2016 at 11:43:59
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 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.

 

RE: You might have to move some tubes around to get the DC balance, posted on June 7, 2016 at 12:16:53
Kasanay
Audiophile

Posts: 11
Location: 53207
Joined: June 2, 2016
 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,

 

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