bogen mo-200 modded for stereo
- Thread starter chefg1
- Start date
Something must be very strange about the OPTs on these amplifiers. They appear to require a decidedly unbalanced drive to achieve best performance. Take a look at the plate and cathode load resistors in the phase splitter circuit. At 33K and 22K respectively, the set of output tubes driven from the cathode leg are only receiving 67% of the drive that the output tubes driven from the plate are. It's like that in both versions. I've never had any experience with either of these amplifiers to performance test them, so does anybody who has know anything about this aspect of the design? It really is very strange.
Dave
Dave
jwalker
Well-Known Member
I see another version of the schematic which shows 22k for both the cathode and anode resistors.
http://gregory.webng.com/bb/mo200A.gif
Hot dang this thing has been modified for stereo, the two white stem pots in the back of the control volume for each channel. I hooked the speakers up the way it was suggested by fellow ak member in earlier post. Thanks i would still be hooking and unhooking wires it ok but i think it could better. Since i dont know this amps history im going to shut it down for now. Gonna give it blackie here in L.a. friday so he can check it and work his custom magic on this amp.
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dnewma04
The Healer
If you search around the site for posts by Micah (Bricktop), you'll find his modded Bogen circuit. It's a huge step up from the Bogen's and allows you to run EL34s or KT77 in place of the 8417s. The sale of the 8417s could probably pay for the mods to turn that into one of the best tube amps you'll hear.
This one, perhaps: http://www.audiokarma.org/forums/showthread.php?t=140121...
petehall347
the brandy coffee man
will it run kt88.s
IIRC KT88s should be fine with the voltages but might need some re-bias; 6550s would be on the edge though.
One issue with KT88s that you'd need to check would be whether they would physically fit. The sockets for the eight 8417s are pretty tightly placed together and some KT88 bottles are much wider than the 8417.
Again i would thank everyone for their help i couldnt have gotten thing going on my own. I still cant belive this is stereo but flat sounding i just know it's the caps most likely but that's ok because going right to service fri . And when i get it back i will update everyone on its new sound and functions so keep up the good work all the audio band of brothers.:thmbsp:
zandru
"Hard At Play"
Whatcha cookin' chef?,
As for me; I believe your MO-200 Mod your falls short of It's true potential, but this is your Amp project.
"For your consideration", If you care to ponder, imagine and weigh this (out of the norm) alternitive.
Your Bogie MO-200 has all the right stuff to be effectively modified; dramatically improving It's original
specs. What you absolutely need to have is a pair of these beauties to appreciate ultimate sonic bliss.
With some help from a simple yet elegantly efficient preamp hub along with a pair home spun speakers
created from an old set of cabinets or homemade cabs that can handle 15" Gold series Tannoy dual
concentric loudspeakers. A person "in the know" would be able to construct a frequency / OPT matching
circuit that will operate flawlessly with the Tannoys in a parallel yet spatially rich (unimpinged) phase
coherency.
A properly devised and well thought out plan for a system being similar to the one mentioned
above, can deliver you a truly magnificent core audio system costing a small fraction of what a Top
Shelf system would cost you on any given day.
Modifying a Bogie MO-200 into a stereo rig won't get you there. It seems to be a cookie cutter solutions.
Anway, I hope your Bogie MO-200 project sounds great and that you will enjoy it for many years to come.
Cheers
And why exactly do you believe that? He hasn't said anything about how it's been modified :saywhat:
No.
As the owner of a pair of these, modified and run as monoblocks, I can assure you that the only difference between 2 monoblocks and one run in stereo is the power output and speaker impedances available (with stock OPTs)
Whatever mods you do to the circuitry has nothing to do with how many of them you run. For anything but the most power-hungry speakers one MO200 split into stereo will be plenty.
zandru
"Hard At Play"
Greetings dgta,
After reconfiguring four Bogie MO-200"s myself, of the four; two were each modified for stereo operation
which resulted in a run of the mill "par for the course" modification to the Bogies that were not overly
spectacular pertaining to the mods sound quality and also; they were most certainly nothing to write home
about.
"It's all about striving to rising above and beyond mediocrity".
The task at hand for my third project (as mentioned in my previous post) was to rethink my two remaining
Bogie MO-200's, with both amps operating in unison (side by side) together in a reworked mono dual
channel arrangement including a variable frequency specific OPT circuit stage which allowed me the
ability to greatly exceed (without exception) the overall performance of the paired Bogies in stark contrast
to a typical Bogie MO-200 stereo Mod. A mod "mind you" that has performed and proved Itself worthy in
reproducing fawless music time after time in conjuction with a pair of !5" Tannoy Golds and a quintessential
preamp hub, making this core audio system a force to be rekoned with.
Mediocrity rarely breeds Ideas or brings about challange which leads to inovation.
So I guess "What ever get's you through the night is all right" - John Lennon
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So what are the details of this mod? Please share.
Never heard of such thing. Can you post a schematic so we can all see what you're talking about?
zandru
"Hard At Play"
Yes! you have, but not in the way that I've butchered your electro-speak vernacular or even scientific
nomenclature. What I have Here are a pair of scantily clad circuit mock-ups in order to denote the two
Bogie output transformers signal path through the "variable frequency circuit stages" and into the dual
concentric speaker configuration. You will take note that I have omitted the component values and I have
also left out a few connections mainly because many variables make it a moot point. One should be able
to gain an adequate sense of how these circuits play within the modified twin bogies, If not!, consider this a
mystery of contention to be pondered and investigated till' he or she concludes that the circuit in question
has been around for a long time, and was a seminal electroacoustic device that had been developed over
80 years ago. For those who are still inconclusive, Have fun reconnoitering this mystery circuit.
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Looks like you have configured the mono amp into the two separate amp circuits and added adjustable frequency filters ahead of the gain stage - essentially a variable frequency crossover as a vertically biamped amplifier.
An interesting exercise. However, each section would be configured uniquely for the drivers/speakers it is driving.
Might be a whole lot easier though to optimize the amp sections for driving their respective loads and do the frequency filtering outboard of the amp - especially since there are so many devices readily available to do that and would easily drive the amp circuits without worrying about impedance mismatches and additional signal caps in the circuit of the amp.
An interesting exercise. However, each section would be configured uniquely for the drivers/speakers it is driving.
Might be a whole lot easier though to optimize the amp sections for driving their respective loads and do the frequency filtering outboard of the amp - especially since there are so many devices readily available to do that and would easily drive the amp circuits without worrying about impedance mismatches and additional signal caps in the circuit of the amp.
Yes, hardly a "mistery". Just two passive RC filters.
There are many problems with that. The amps will only be good for one set of bi-amped speakers, at most. In theory.
In reality, 2nd and higher order RC-only passive filters can only have real poles and you have a very limited choice of filter characteristics. None of which are the types used for crossovers. Can you post the filter response you designed for? And what kind of speakers you are driving with that? I can't imagine the response of a 3rd order RC being useful for bi-amping speakers.
There are many problems with that. The amps will only be good for one set of bi-amped speakers, at most. In theory.
In reality, 2nd and higher order RC-only passive filters can only have real poles and you have a very limited choice of filter characteristics. None of which are the types used for crossovers. Can you post the filter response you designed for? And what kind of speakers you are driving with that? I can't imagine the response of a 3rd order RC being useful for bi-amping speakers.
