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The Mighty Bogen MO-200A

Sound - The two transistors make up the EFB™Grid Bias Regulator, which is a large part of the amplifier's improved performance.

In a pentode tube, the operating point is set as much by the screen grid voltage as it is by the control grid voltage. In typical fixed bias amplifier designs, the screen grid voltage drops fairly significantly at high levels of power output which elevates distortion, and reduces power output. The ideal answer of course is to regulate the screen voltage which certainly helps, but the plate voltage still drops which limits the effectiveness with that type of regulation. Now in the Bogen design, because the plates are powered directly from the high voltage supply, and the screens are powered almost directly from the mid voltage tap of the doubler supply, the relationship between the plate and screen voltage is maintained pretty well as power supply voltages begin to droop. But the control grid voltage remains quite stable as B+ voltage is dropping, which again, upsets the operating point of the output stage, and increases distortion.

EFB solves this problem by causing the grid bias voltage to track in a set relationship to the screen grid voltage, which itself tracks in a set relationship to the plate voltage. With EFB applied then, the main B+ will still droop as power output is increased, but the screen voltage and the control grid voltage will decrease in precisely the same relative amount along with it. The end result is that yes, power output is still lost because of the droop, but because all the elements drop proportionately together, the operating point is maintained in spite of the droop, and distortion drops dramatically. This concept has been proven out in numerous different units, and verified by numerous folks world wide now. The 470K resistor at the base of the inverter transistor connects to the screen grid supply, so that the EFB Control Grid regulator then controls the bias supplied to the output tubes, based on the screen grid voltage applied to the tubes. It's a win-win approach, because compared to traditional high voltage regulators for vacuum tube service, the regulator is quite simple, yet delivers the same amount of distortion reduction that traditional full regulator delivers.

The transistors used are conventional 160 volt PNP silicon devices. The Zener is also a conventional 100 Volt 5 watt device. Mouser part number 863-1N5378BRLG will fill the bill nicely. $0.45 each.

If you have further interest in EFB, you can reference my white paper where I first introduce the concept, which can be found on the web with a Google search for "A New Look at an Old Friend - Tronola".

I hope this helps!

Dave
 
Dave, Great article on the dyna SCA35 excellent read.also couldn't find you other article on Audioxpress..so I looked for the 2sa1013 on mouser, the original one is obsolete and others have to be order non-stock, checked digi-key obsolete..and parts express only has a NTE32...if you can help find a supplier i'd be very appreciated..
 
Search for KSA1013OBU, KSA1013YBU, or KSA1013YTA. The only difference between them is how they are packaged for machine insertion. Any of those would work. Mouser has thousands of them.

But any PNP type would work with -160V collector-emitter rating.
 
Dave
Did you happen to record the voltage output from the feedback winding of the bogen transformer during the test at the input of the RC network during a full load test?
I'm going to get the dynaco output trannies for my altec chassis and use the speaker loaded winding 4 or 8 ohm to inject the feedback.
Any tips would be very appreciated.

Thanks John
 
John -- The output of the NFB winding is literally identical to that of the 70 volt output tap. As I recall, this is discussed in more detail at some point buried somewhere in this thread........

Dave
 
If you're using the Dynaco transformers, I suspect you'd be better served using the Dynaco schematic. That will be tuned for the specifics of those transformers.
 
Has anybody used bogen m120 transformers for Dave's 6550 circuit? Would there be mods to make them work?
 
WOW what an awesome thread!!! I realize this is quite old now being we are almost in 2025, But...

I picked up one of these MO200a amplifiers last year. it caught my eye on Facebook Market place and i scored it for a pretty good price. no tubes however and I have never been a fan of 8417's anyway. My interest is in the output transformers only. I want to build a pair of mono block amps using KT-120 or KT-150 tubes of which i have plenty on stock. I have some 6550's but I wonder if the KT120/150's would do better??

I have no desire to keep the stock bogen circuitry, chassis or even power supply. Removing all those constraints, I am curious what can be done with these Output transformers for the best performance possible. dcgillespie are you still around??
 
KT120 work fine, I ran them for a good while in mine. Currently running a KT90, but I also have KT88. They all do the same power, just some tonal difference like you might expect from rolling tubes in general. The KT120 is a little better on the bottom end, the KT90 I think has nicer mids. The KT88 is sort of between them. Since I'm running a sub the bottom end doesn't matter as much so I went for the better mids.

The stock power supply is more than up to the task. Pretty hard to find transformers that will supply this much voltage and current, especially if you already own one from an MO-200A. Chassis isn't anything special though.
 
If i could find a 2nd MO200A power transformer i would consider keeping them. but I can find power transformers. I used to work for ARC so i have sources i can get whatever i need from. Output transformers is much harder.

So...let's talk about the ultimate build using just the 200/100 Output transformers. I am looking for 100 watts 8 or 4 ohms. two KT120 or KT150 tubes. and the input and driver stages can be anything. I like 6H30's and I have plenty of those. 6SN7's are a favorite tube. etc.
 
Mr Overkill said:
So...let's talk about the ultimate build using just the 200/100 Output transformers.
Click to expand...
Not sure there’s a lot to add re: the original designs Dave put forth. A couple of thoughts though:

a) do you want to build monoblocks?

b) the one thing I’ve found with the KT120’s (and I assume the KT150’s as well) is they tend to exhibit some LF parasitic oscillation that you don’t see with KT88s. That needs to be dealt with somehow. I used some local feedback on one of my builds but you might also address it with a plate stopper. Like a 22 ohm resistor in series between the OPT and plate.

Other than that, I’d suggest just following the 6550 circuit design already put forth.

For true overkill as your avatar name suggests, you could add fully regulated power supplies. :)
 
Mine are MO-100a's, the mono version. Much lighter, but the power transformers put out a little less voltage so mine make about 95 watts where Dave got a little over 100. Not enough to care about unless we're spec racing.
 
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I worked on my 200a today and wow what a mess. it has had a fire at some point. and its obvious that someone has modified it quite a bit some time ago. the mods look old. old parts etc. someone added a choke, and changed some wiring. changed it to stereo. added a tube socket. etc etc. the chassis is not worth keeping so I pulled out the transformers.

I did some testing and found that on the 17 ohm tap, there is about a 17:1 winding ratio, resulting in a 289:1 impedance ratio and about a 4600 ohm plate to plate impedance. if my math is correct. an 8 ohm load on the 16 ohm output would be about 2300 ohms which is interesting. applying a 4 ohm load to the 25v tap gets around 2900 ohms. ARC uses output transformers around that impedance with the KT120/150 tubes so that might be interesting to play with.
 
One of my 100a came to me with a pair of 8417 and a pair of 6L6G. The balance pot and everything around it looked like it had been blasted with a torch.
 
I don't think those AB pots are original, looks like dual section but wired in parallel. Possibly the original let the magic smoke out and it just wasn't cleaned up. Carbon on those output tube sockets with the voltage they see is not exactly ideal though
 
The 8417 is a very high Gm tube, and in the MO-200A, is rather famous for creating parasitic oscillations that invariably blow the guts out of at least some of the plate stopper resistors. You'd be hard pressed to find an example where that has not happened.

At least the KT120 also is prone to parasitic oscillations -- although not nearly to the extent that the 8417 is.

In the Bogen design, any plate load resistance much below 5000Ω plate to plate is going to seriously overload the tubes -- unless you plan on keeping the push-pull-parallel topology of the original design, where then it starts looking pretty good. You might be able to get away with the 4600Ω configuration with two tubes, but going lower than that is just is really putting a beating on the tubes.

ARC did use primary impedance levels of less than 3KΩ for their multi-output tube models, but remember that was modified then by their partial cathode coupled output stage configuration they always used.

Good luck with your project!

Dave
 
dcgillespie said:
mech -- I've read the info on baking tubes, but frankly have never tried it. While the concept does make sense, it would be hard for me to imagine that baking the one tube could save it. It was hard enough just to snap the pic I presented of its performance, as the power output of that tube sank very fast from the moment the drive signal is applied.

Based on the otherwise completely normal appearance of that tube, I would strongly suspect that it was damaged (arced) somewhere along the line. I can't tell for sure, but it appears that the original wiring to one output tube has been worked on, so this, plus the obvious blowout in the power supply area may explain the historical failure of this particular tube, and the amp in general.

It is also interesting that the strongest tube of the bunch is a Bogen labeled tube, while the worst was a garden variety GE labeled tube -- obviously therefore already a replacement tube, while the Bogen tube was almost certainly an original one.

As for taking out the transformers, stripping down the chassis, repainting, etc., etc., etc., there are an awful lot of folks who do a wonderful job of that kind of work here on AK, but that is not my particular goal here or forte even. My goal is to first go through the steps necessary to place the unit back in best possible as designed operating condition, from which a baseline of performance can be developed. From that information, the unit can then be analyzed for what modifications would best be suitable --within what the unit basically is -- for converted use to stereo operation, as used in a home environment. Once that basic effort is all developed, proven, installed, and documented, then any personal preferences for the unit regarding appearance or boutique components can be addressed by the owner, while keeping the modifications developed in place. My goal then is to place the unit into best possible performance condition for its new intended use environment. Along the way, the wiring, and any components of concern will certainly be addressed in working towards the stated goal.

rpampt --in a well designed amplifier, output tubes that otherwise start out as very similar well made devices will in fact wear out fairly evenly throughout their lifetimes. Like a tube curve, they may not wear identically initially, or at the near end of their life point. But throughout their useful service life, they will generally wear out at an even rate. However, circuit design can affect how output tubes wear considerably. Most typically, turn on/turn off burps and thumps presented to the output stage are classcially non-symmetrical, and very hard on the tubes when they happen -- which can cause otherwise equal tubes to wear at vastly different rates.

I have often seen units where the power supply was upgraded with massive amounts of capacitance, which may be good for the perceived sonic experience, but all too often can alter the charge/discharge rate of the power supply, causing the amplifier to now produce the burps and thumps, where none were present before. The discussion of such equipment often turns quite quickly into questions as to why the output tubes are wearing out so fast now......

Dave
Click to expand...
The peak current from all that capacitance can clobber the power transformer.
 
The only part of the Bogen I am keeping is the output transformers. if anyone wants the chassis, i will send them the chassis for the cost of shipping. I may consider letting go of the power transformer as well.
 
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