• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

The Mighty Bogen MO-200A

Now that I've set the stage, I unboxed the Bogen and for the most part, it arrived in the same state that it left here. Dave had mentioned a couple minor imperfections that appeared during the build process, but they are, quite frankly, inconsequential. After unpacking, I unplugged the Maggie and plugged in the new Bogen. I put my probes in the test jacks and fired it up. My only initial disappointment was the amount of hum! Dave had mentioned a certain amount of hum would be present due to the design of the tranformers. But this was more than I expected. After some thought as to what Dave had mentioned in his thread, I changed out the input RCA's for a better shielded pair. Viola, hum gone. The original jacks on the amp are located very close together and do no allow for very large plugs, so I may have to replace those and separate them a bit farther. But back to the listening... Once interconnects were changed, it didn't take long to tell that this was certainly in another league compared to the Magnavox 9304. I don't typically do reviews and don't follow some of the reviews terminology. What I have found, is that the power just seems effortless. Regardless of the types of music I have thrown at it. The bass tracks are solid, and clean. The clarity of the highs and the detail it provides is much better. I listened to an Elton John track and you could tell that he just slightly laughed when phrasing a word. Have never noticed that before. The bias and balance was on, right out of the box. As for the transformers, I've heard of the 3 second rule. If you can keep your hand on them for 3 seconds, they aren't too hot. The Bogen would probably allow me to put my cheek on them and not be a problem. I've been listening for over an hour now and the OPT's are not even lukewarm. The power transformer is cool enough that I can lay the back of my hand or my cheek on it and leave it there indefinately. It is no doubt a cool customer. I've driven it at times for hours and it never breaks a sweat. Funny thing about listening critically. The better the gear, the more the weaknesses show up. I noticed one of the Cornwalls seemed a little muffled in the higher frequencies. So I went through the whole cabling, wiring on the amps and speakers, (fearing it could be an amp issue) but it stayed with the same speaker. So off came the backs of the Cornwalls. After switching the tweets, it still stayed with the same speaker leading to believe it was a crossover issue. Then I remembered that one of the tweets screwed on farther than the other. Looking closer, threads had been crossed on one Cornwall and the tweet did not screw on near as far which affected the highs. I pulled the horn, got the tweeter threaded on straight and problem solved. I didn't realize that the depth of a few threads could make that big of a difference, and hadn't noticed it at all with the Maggie.

So I will continue to test and review. I have yet to get the Dynaco's hooked back up, and quite frankly, I feel no need to do that. This amp is so solid, clean and powerful, it has no doubt taken it's place as my main player. I will at some point attempt to compare it head to head to the MK III's but for now I am very, very happy with the new Bogen. I took a peek under the hood and it is more like artwork. Positioning of components and lead dress is impeccable. The amp performs flawlessy on all types of music with detail and strength. I can try the 3 different pairs of speakers and that each add their own character but can all be driven to ear damaging levels if asked. Anyone considering if this amp is a worthwile candidate for HIFI, I can say yes! YES!!! Sorry my review is not more eloquent, but it doesn't just sound good, it sounds great. I am forever indebted to Dave for his masterful work and his generosity of time. I'm glad he loves his work, because I could never afford anything like this if I would have to pay by the hour. I don't know what the $2k and $3K amps sound like, but I doubt they are going to sound any better than this big guy. Thanks Dave and thanks all for following along. When and or if I get to run it side by side with the MK III's, I will give a comparo here.
 
Last edited:
dcgillespie said:
Definitely one chassis for the power supply (power supplies in your case), and one for the amplifiers!

Dave
Click to expand...

It was mentioned in the first post that 8417 was a "cavity anode" tube.
I have searched for info but find nothing.

What was the "cavity anode" design ? Any explanation / pictures ?

BTW:thanks for the nice articles !
 
Awesome. Glad to hear that it worked out to your satisfaction. This ought to give me more motivation to get off my arse and get on with doing mine up.

Big thanks to both you and Dave for all the work that was done on these. The data simply wouldn't exist without the both of you, and all of us who own or will own one of these beasts owe you a thanks for that.
 
Peter -- The "Cavity Anode" design is a term Fisher used, in describing this tube as used in their highest powered basic stereo amplifier offering, the SA-1000. Better yet, Fisher even claims to have commissioned this particular tube to be made. With Avery's drive to be the "first" at anything/everything/all things stereo/hifi feature wise, and considering the size of his company, this is entirely possible -- although I have not been able to independently confirm or deny this claim.

As for the design itself, if you look at the tube, the plate (anode) design is in fact quite unique. Rather than the more boxy looking anode assemblies -- as exemplified by (say) the KT88 -- the 8417's anode is squeezed together so that it pushes out in two channels on each side of a center line. On each side of this line, there is first a wider channel, followed by a much narrower channel that jets out (or stair steps out if you will) from the center of the wider channel. The inner elements then extend between these two narrow channels inside this assembly.

As such, the spacing of the elements inside an 8417 is quite small, producing the almost unreal Gm figure that this tube is capable of. Other than its high Gm rating however, its other performance features are in line with that of the 6550 family of tubes.

Dave
 
Last edited:
Rpampt,

Glad to hear that you are liking the new DG MO200A. Both Dave and you have put a lot of efforts in developing the mods and giving a new life to the boring MO200A. I also have a pair that I am not sure what to do with.

Dave,

I know it's more than a few weeks now since I said I will measure the other MO200A. In fact, I variac'ed the other one a few weeks ago and benched it. I have extreme crazy roll off from 20Hz-700Hz (about 4dB down) and from 16KHz-20KHz (about 2.5dB down). So, there are caps probably dying and needs replacement before I re-measured it again. Otherwise, the transformers all seems good and I don't know how they behave (distortion wise) till I have fixed the problems.

Cheers,
James
 
This one is for Dave... gadget73' MO100A thread prompt me to scan through this great thread agian, in some earlier posts, you mentioned applying cathode feedback from the 70V winding, so what were your findings, if any?
 
Jaz -- the coupling between the windings was too poor to make a viable go of using the CF configuration. Basically, it became a HF oscillator, even with the polarity correctly observed. I didn't pursue it to the ends of the earth, as the real issue was going to end of being drive capability if it did work anyway -- but it didn't readily work, so that kind of ended that effort real quick. If I ever decide to revisit it, I suppose some frequency tailoring could stabilize the configuration, but that would take more development -- and still of course, a better driver as well.

Dave
 
I see, that's too bad... since the amp was intended for PA distribution, it seems kinda odd that the 70V winding would have poor coupling. Oh well...
 
The coupling was plenty fine to transfer power in the midband portion of the audio frequencies. It was at frequencies above about 15 kHz where the problem resided (iirc).

Dave
 
Hello Mr. Gillespie

Newbie Alert (Me). I must say, this thread has been absolutely riveting. Mr. Gillespie is an amazing dude and I am incredibly thankful for his articulate explanations. Why am I here? I happened to acquire an M0-200a and started looking for information on it.

I know I'm dredging up a two year old thread. I have spent well over 10 hrs reading every single posting and digesting as much as I can. I am still left with one question: For those of us "crazy" guys who want more power, those who are looking to build a poor-mans "silver seven" (in a steampunk style), can this unit be designed to run all eight tubes and get something well north of 150 watts per channel?

Not being a tube amp expert (I'm learning), the logic would seem to imply that if I ran Parallel Push Pull, the OPT load would get split between the two tubes of each side. Thus effectively halving the current load and doubling the impedance seen by each tube, right? Therefor, I could place a 4-ohm load on the 8-ohm tap, get increased voltage to my load, while returning the proper impedance load to each tube?? Isn't that how it works? Please help me understand where I've gone off the rails on my logic.

In any case, I'm thrilled with this thread and would love to see an iteration for us crazy's who don't have the luxury of huge efficient speakers (WAF) and could use the increased power. Is it possible to get a step function more power by going from two to four tubes per channel? And could you sketch out the possible circuit mods to make it happen? I may be willing to compromise the three trimming pots for absolute minimum distortion (or rely on well matched tube pairs).

Thank you for your time.
 
Hi Jeff -- Mo' Power! -- In the classic Tim Taylor fashion! I've been there myself.

What you suggest is certainly possible in theory, and it does work in practice -- although not perfectly. Factors to consider include:

1. Your output impedance options will go to 10.25 Ohms -- which would still be workable for most 8 Ohm loads, 4, and 1 Ohm. These options should work well for most applications.

2. Single channel power at mid frequencies should in fact double to 200 watts -- although power at the lowest frequencies will likely show only a 50% increase at best.

3. Since you never get anything for free, the power supply of course will be the ultimate limiting factor. Being designed to support a 200 watt power output capability -- and almost certainly under a continuous condition -- it is doubtful that it will support an output of 400 watts RMS -- continuously or otherwise -- although it will surely make a heck of a go at it. I would more expect more on the order of 150 watts at midband, with both channels being driven.

I have participated in such projects in the past, not only with my own experiments, but also in running labs tests for others doing similar experiments to determine the absolute maximum power output an output transformer can handle at 20 Hz before saturating.

I would start by conducting your own experiments on the original design -- but first addressing the inadequate bias for the AF Amplifier stages as discussed in this thread before you do. Then, you can run different power output tests at various test frequencies, with the amplifier loaded at one half the normal load rating. Divide the results by 2, and that will give you a very good idea of how a single stereo channel will perform under conditions of both channels being driven. The test would likely be easiest to conduct using a 4 Ohm 400 watt load connected to 8 Ohm tap of the original design. If you like what you see from those tests, then split the two channels apart, load the output taps accordingly, and there you go.

In any push-pull parallel fixed bias design, it is usually possible to simply remove two output tubes (one on each side of the push-pull signal), and reconnect the load to a lower impedance tap to maintain the same (or similar) loading conditions for the remaining tubes installed, and then operate the unit at 50% power output capability. At best, only a slight adjustment of the bias controls might be necessary.

Good luck with your project!

Dave
 
If he picked up a second one, he could bridge the outputs like the original. Would that reduce some damping or introduce some distortion?
 
That's certainly a way to go as well. There is no doubt that trying to push twice the power through one or each OPT will clearly compromise the distortion, and the bandwidth over which rated power output can be achieved. In the modified design, these were two performance areas that were very much targeted in working to turn an industrial grunt machine into a polished, full response piece with enough manners to mingle with high society.

I the end, it comes down to expectations. In achieving them, there are always trade offs depending on what you chose to emphasize. Going the power route will take you one direction, while quality will take you another. As long as the compromises don't cause the results of any one performance area to fall below the expectations for the application, then the compromises work. The effort of this thread was to achieve maximum quality, while still maintaining the original power output specification of each amplifier when isolated from the other.

Dave
 
dcgillespie said:
The effort of this thread was to achieve maximum quality, while still maintaining the original power output specification of each amplifier when isolated from the other.

Dave
Click to expand...

And you certainly succeeded at that with flying colors!!!
 
Indeed. Having essentially duplicated the work on my pair of MO-100A amps (half the 200A) I can say it works very well. The 200A runs at a bit higher voltage, so it makes a couple watts more. On the upside, the 100A's are a bit easier to move around :)
 
Thank you. I figured that cleaning up the phase splitter would be the first place to start. I must have missed a schematic for the 8417 version using the original feedback winding, but revised with all the final mods. Could you repost that? And maybe if it's not too much trouble, could you confirm what the control grid resistors should be if it's a parallel-push-pull?

I'm thinking that I'll stick with the original tube balancing circuit, but add the phase splitter balance, and a control for bias voltage. I'm torn as to jumping to the 6550 version or sticking with 8417. Mine came with only three good 8417 tubes and I have no 6550 at all. So I'm in for a big investment in tubes no matter what.

Thanks again for such a detailed analysis and design. I have learned a ton just by reading all of this and can't wait to exercise my learnings on this Bogen.
 
Oh, one other question for Mr Gillespie, Is there a place where your custom made tube power tester schematic is published (is it called the TT-10)? I too am very frustrated with the wide range of results I get from various testers and would like to build a real-truth tester.
Thank you.
 
Hi Jeff -- No, I've never published it, although I've discussed the topology of how it operates, and at some point (I think) I even published a block diagram of it -- maybe even in this thread. Each portion of the circuitry is fairly straight forward, but when all the necessary circuits are then brought together, it does become quite complex, consisting of numerous and separate regulated power supplies, signal generation, amplification and driver stages, adjustable load and peak indicator circuits, various indicators, and not to mention the external scope and DVM required for digital readout and visual indications. By modern engineering standards, it is hardly SOTA, containing zero processors, integraters, buffers, and the like, in fact containing no digital circuits of any kind at all. It is entirely an analog design. One of the biggest challenges of designing it was to determine the appropriate load into which each tube type operates during the test, that would enable the cathode of the Device Under Test (DUT) to (potentially) emit its maximum rated current flow (if it was capable of doing so), consistent with producing maximum power output. There have been a number of modern tube testers come on the scene -- both rather simple, and some that are quite complex with processor control and digital interfaces for programming. Being old school, I have no such advantages in my unit -- but then I'm not sure that any of the modern units perform a power output test quite like mine does, either.

Thanks for the interest!

Dave
 
You must log in or register to reply here.
Back
Top Bottom