The Mighty Bogen MO-200A

Thank you Mr. Gillespie. I would be very interested in getting the information on your jig and configurations for testing performance for a couple of key tubes, the 6L6, 6550, 8417 (6BQ5, EL34 also). Even some small tubes like 12Ax7 (I fear I've thrown away a lot of great Funkin, Amperex, Mullard tubes due to "bad" measurements on my current tester). Maybe a schematic for how you powered, drove, loaded, measured and interpreted the results on one of those tubes, then some guidance on how the test changes for other tubes - - how to convert the tube data-sheet values into test configurations.

I can certainly understand where multiple precision regulated power supplies are needed, as well as several DMM, scopes, and signal analyzers. I have all the measurement tools (including an Audio Precision unit and a bench high voltage DC power supply). But as I'm still learning tube gear, I just need some guidance on exactly how to configure the test set up for such a measurement, or derive the test configuration from the data sheets.

I thank you yet again for sharing with us with your extensive knowledge!
 
I greatly appreciate this thread, especially the work of Dave Gillespie, but everyone's contributions have been a great education to me. I certainly don't have any of the test equipment needed to do this sort of rework. My brother in law dumped 3 of these amps in my lap a couple years ago (figuratively, if literally I'd still be sitting in the same chair today). I used two of them as monoblocks with very minor modifications at first. They sounded pretty darn good to me even then. After reading through this thread a few times I used the recipes here to split one amp into stereo using the 8417s (they came with 16 or so of them). I still have some work to do in the power supply, but it still out performs anything else I have, or anyone else I know has, as it drives a few small classic Klipsch speakers I have at reasonable levels. I've heard a new MacIntosh SS amp sound better, but no wonder. Most of the stuff I play through it is acoustic Jazz stuff and Classical. I can hear its limitations when I try to blast Rock or similar sonicly thick stuff, but it certainly has changed my HiFi life as my first project tube amp.
 
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For their fairly pedestrian origins they really do sound good. I'm very pleased with how my MO-100A's perform.
 
Gravicen said:
I greatly appreciate this thread, especially the work of Dave Gillespie, but everyone's contributions have been a great education to me. I certainly don't have any of the test equipment needed to do this sort of rework. My brother in law dumped 3 of these amps in my lap a couple years ago (figuratively, if literally I'd still be sitting in the same chair today). I used two of them as monoblocks with very minor modifications at first. They sounded pretty darn good to me even then. After reading through this thread a few times I used the recipes here to split one amp into stereo using the 8417s (they came with 16 or so of them). I still have some work to do in the power supply, but it still out performs anything else I have, or anyone else I know has, as it drives a few small classic Klipsch speakers I have at reasonable levels. I've heard a new MacIntosh SS amp sound better, but no wonder. Most of the stuff I play through it is acoustic Jazz stuff and Classical. I can hear its limitations when I try to blast Rock or similar sonicly thick stuff, but it certainly has changed my HiFi life as my first project tube amp.
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Welcome to AK! And nice to hear how you've worked on your MO-200's. When you get a chance, pop some pictures into this thread to show off your restoration.
 
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One of my goals was to re-use old stuff (as long as it still measured OK), and whatever I had lying around. It resulted in a project that's pretty messy inside, but didn't cost much, and it still sounds pretty good.
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I'd like to eventually fit in the better biasing as Dave Gillespie redesigned it, and so on, but it's time for me to move on to other projects: I need a good preamp to go with this.
When I first posted here I had just started moving it from the original Bogen steel frame into this humidor (never kept my cigars at 70%, may as well use it for something else), thus the delay.
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Unbelievable!! I've seen some different approaches over the years, with separating the power supply and audio circuits one of the more popular different approaches to use. But absolutely never have I seen them split up between a humidor and an ammo box. Very well done!! I cannot imagine anything winning out over this entry into the "unique and different" category!

Dave
 
I know I'm a couple of years after the fact, but I have questions...

For example, the 6550 markedly improved frequency response, but what about a tube like the KT-120? Is that tube a consideration? Would it be better, worse or sideways?

Also, I've heard people disparage the 6U8 and their brethren, as they are not audio tubes (like, for example, the 7199). Some say that this tube choice is a failing of the Scott 340B. I know that there were limitations on the project, but in a "no holds barred" approach, would there be a better choice than the repurposed TV tubes?

Additionally, the choice for the primary? winding was also selected based on best frequency response, but as it turned out, the Xformers on the Test Mule were damaged. Is it possible that on the second unit that the more traditional winding could have measured better and been used instead? (Forgive my poor memory, I think it was the 70v winding that was used, as opposed to the 8ohm winding? My knowledge of this technology is zero :( )

And the last question I have is if it is possible, worthwhile or advisable to redesign the circuit with tube rectification? I have heard the opinion that ANY amp or receiver without tube rectification isn't worth its weight. (I think this is a pretty common argument.) But you were intentionally staying close to the design, plus there simply may not be room in the chassis and it may result in redesigning everything, so I have no idea.

Those are my questions for now, but I have to end by saying that Dave Gillespie is not only one of the premier tube designers in this day and age, but he is exceptionally generous with his time knowledge and instruction - what I would call a rare and priceless bird. Gives me faith that there are still a few good ones out there. Thanks to him and all who participated in this thread so that those like me can begin to learn, even just a little...

Dan
 
I run KT-120's in mine. I also have run EH KT88, and honestly I like the KT-120 sound better. The 120's seem to have a little more bottom end. I have not tried old production tubes or 6550's though. This should happily support most any of the big power tubes, though its not likely to perform much different or better with them.

I guess you could use a different front end design, but it would need more tubes. The pentode/triode is an efficient way of getting the job done with the available space, but if you built on a new chassis you wouldn't be limited to that. A Mullard style front end with triodes should be possible, or you could keep it a split load inverter using triodes, a Williamson, or really whatever you wanted as long as you were prepared to do all the performance and stability testing to make sure its going to behave itself. You could also keep the same basic design and run an EF86 and a 6C4. Personally I don't mind pentode small signal tubes, other people think they absolutely awful in terms of sonics. I do feel they can add a bit of extra something up at the high frequencies but not everyone likes that.

Tube rectifiers would be a very difficult thing. The secondary voltage isn't high enough to make it work, there isn't a heater winding for them, and the amount of current required would need several parallel rectifier tubes. I'd figure at least 2 per channel, but 3 may be a safer bet. Basically you'd need a different power transformer for this to have much chance of working at all, and I suspect the results would not be worth the trouble.
 
Thanks gadget, you break it down very well and make it easy to understand. I read your thread on your 100As (which is what I have) and have learned a lot from you and DG.

I'm sure I'm not alone when I say that I'd love to see a detailed Listening Review of your amps, and maybe rpampt could get in on this too. He did a bit of a summary in this thread, but all of us who have been curious about these big, ugly beasts would surely appreciate a more serious review session, taking on some of your favorite LP and CD tracks, and if you have the luxury, even an A/B with other well-known or highly regarded gear.

I loved seeing how your and rpampt's amps came together, well above expectations, but in the end it's all about SOUND, so if I could talk you into it, I'd sit perfectly still on the rug and listen to your story...

Dan
 
I'm pretty bad at doing that sort of description honestly. Other people have heard mine and thought they sounded good too, but beyond "it sounds good" I don't know how to really talk about them. My listening setup and speaker situation is far less than ideal too, so I'm not even able to get all that they can deliver.
 
Hey gadget, fair enough. I plan to eventually add mine to the ranks, so at that point perhaps I'll be able to add to the discussion, and if all comes to fruition, I hope you'd come by and give them a listen.
Dan
 
MasterHiFi said:
Hey gadget, fair enough. I plan to eventually add mine to the ranks, so at that point perhaps I'll be able to add to the discussion, and if all comes to fruition, I hope you'd come by and give them a listen.
Dan
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I, like Gadget, am not great at listening reviews and usage of the terminology so eloquently spoken by those of refined audio heritage! :)

Awesome, da bomb, great, lousy...much more likely than velvety tenderness, refined sibilence to come from my mouth.

I can say the Mighty Bogen continues to perform flawlessly. Right now usage is only about 2-3 hrs per month. It's only being used on mids in a tri amped system, but with hopes of using it full range is a smaller system again soon. The 100's may be a different animal, but the chassis of the MO 200a places the tube sockets close together and while 6550s fit ok, I believe I determined the Kt88 would not fit if 8 were used. If reduced to 4 output tubes, they could fit if the right sockets were used. I haven't used 120 b4, but if the diameter is larger than the 88s, then I don't believe they'd fit on my "little red hot rod" as Dave aptly called it.

I don't have any really high end amps to compare too, but sometimes I feel that having an amp all in spec is more important than all high end parts.

A high end amp with caps and various parts that are out of spec vs a vintage amp that's had all out of spec parts replaced may be enough to give the vintage amp an edge in sonics. Making upgrades or changes to existing gear with great attention to performance testing and stability like DG does, along with constant measurement against original, is going to naturally be an improvement. If compared to a modern amp that is in perfect working order, well, only listening can likely settle that. DG is definitely a "rare bird" as you mentioned! I was very lucky!

Wish I could help more.
 
I recently picked up one of these beasts in an audio gear pick. Oddly the owner said he pulled it out of AT&T surplus back in the 90s. He had several spare transformers and some parts chassis that I picked up as well.
Might have to follow Dave's lead on this one and bring it to life.
 
A question for Gadget, if he is reading this -- as you told me that you were using KT120 tubes in your modified MO-200As. I am currently in the process of having my tech rebuild a pair of MO-100As per Dave's schematics and notes, for use as a pair of mono blocks for a stereo system; and using Tung Sol KT120s (2 each) for the output tubes. One thing I am changing is using gas regulator tubes in place of the zener diodes, and putting those tubes in the rear row, in place of the original rear sockets/tubes. I think this will make for a lot more bling!

My question is: is there anything he should be aware of when going to the KT120s from the 6550s as Dave's schematic is designed to use? Or, other than adjusting bias, are we pretty free to roll tubes from the KT line?

Thank you for any information on this!

Dennis
 
DennisConger said:
A question for Gadget, if he is reading this -- as you told me that you were using KT120 tubes in your modified MO-200As. I am currently in the process of having my tech rebuild a pair of MO-100As per Dave's schematics and notes, for use as a pair of mono blocks for a stereo system; and using Tung Sol KT120s (2 each) for the output tubes. One thing I am changing is using gas regulator tubes in place of the zener diodes, and putting those tubes in the rear row, in place of the original rear sockets/tubes. I think this will make for a lot more bling!

My question is: is there anything he should be aware of when going to the KT120s from the 6550s as Dave's schematic is designed to use? Or, other than adjusting bias, are we pretty free to roll tubes from the KT line?

Thank you for any information on this!

Dennis
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The tubes with names starting "KT" ( KT88 KT120 KT150 etc) has NOTHING in common ! They are different
beam tetrodes where marketing creatures adopted the prefix to increase sale.

Origin of prefix "KT" was MO, inventor of beam tetrode, and the meaning was "Kinkless Tetrode"

Many amps designed for KT88 may accept KT120, but distortion will increase and power will not increase .
Amps power is limited by available B+ and transformer impedance.

Note : "MO" refers to "Marconi-Osram Valve"
https://en.wikipedia.org/wiki/Marconi-Osram_Valve
 
Kt88 , 120 are pin compatible. The 120s draw more filament current.
I don't have the tube data sheets in front of me and I don't know the operating point of the mo. It might be an ok swap. Operating points have to be verified.
 
It doesn't make any more power, but they work fine. I have KT88's and KT120's. Other than a bias adjustment performance is virtually identical between them. Yes the KT120 does pull a little more heater current but considering each of these amps ran 4x 8417 tubes originally, its got heater supply to spare.

I made no changes to the schematic specific to the KT120 tubes. I did make some small changes just for physical fitment though. The choke and bias supply transformer are smaller, and I think thats the only difference.

https://audiokarma.org/forums/index.php?threads/the-slightly-less-mighty-bogen-mo-100a.685890/

has some more details but it looks like I didn't actually mention what choke or transformer I used. I can dig that up when I get home, I'm sure I have the notes, or at worst I got them either from Digikey or Mouser so it will be in my order history.
 
Thank you, Gadget, for your confirmation. I showed the last few comments to my tech, and he mentioned that it would be nice to know what specifically was used for choke/transformer. If you could get that to us, that would be greatly appreciated!

Regarding peterh's comments on interchangeability, I did some research. There was a page from The National Valve Museum (Southerland and Wyatt -- "On the Non-Equivalence of Beam-Tetrodes vs Pentodes), but didn't specifically address the 6550/KT "series" (that I could find). Most of the information I downloaded seemed to indicate that they are largely interchangeable, but it depends on the amp, some mods may be necessary, and as in a lot of things, the devil is in the details.
 
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