My Winter Project

Converting a Bogen MO-200A into something useful...

I got a beat-up bogen a while ago for $60 from a friend (which he got from Douglas). Finally got around to rebuilding it. I know there are a bunch of these things around, so I'm posting my refurb here for anyone that wants to a refurb on theirs, or just people who like tube eye candy . I'm just posting now because I hope to get it up and running soon. Sometimes they pop up on that auction site here and there.

Amp specs:
plate voltage: 680 VDC
Screen voltage: 340 VDC
Power tubes: originally 8x 8417, converted to 8x JJ E34L
class: little AB, mostly B
power output: transformers are good for 100W each 20Hz-50KHz from what I gather here: http://www.montagar.com/~patj/bogenspecs.htm, but I'll see what it actually does soon.
drivers: originally two 7247s, now 4x 6CL6 LTP w/CCS

I took some pics here of the chassis and fit-testing the caps. I like how they go in here, pretty proud-a that since you can't see the clamps from the top. I'm gonna go with the blumlien cathode bias scheme so there will be no bias potentiometers to adjust. thanks for the suggestion Douglas.:thmbsp:

The little spots of gray on the chassis is JB weld that I used to fill in the old holes...seems to work well and paint goes over it nicely. Anyways, Happy Holidays everyone. This will probly be one of last posts before Christmas b/c I'll be going home where there is only dial-up, and that connection just makes me mad so I don't use it:sigh:
bye yall'

Looks like a winner. I want to see the whole project. Are you going to post pic's as you go? That'd be cool.

Wow 680v on the plates, and I thought the 520 I was running on EL34s was a bunch.:thmbsp: I remember reading somewhere that the big deal about EL34s (when they were first invented) was the fact that none of the HV internal wiring had to cross paths, which enabled them to have the "high" high voltage ratings. I see that you have both positive and negative wire on your caps, are you planning some sort of Star grounding? I'll be watching this thread very carefully, and paying close attention to your grounding ideas and lead dress (two things I'm struggling with). Please try to go kinda slow and give detailed explanations if you don't mind. I see a good learning opportunity here. Thanks for starting the thread, Jay

bricktop said:

The little spots of gray on the chassis is JB weld that I used to fill in the old holes...seems to work well and paint goes over it nicely.

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It should work fine. When the holes were punched originally they usually have a small amount of burring which will help hold the epoxy in place long term.

Here's a trick I do to make it even more secure -

Put a slight amount of countersinking on each side of the hole you want to plug. The epoxy will be wider on both sides and taper to the middle. It won't go anywhere once it cures. For tools, this is all you need -

gonzomeep said:

Are you going to post pic's as you go?

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Yup, I planned to have a build thread like Jay's 6GK5 amp.

jaymanaa said:

I see that you have both positive and negative wire on your caps, are you planning some sort of Star grounding?

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Those leads are left over from testing to try to find my optimum resistor for the blumlien balancing (470ohms works nice here). For grounding I plan on using a brass buss-bar then electrically connecting that to the chassis at one point.

soundmotor said:

Put a slight amount of countersinking on each side of the hole you want to plug. The epoxy will be wider on both sides and taper to the middle. It won't go anywhere once it cures. For tools, this is all you need -

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Good idea. I have a countersink drill bit I can use for next time:scratch2:


I'll be heading out soon, so no posts for a few days. At least at home I can make the wooden frame to go around the edge. Got the woodworking tools there. There are so many factory punched holes in this chassis it's like swiss cheese. Good thing it's steel and not aluminum, else it would bend to pieces.

bricktop said:

Yup, I planned to have a build thread like Jay's 6GK5 amp.

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SaaaaWeeeet!

I'll be looking forward to this one. I think you will like the Blumlein circuit, it seems to work awfully well, and couldn't be simpler. If you do any work with a scope, I hope you might take some of us rookies through the hows and whys. I have a couple older scopes, but don't have a clue as to how to operate them. Have a good Holliday, Jay

Out of curiosity, is the main reason for switching tubes the expense of the originals?

My understanding is that the amp itself is very good, with incredible power output for a tube amp, in its original configuration. But I also know that the tubes are ridiculously expensive. What performance differences do you expect with the modified tube arrangement if everything goes well.

By the way, I have also heard that the transformers were excellent for audio applications and were linear well beyond the normal hearing frequencies.

hammr7 said:

Out of curiosity, is the main reason for switching tubes the expense of the originals?

My understanding is that the amp itself is very good, with incredible power output for a tube amp, in its original configuration. But I also know that the tubes are ridiculously expensive. What performance differences do you expect with the modified tube arrangement if everything goes well.

By the way, I have also heard that the transformers were excellent for audio applications and were linear well beyond the normal hearing frequencies.

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I'm back from break. Yup, I chose EL34 types because the new ones are affordable. I suspect that it will have slightly less power output and more class B range with EL34s since they are 25 watt tubes and the 8417 is a 35 watt tube.

I'm diggin' that news about the transformers, makes me wanna finish it asap. Over break I didn't get much done with the amp, but I did get the wood frame finished. Solid oak with Minwax "Dark Walnut 2716" wood stain and a coat of urethane. I like the way it turned out. I have a few more holes to drill and placement decisions to make then I'll be priming and painting the chassis. more news to come...

Welcome back, the frame looks outstanding. I'm curious as to what the holes are far.:scratch2: Try to give us lots of details on the build if you don't mind the typing.

jaymanaa said:

Welcome back, the frame looks outstanding. I'm curious as to what the holes are far.:scratch2: Try to give us lots of details on the build if you don't mind the typing.

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And post the photos to go with it please???

The holes to hold it together with #4x3/4" flat brass wood screws. For the wood, I used 1/2" x 3" x 36" oak pieces from Lowe's. I could have glued them, but I don't like the permanence and the brass is a nice accent. I'll be sure to put more details from here on out

I made sure to drill ample pilot holes from all screws, else it will split, especially with oak. Also a countersink bit was used for the holes as well.

Hi, I'm a noob here, but have built amplifier gear way back when tubes were affordable from GE to Genalex. This is eye candy for me now, as I am trying to revive the interest in such hobby once again.

Your handling of grounding seems to be making good sense, so it's apparent to me that you know what you're doing. Keep up the good work! I'll keep watching.... :thmbsp:

Nice work, the screws do make a nice accent.

Welcome old1625. that's another tube I'd like to build around someday. They are still affordable and plentiful. I think people are scared of the anode caps sometimes.:thmbsp:

hey-Hey!!!,
That is some nice work you've done there... I hardly recognize it. I too am pleased with your discovery on the output TX bandwidth. Let us know when it's up and running!
cheers,
Douglas

primetime! (pun intended)
used Rust-oleum gray primer 7582

a few extra holes since the last picture:
- holes by the driver tubes for RCA inputs
- also holes on the far corner for a power switch, next to a hole already there which i'm using for a fuse holder
- near the caps for speaker output screw terminals (used dremel to make it look nice)
- a few extra holes on the sides to hold the wood fixture in place


just curious, does anyone out there who's reading this also have an old bogen MO-200??

bricktop said:

just curious, does anyone out there who's reading this also have an old bogen MO-200??

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No, but now I want one really bad (thanks alot). The chassis looks really sharp. Did you mention a color?

bricktop said:

primetime! (pun intended)

just curious, does anyone out there who's reading this also have an old bogen MO-200??

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hey-Hey!!!,
I don't have one, but I wish I did now... I think I know where a pair of MO100A's live though.
cheers,
Douglas

jaymanaa said:

Nice work, the screws do make a nice accent.

Welcome old1625. that's another tube I'd like to build around someday. They are still affordable and plentiful. I think people are scared of the anode caps sometimes.:thmbsp:

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Thanks! I've got a few of them in cartons here. I hope to fiddle with them some day. Finding good ceramic or porcelain sockets for those bad boys may prove to be a challenge....:scratch2: Big thing about these--as well as the typical sweep or TX tube--is that (IIRC) unlike the 6L6GCs and 6550s the screen grid maximum voltage rating is significantly lower than that for the plate. This can complicate matters to a degree if ultra linear or unity-coupled design is one's goal.

Old1625 said:

Thanks! I've got a few of them in cartons here. I hope to fiddle with them some day. Finding good ceramic or porcelain sockets for those bad boys may prove to be a challenge....:scratch2: Big thing about these--as well as the typical sweep or TX tube--is that (IIRC) unlike the 6L6GCs and 6550s the screen grid maximum voltage rating is significantly lower than that for the plate. This can complicate matters to a degree if ultra linear or unity-coupled design is one's goal.

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I was looking at those too for use with a pair of TO-300's I have lying around, but once I noticed the screen voltage rating like you mentioned, I was a bit bummed out. I still haven't decided what to do with them, maybe use KT66s since they make them new now

spose I could use them in pentode but I'd feel like i'd be wasting the transformer