Bogen MO100A restoration Pt. 1

[scuba]

I have a pair of Bogen MO100A's that I want to restore, one of which (the one pictured here) is supposed to be unmodified. I have all the necessary tubes as well as the test gear shown below (am I missing anything? ). The first question I have concerns the use of the DBT. Do I need to have all the tubes in place before testing for shorts with the DBT, and for how long. The Amp draws 250 watts and the bulb is rated at 250 W also. The tubes are expensive and somewhat hard to come by and I would rather not blow them up if at all possible.
The second question is: Assuming the DBT test is successful, for how long and at what rate of increase should I use the Variac?
One thing of note though is that this unit had been subjected to the usual highly scientific test used by most ebay sellers prior to my purchasing it; i.e.: I plugged it in and it lit up, therefore it is in working condition.


Note the resistor with a lovely blob of solder on it. It is labeled 510 ohms, 5%, 7Watts. Could someone tell me what kind of a resistor this is as it will definitely need to be replaced. I am assuming some form of ceramic resistor.

Any and all help is much appreciated.

 

[scuba]

How do I edit my title. Should read Bogen and not Borgne

 

[drtool]

^ A mod can edit.

 

[arts]

Scuba,your post might be better off in the Tube Audio forum. You'd probably get a lot more ''viewers''

 

[scuba]

Thanks arts. I'll ask the mods to move it. Not quit sure how to do this so hopefully a mod will see this post and move it.

 

[gadget73]

You can test it with no tubes in it for basic life. I'd probably use a lower rated bulb though.


Are you planning to refurb these in stock form ? If you've got no specific plans, there is a rather lengthy thread on how to re-work this circuit for quite good results. Basically you re-use none of the original parts and only half of the tubes. If you've got 8417's in good shape, there is an option for using them. If you don't have a set, the other method uses a 6550 or KT88 tube. I have KT120's in mine since I was able to get them for a slightly better price.

http://www.audiokarma.org/forums/index.php?threads/the-mighty-bogen-mo-200a.556575/

The 200A is basically two 100A's on a single chassis. The schematic ends up the same. The only real difference I notice with mine is that the B+ voltages are a bit lower, and my power output is a bit down as a result. I get 90 watts, Dave's 200A got I think 107. Mine might actually do a smidge more, at the time I had a meter that wasn't really accurate. I have not bothered to re-measure things since I upgraded.

This is what it ends up looking like in a 100A

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

 

[scuba]

My ultimate goal is to end up with a pair of 100 watt amps strictly for music. I have 6 8417's as well as a quad of Psvane KT88s, so I could go either way with the tubes. I will have to take a long look at the threads you linked before deciding wich way to go. My electronics knowledge is still in its infancy, so I will also have to take things slowly and in bite sizes.
My first inclination, given the state of the amps is to strip them down to the bare chassis and build back up from there. Probably getting rid of everything not directly connecred to amplifying music.

 

[mech986]

Scuba, I notified the mods for you. You can use the "report" link in any post to bring something to the attention to the mods.

 

[knockbill]

Note the resistor with a lovely blob of solder on it. It is labeled 510 ohms, 5%, 7Watts. Could someone tell me what kind of a resistor this is as it will definitely need to be replaced. I am assuming some form of ceramic resistor.

Any and all help is much appreciated.

Resistor is a 7W/510R/5% wire wound, you can use a 10W if its fits... Mouser should have em...

 

[scuba]

Scuba, I notified the mods for you. You can use the "report" link in any post to bring something to the attention to the mods.

Thanks mech986. I'm going to need quite a bit of guidance for this project and having it in the right forum will help a lot.

 

[scuba]

Bogen #2. Highly modified. Looks like it may have been set up as a Guitar amp. I has a VU meter at one end and a 1/4 inch jack as well as an electronic switch at the other end. Given its current condition, I believe that I will start with this one. Does anyone have any idea what that switch-like thingy on the bottom left of the first pic might be?
The small 9 pin socket for the TL600 has been replaced with a much larger novel socket for some reason.
Going to start stripping it today.

 

[scuba]

Assuming that the input from the pre-amp would use the high impedance (0.5 megohms) input and that I don't see the need for a volume control on the amp itself, would these modifications make sense?
The first scan is the schematic for the amp and the second shows proposed changes to date.
Am I correct in assuming that the pre-amp will use the high impedance input and can I connect it directly to pin 7 of the 7247?

 

[gadget73]

If you remove the volume control, you'll need to put a 470k resistor from pin 7 to ground to take it's place. The volume pot is being used as the grid resistor.

If you end up changing it to Dave's recipe, basically you start by removing everything from the amp. When I did mine, the only things left in place were the tube heater wiring, the pilot light, and the main 120v power wiring. The switches were both bad, so they even got the boot. The extra sockets were removed, the extra tube heater wiring was removed, and the caps were all removed since they weren't any good or the right values. Honestly it makes for a nice place to start when the chassis is empty. No worries about trying to fit new parts in place of old parts.

 

[scuba]

In for a penny, in for a pound. Which ever way I ultimately decide to go, I'll have to strip it bare. So here goes nothing!
Thanks gadget. It'll take me a couple of days to remove all the caps and resistors etc.
Fun, fun, fun.

 

[scuba]

This is about as stripped as it's going to get. I managed to break a pin on the 9 pin socket for the 7247 while attempting to desolder a connection and as a result decided to scrap and replace all the sockets. There was such a cluster event of solder and wire that this was pretty much the only path that made sense. Still undecided wether to sand and repaint the chassis or possibly build a wood casing for it. On the other hand, the rat-rod look is pretty cool too.

If you remove the volume control, you'll need to put a 470k resistor from pin 7 to ground to take it's place. The volume pot is being used as the grid resistor.

If you end up changing it to Dave's recipe, basically you start by removing everything from the amp. When I did mine, the only things left in place were the tube heater wiring, the pilot light, and the main 120v power wiring. The switches were both bad, so they even got the boot. The extra sockets were removed, the extra tube heater wiring was removed, and the caps were all removed since they weren't any good or the right values. Honestly it makes for a nice place to start when the chassis is empty. No worries about trying to fit new parts in place of old parts.

You were right about having an empty chassis. It'l really make it easier to get a clean and neat build.
I've attached a scan of the modifications I am currently thinking of using. Basically, I want to remove anything that is not strictly needed to make a music only amp. I.e. the low impedance, volume, accessory and remote standby controller circuits. Although this is not necessarily the way I will ultimately go, could anybody take a look at it and tell me if it makes sense and if it will work?



P.S. Does anybody know what this doohickey is?

 

[scuba]

Here is the schematic in a more practical format.

 

[gadget73]

doohickey looks like a rectifier of some fashion. Mine didn't have that type.

Schematic changes look right if you are keeping the stock circuit. I wouldn't, but thats just my 2 cents.

Looking back on mine, I sort of wish I'd at least yanked the transformers and painted them, but I can do it in place I guess. I do sort of wish I'd had some way of removing / changing the tube numbering on the chassis so it reflects what it actually takes without looking like anything changed. I think I'd have had to repaint the whole thing, and then number the tubes. Not much chance of getting the old numbers off or applying paint over it without it being obvious.

 

[scuba]

When I was a kid (eons ago, I know) there used to be this stuff called Letraset where you applied the letters like you would a rub-on decal. Don't know if it still exists but might be worth a look.
As far as the final circuit goes, I would like to use Dave's, but I'm having a hard time wrapping my mind around it. I work with plans on a daily basis and until I can close my eyes visualize it in my head and understand how things connect or fit together, I find it very difficult to move forward. I need to stare at Dave's schematic for a few more hours before things start to click.
Unfortunately, since I work thousands of miles from home And have to go back there in a few days, this is about as far as I can go for the next month. I also have to order the parts anyway, so no great loss there. Bills must be paid yada yada yada.

 

[gadget73]

Its actually not all that involved once you get to building it. The amplifier part of it is really straightforward.

 

[scuba]

Its actually not all that involved once you get to building it. The amplifier part of it is really straightforward.

The part of this design that is giving me the biggest headache is the power supply. Dave's PTO shows two sets of heater windings while the PTO on the MO100 only has one. He also shows a right and left channel. I'm assuming that his schematic shows only one side to the MO200 and therefore both heater winding circuits are required. If that's the case, can I splice the two channels to the one set of heater winding taps on my PTO? He also shows a choke as part of the main power supply circuit and a secondary transformer on the left heater circuit. Do you know what type of transformer it is (looks like a step-up transfo) and could you make any recommendation as to which choke and transfer to buy as well as where (Mouser? ) to get them?
I have a butt load of other questions, but at the moment I'm just trying to draw a parts list as I have to order the various bits and pieces.
Since the 6550's are so close to the KT88's, can I substitute them? I have a quad of cheap Chinese KT88's I can use as guinea pigs as well as a quad of amazing Psvane's which I would like to use once the project is finished.
My apologies for being such a pain with all these questions, but the deeper I delve into this project, the harder the bug bites me.

Thanks for all your help.