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Old 30th October 2006, 10:48 PM   #1
apehead is offline apehead  United States
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Default Bogen-Stein! mx60a mx-60a mo100a mo-100a

Hello everybody,
This is my first post, and first real project (apart from assembling a crappy PAIA tube pre-amp). It's not a very complicated one, but need a few pointers...

1. THE PROJECT:

Dismember two Bogen MO-100A's and one Bogen MX-60A, replace the 8417 tubes with 6550's, hardwire them all together, place all onto a new chassis, and run them all with one power cord. More specifically, I plan to use the MX-60A as a first stage, then run the MO-100A's in parrallel.

2. THE QUESTIONS:

Each amp contains one 7247 driver. Should all three be kept in circuit? If so, series or parallel? If not, how many and where in the circuit? Also, I'm not sure the best option for a clean maximum gain. Place all the amps in parallel maybe?

P.S. I've read all the literature I could find on this website (and others) pertaining to these pieces of equipment (including the QS 8417 to 6550 conversion procedure) but have only been able to get this far alone. I've ordered a MX-60A manual, but did manage to find a MO-100A manual online. The link is posted below:

MO-100A manual

Please help, for I can hardly wait to blow the windows out with an estimated 260W of head-popping bass!!

-Jack
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Old 30th October 2006, 11:05 PM   #2
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Hey-Hey!!!,
I had most of what you're planning on creating. It was the MO-200. They are two MO-100's, parallel at the input, and parallel at the output.

I would not recommend applying the different MX60A( had a few of them too, nice bass amp once the NFB winding is disconnected ) into the mix.
cheers,
Douglas
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Old 31st October 2006, 05:29 PM   #3
apehead is offline apehead  United States
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Thanks for the advice. Now I have a new question. Could I go directly from the output of the MX-60A into the input of the paralleled MO-100A's? The power stage of the MX-60A consists of two 8417 tubes (modded by the former owner to EL34's). Will this overdrive/damage the MO-100A's?
-Thanks,
Jack
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Old 31st October 2006, 06:17 PM   #4
apehead is offline apehead  United States
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P.S.
I am planning to replace the EL34's with 6550's
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Old 2nd November 2006, 09:32 PM   #5
apehead is offline apehead  United States
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Ended up only getting one MO-100A... guess I'll have to settle for 160W...(sigh). On a different note, I was thinking of adding a tone circuit to each of the inputs. Low noise is very important to me, and was wondering if anyone had any suggestions for designs. I was thinking of going with the passive Baxandall. Please somebody respond, before I lose my faith in this project or even, gasp, tubes!
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Old 3rd November 2006, 04:28 PM   #6
kevinkr is offline kevinkr  United States
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Can't parallel two amplifiers based on dissimilar output transformers. Sorry.. So you've got a 100W amp and a 60W amp. I'd go with the 100W amp - plenty loud enough with the right driver.. (Say vintage series Jensen or Celestion)

Baxandall configuration by definition is a feedback based tone control circuit, and a very good design at that.. By definition not passive.

Western Electric stack used by Marshall and Fender is a passive tone control and quite different. (Most suited I think to an electric guitar amp while the Baxandall seems more suited to acoustic instrument amplifiers.)

I used to be Fishman's (FTI) chief ee so take the above with a grain of salt..
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Old 5th November 2006, 02:49 AM   #7
apehead is offline apehead  United States
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Thank you for the advice, I'll have to get a pre-amp for the MO-100A. The manual mentions the Bogen MXM. Oh well... One final question. Would it be possible to use the MX-60A as a pre-amp, i.e. take the power tubes and driver out of the circuit? Would the output transformer explode? Had that happen once with an old radio. We took all the load (radio, record player) out of circuit and plugged my bass into it. Needless to say, it snapped, krackled, and popped! So, would history repeat itself?
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Old 5th November 2006, 03:07 PM   #8
kevinkr is offline kevinkr  United States
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Better I think to find a guitar pre-amplifier out there, some effects boxes have enough gain to serve this purpose as well.

Using the MX-60 as a pre-amp even removing the driver and output tubes doesn't seem like the right way to go as the power supply would be rather lightly loaded causing voltages to soar - probably beyond the ratings of the supply electrolytics. I also doubt that the pre-amp circuitry in that amp is well suited to bass applications without a lot of reworking.

One thing you might consider is copying the front circuitry of your favorite fender or marshall amp (separate chassis) and using that with the MO-100's.

One pre-amp you could probably use provided the input sensitivity of the MO-100A is high enough is the Fishman Platinum Pro Bass which I designed about 4yrs ago. It's highly suited to use with uprights, acoustic bass and electric bass. Sort of universal.

Lest you think I'm a big fan of Fishman I'll tell you I left there under less than happy circumstances, and will never forget that, but I think this particular pre will fit the bill pretty well. Try to find a used one if you can, make sure if you do that it is the bass model, the other one is for acoustic guitars and is not suitable. Should you need some additional gain it should not be too hard to add an op-amp or simple tube buffer between the pre and the amp.
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Old 5th November 2006, 03:27 PM   #9
kevinkr is offline kevinkr  United States
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One thing to be aware of in converting from 8417 to 6550 is that the transconductance of the 8417 is about twice that of the 6550 which means open loop gain will be reduced by about half..
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Old 8th November 2006, 02:40 PM   #10
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Well I finally got the MX-60A. Quite impressive! Sounds great! It currently has EL-34's in it now, and I'm content, but the mid-range could maybe have a little more umph. Would 6550's help out in the mid department? If so, what brand? I don't have a bloddy schematic yet, I should be getting it in the mail any day now. So without diagrams, does anyone think I will need to modify it to accomodate 6550's? I have to contact the guy who sold it to me and ask him what he fiddled with to alter the amp.
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