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
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
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
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
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
-Thanks,
Jack
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!
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..
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..
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?
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.
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.
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.
IMO The EL34 and 6550 really aren't that interchangeable, the EL34 has twice the transconductance and requires roughly half the drive voltage for a comparable level of output power.
I have never heard anyone accuse the EL34 of lacking oomph in the midrange, and IIRC my extensive experience with the 6550 (GE/SYL/Tungsol/SV) if anything indicates it might be very slightly more reticent in the mids than the EL34.
I would look at things like coupling caps, condition of power supply caps, etc.. Make sure that the amplifier is actually totally up to snuff before making major changes.
Also consider the possibility that the speaker(s) play a role in your midrange performance as well. Perhaps they aren't that good in the mids. You could try a separate mids driver - its been done before.
I have never heard anyone accuse the EL34 of lacking oomph in the midrange, and IIRC my extensive experience with the 6550 (GE/SYL/Tungsol/SV) if anything indicates it might be very slightly more reticent in the mids than the EL34.
I would look at things like coupling caps, condition of power supply caps, etc.. Make sure that the amplifier is actually totally up to snuff before making major changes.
Also consider the possibility that the speaker(s) play a role in your midrange performance as well. Perhaps they aren't that good in the mids. You could try a separate mids driver - its been done before.
I actually figured out the problem... there is a bright channel control on the amp that I hadn't found. It's totally cool now. I did have some more questions though, so I began some new threads...
Bogen MX-60a Tune-up
bias adjust installation, bogen mo-100a mo 100a quicksilver
Bogen MX-60a Tune-up
bias adjust installation, bogen mo-100a mo 100a quicksilver
Bogen 8417 to 6550
Hi, gEo the Large here
I just got a Bogen Mx60a in decent shape from a studio owner in exchange for some fixing guitar amps stuff...
I cleaned it up for sale, hoping to ebay it for like $80 bucks??
I have one good 8417, the other has "Air" and is poopy!
I have two Ruby 6550s so I modded the bias up to 33 volts and the 6550s seem super-duper and the amp sounds great. You can go line in at the AUX jack and bypass all the Mikey preamps, then just EQ, and out.
I changed the 2.2k resistor across the 100/25 volt cap, to 5.1K
I changed the 100/25 to a 100/50V cap.
The 8417 was almost the last tube designed. Itwas for super-AB1/2 operation, so the grid bias it needs is 1/2 what a 6550 needs. Then when running 600 on the plates, 300 screens and -15 bias into 4200 ohms, 120 watts.
i used to build Killer 240 watt maps on these Plush guitar amp chassis that had 1900 ohm Hammond output transformers with four 8417s. Back when you could buy them.
If anone is interested in buying this beast, let me know.
I don't have any spare 12ax7's, so it comes with the 7247s and two 6550s.
Any other tech help, I can when I have time.
I am in North San Diego, CA 92084, Vista
Here is a link to a photo-album with some of my work on it.
http://photobucket.com/albums/b121/SalesBoy/
Hi, gEo the Large here
I just got a Bogen Mx60a in decent shape from a studio owner in exchange for some fixing guitar amps stuff...
I cleaned it up for sale, hoping to ebay it for like $80 bucks??
I have one good 8417, the other has "Air" and is poopy!
I have two Ruby 6550s so I modded the bias up to 33 volts and the 6550s seem super-duper and the amp sounds great. You can go line in at the AUX jack and bypass all the Mikey preamps, then just EQ, and out.
I changed the 2.2k resistor across the 100/25 volt cap, to 5.1K
I changed the 100/25 to a 100/50V cap.
The 8417 was almost the last tube designed. Itwas for super-AB1/2 operation, so the grid bias it needs is 1/2 what a 6550 needs. Then when running 600 on the plates, 300 screens and -15 bias into 4200 ohms, 120 watts.
i used to build Killer 240 watt maps on these Plush guitar amp chassis that had 1900 ohm Hammond output transformers with four 8417s. Back when you could buy them.
If anone is interested in buying this beast, let me know.
I don't have any spare 12ax7's, so it comes with the 7247s and two 6550s.
Any other tech help, I can when I have time.
I am in North San Diego, CA 92084, Vista
Here is a link to a photo-album with some of my work on it.
http://photobucket.com/albums/b121/SalesBoy/
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