I'm probably going to be trying to convert a Bogen MX30A into a somewhat nice guitar amp. I've been searching desperately online for some info or a schematic on this amp but turned up nothing except for a single thread on this forum:
BOGEN MX60A Schematic - diyAudio
Anyway, I'm usually pretty good with electronics but I've never worked with vintage tube amps. I have a few questions regarding this project:
1. The amp is missing the preamp tubes, and the type of tube is not specified. It looks like a standard 9-pin connector, would trying a set of 12AX7s be a good idea? Does anyone here know what kind of tubes the amp used originally?
2. What about "cascading" the inputs? That just means the output of one tube goes straight into another, right? Or does it go to one of the "mic" knobs between each tube?
3. How exactly do you insert the signal from a 1/4" jack? The mic inputs are a strange 3-pin design that I've never seen before. Which wires would I splice?
Sorry, mostly n00b questions but like I said, I'm not familiar with tube amps. If anyone has step-by-step instructions, feel free to post them
. I'm sure I could figure out a bit more myself but I don't have the amp yet (all I've seen are pictures) so I can't play with it, right now I'm just trying to get a good plan of what to do when I get it. Thanks.
-Darren
BOGEN MX60A Schematic - diyAudio
Anyway, I'm usually pretty good with electronics but I've never worked with vintage tube amps. I have a few questions regarding this project:
1. The amp is missing the preamp tubes, and the type of tube is not specified. It looks like a standard 9-pin connector, would trying a set of 12AX7s be a good idea? Does anyone here know what kind of tubes the amp used originally?
2. What about "cascading" the inputs? That just means the output of one tube goes straight into another, right? Or does it go to one of the "mic" knobs between each tube?
3. How exactly do you insert the signal from a 1/4" jack? The mic inputs are a strange 3-pin design that I've never seen before. Which wires would I splice?
Sorry, mostly n00b questions but like I said, I'm not familiar with tube amps. If anyone has step-by-step instructions, feel free to post them
. I'm sure I could figure out a bit more myself but I don't have the amp yet (all I've seen are pictures) so I can't play with it, right now I'm just trying to get a good plan of what to do when I get it. Thanks.-Darren
Mic inputs work well as guitar inputs, they are about the same level. Since its a mixer, just turn down all the other inputs, no casacading necessary.
You need to find out what tubes they were, if you just assume 12AX7 you might get fireworks. (from google groups:
2x7868 12DW7 12AX7.) It is usually written on the chassis somewhere though, so look for that. You can probably bodge something together with help from people here. You also only need to get one or two inputs running so you can probably just omit a few of the preamp tubes from the unused channels.
You might want to eventually strip the thing down and build a proven circuit like a Fender of some kind... Thats what I have done in the past with old PA amps.
And if you are familiar with electronics, tube circuits are about as simple as it gets, just mind the high voltage. And change the old capacitors to the output tubes, the signal ones are always leaky and fry the output tubes, and the power supply ones sometimes swell up and get hot. Tubes themselves last for a loooong time.
Good luck and post some pics!
You need to find out what tubes they were, if you just assume 12AX7 you might get fireworks. (from google groups:
2x7868 12DW7 12AX7.) It is usually written on the chassis somewhere though, so look for that. You can probably bodge something together with help from people here. You also only need to get one or two inputs running so you can probably just omit a few of the preamp tubes from the unused channels.
You might want to eventually strip the thing down and build a proven circuit like a Fender of some kind... Thats what I have done in the past with old PA amps.
And if you are familiar with electronics, tube circuits are about as simple as it gets, just mind the high voltage. And change the old capacitors to the output tubes, the signal ones are always leaky and fry the output tubes, and the power supply ones sometimes swell up and get hot. Tubes themselves last for a loooong time.
Good luck and post some pics!
Re: Bogen PA--->Guitar Amp, few questions
Hi,
not in this case, the mic inputs are I presume XLR, 600 ohm, entirely unsuitable for guitar.
(high impedance mic inputs, usually 1/4" jacks, do work with guitars,
though the impedance (~50K) is somewhat lower than a guitar input.)
/sreten.
edit : if the 3 pin inputs go to what looks like transformers your
back in business, as the input for the output of the transformer
(which should be only two connections) is your new input.
Turbo7MN said:
SHiFTY said:
Hi,
not in this case, the mic inputs are I presume XLR, 600 ohm, entirely unsuitable for guitar.
(high impedance mic inputs, usually 1/4" jacks, do work with guitars,
though the impedance (~50K) is somewhat lower than a guitar input.)
/sreten.edit : if the 3 pin inputs go to what looks like transformers your
back in business, as the input for the output of the transformer
(which should be only two connections) is your new input.
Thanks for the help. I emailed Tom for the schematic.
So about the impendance of the input: would it be possible to insert resistors after the new 1/4" jack to get the impendance up to the 50k range?
I asked about cascading because in the thread that I linked to, that is what the guy said he did. It's a shame he's not a regular on here so I could ask him specifiaclly what he did.
So about the impendance of the input: would it be possible to insert resistors after the new 1/4" jack to get the impendance up to the 50k range?
I asked about cascading because in the thread that I linked to, that is what the guy said he did. It's a shame he's not a regular on here so I could ask him specifiaclly what he did.
Well, scratch that. I fully intended on winning the ebay auctuion but I was about to put in what I thought would be a high enough last-minute bid when it got higher than I planned.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5861981544
Now that I'm looking at the schematic (which is for the MX30, not the MX30A, but it should be close enough), I see that all the tubes actually appear to be there. I figured the empty 9-pin sockets right next to the inputs were tube sockets, perhaps I was wrong? The only verification I got for missing tubes was from a guy on a car forum.
Maybe I should have posted the ebay link earlier. Ah well, too late now. I wish I would have checked emails before I logged on to ebay tonight, knowing all the tubes were there would have compelled me to pay a fair bit more for it. I guess I'm back in the hunt. Thanks.
By the way, how long does it take for your posts to stop being "moderated" when you're a new member?
-Darren
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5861981544
Now that I'm looking at the schematic (which is for the MX30, not the MX30A, but it should be close enough), I see that all the tubes actually appear to be there. I figured the empty 9-pin sockets right next to the inputs were tube sockets, perhaps I was wrong? The only verification I got for missing tubes was from a guy on a car forum.
Maybe I should have posted the ebay link earlier. Ah well, too late now. I wish I would have checked emails before I logged on to ebay tonight, knowing all the tubes were there would have compelled me to pay a fair bit more for it. I guess I'm back in the hunt. Thanks.
By the way, how long does it take for your posts to stop being "moderated" when you're a new member?
-Darren
Don't worry, it probably wasn't the best candidate for a guitar amp conversion anyway. Look out for a more standard one, maybe a 6L6 push-pull or similar with 4-8-16 ohm outputs. I once got two old PA amps, with tubes, for about $20.
Maybe have a look around old junk stores in your neighbourhood, or ask some old ham radio guys.
Maybe have a look around old junk stores in your neighbourhood, or ask some old ham radio guys.
Okay, back in business! Check out what I just scored!
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5864748037
And so the fun begins. Anyone have the schematic for the M60A? Thanks.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5864748037
And so the fun begins. Anyone have the schematic for the M60A? Thanks.
The sockets next to the inputs were probably for input transformers.
On a similar note,
I have a "gutted" chassis+output trans. from a Bogen M120 that I was thinking of building a "bass" amp with (meaning bass guitar,and possibly subwoofer duty also.)
Does anyone have the schematic of the M120? There's some interesting windings on the OPT,and I havn't figured out what they're for yet..(seperate feedback winding,and a 70V output,perhaps?)
I found the plate and speaker leads,and I've crudely figured the impedance to be approx 3.5K. (can anyone confirm this?)
</thread jacking>
On a similar note,
I have a "gutted" chassis+output trans. from a Bogen M120 that I was thinking of building a "bass" amp with (meaning bass guitar,and possibly subwoofer duty also.)
Does anyone have the schematic of the M120? There's some interesting windings on the OPT,and I havn't figured out what they're for yet..(seperate feedback winding,and a 70V output,perhaps?)
I found the plate and speaker leads,and I've crudely figured the impedance to be approx 3.5K. (can anyone confirm this?)
</thread jacking>
M120 is not listed in Sams index. But Bogen is still in business - I have heard that they sometimes will provide info on their older products...
M60A is not in Sams either. But..
M60 is in 488-5
MX60 is 482-6
MX60A is in 891-Servicer (probably schematic only)
Your library may be able to get these, or try Walt at infotronix.us or Bob at findatube.com
M60A is not in Sams either. But..
M60 is in 488-5
MX60 is 482-6
MX60A is in 891-Servicer (probably schematic only)
Your library may be able to get these, or try Walt at infotronix.us or Bob at findatube.com
Do you guys think one of these would be a worthwhile investment?
http://www.tech-wiz.org/store/view_...0A&PHPSESSID=e7e71b847cdfe9bce31bba1896099290
http://www.tech-wiz.org/store/view_...0A&PHPSESSID=e7e71b847cdfe9bce31bba1896099290
That seems like for a schematic to me.
Go over to Ampage and ask in the guitar amp forum if someone has that schematic. They often discuss rebuilding Bogens into guitar amps, it is a popular project. It is a forum devoted to guitar amp building and tweaking.
Schematic Heaven would be worth checking but they seem to be down lately.
http://www.ampage.org
Go over to Ampage and ask in the guitar amp forum if someone has that schematic. They often discuss rebuilding Bogens into guitar amps, it is a popular project. It is a forum devoted to guitar amp building and tweaking.
Schematic Heaven would be worth checking but they seem to be down lately.
http://www.ampage.org
oops.
It appears that the M60A uses 8417 power tubes, which haven't been in production since 1988. Looks like I have to pray the tubes in there still have a lot of life left.
I searched around for other tubes to convert it to, and it looks like the 6550 is the ideal option. Does anyone have any suggestions on how to do this? Thanks.
-Darren
It appears that the M60A uses 8417 power tubes, which haven't been in production since 1988. Looks like I have to pray the tubes in there still have a lot of life left.
I searched around for other tubes to convert it to, and it looks like the 6550 is the ideal option. Does anyone have any suggestions on how to do this? Thanks.
-Darren
I once had a MO-100A (quad 8417) on the bench. For giggles I measured the primary impedance of the output transformer and found it to be about 4.5k. If the MO-60A is similar, but with just a pair of 8417 then I would expect the primary Z to be about 9k.
9k is pretty high if you wanted to try other output tubes. The good news is that guitars only go down to about 80Hz, so you can easily get away with putting an 8 ohm speaker on the 4 ohm tap to get a 4.5k load. That's not too far from where most 40W or 50W amps run class AB pentodes (ie 6L6/EL34/KT66.) 6550 and KT88 ought to be pretty happy there too.
On the other hand, you might really like a 9k load. I understand that the early 50W marshalls used something like 7k or 8k on a pair of KT66/5881/EL34 and nobody ever complained about the sound of those amps!
-- Dave
edit: errrp, I should have said put an 8 ohm speaker on the 16 ohm tap.
9k is pretty high if you wanted to try other output tubes. The good news is that guitars only go down to about 80Hz, so you can easily get away with putting an 8 ohm speaker on the 4 ohm tap to get a 4.5k load. That's not too far from where most 40W or 50W amps run class AB pentodes (ie 6L6/EL34/KT66.) 6550 and KT88 ought to be pretty happy there too.
On the other hand, you might really like a 9k load. I understand that the early 50W marshalls used something like 7k or 8k on a pair of KT66/5881/EL34 and nobody ever complained about the sound of those amps!
-- Dave
edit: errrp, I should have said put an 8 ohm speaker on the 16 ohm tap.
I'm probably getting ahead of myself worrying about the tubes, but when you buy something untested off of ebay, you always have to assume the worst .
Anyway, do output tubes have to be the same brand? I found a few 8417s on ebay, and after an hour of searching, that's the best I'm probably going to get. I could't find any tube pairs, only single NOS tubes and they are all different brands. I'm just wondering if brand mattered, because it might not be a bad idea to snag one or two as backups anyway.
A guy on Ampage said that the schematic of the M60A is almost identical to that of the CHB-50. Can anyone give me a schematic of that one? Thanks.
-Darren
.Anyway, do output tubes have to be the same brand? I found a few 8417s on ebay, and after an hour of searching, that's the best I'm probably going to get. I could't find any tube pairs, only single NOS tubes and they are all different brands. I'm just wondering if brand mattered, because it might not be a bad idea to snag one or two as backups anyway.
A guy on Ampage said that the schematic of the M60A is almost identical to that of the CHB-50. Can anyone give me a schematic of that one? Thanks.
-Darren
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