Bogen PA--->Guitar Amp, few questions - Page 2

Turbo7MN · 9th February 2006, 03:39 AM

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...tem=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

SHiFTY · 9th February 2006, 11:49 AM

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.

Turbo7MN · 9th February 2006, 04:18 PM

Okay, back in business! Check out what I just scored!

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=5864748037

And so the fun begins. Anyone have the schematic for the M60A? Thanks.

DigitalJunkie · 9th February 2006, 04:35 PM

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>

Tom Bavis · 9th February 2006, 06:26 PM

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

Turbo7MN · 9th February 2006, 10:08 PM

Do you guys think one of these would be a worthwhile investment?

http://www.tech-wiz.org/store/view_p...1bba1896099290

Enzo · 10th February 2006, 04:58 AM

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

Turbo7MN · 10th February 2006, 04:39 PM

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

Dave Cigna · 10th February 2006, 06:16 PM

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.

Turbo7MN · 10th February 2006, 10:08 PM

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

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9th February 2006, 03:39 AM	#11
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	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...tem=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

9th February 2006, 11:49 AM	#12
SHiFTY diyAudio Member Join Date: Jul 2002 Location: New Zealand	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.

9th February 2006, 04:18 PM	#13
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	Okay, back in business! Check out what I just scored! http://cgi.ebay.com/ws/eBayISAPI.dll...tem=5864748037 And so the fun begins. Anyone have the schematic for the M60A? Thanks.

9th February 2006, 04:35 PM	#14
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 3	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>

9th February 2006, 06:26 PM	#15
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	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

9th February 2006, 10:08 PM	#16
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	Do you guys think one of these would be a worthwhile investment? http://www.tech-wiz.org/store/view_p...1bba1896099290

10th February 2006, 04:58 AM	#17
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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

10th February 2006, 04:39 PM	#18
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	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

10th February 2006, 06:16 PM	#19
Dave Cigna diyAudio Member Join Date: Oct 2003 Location: Finger Lakes, NY	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.

10th February 2006, 10:08 PM	#20
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	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

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