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Old 8th February 2006, 06:58 AM   #1
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Default Bogen PA--->Guitar Amp, few questions

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

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
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Old 8th February 2006, 09:04 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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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!
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Old 8th February 2006, 10:08 AM   #3
sreten is offline sreten  United Kingdom
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Default Re: Bogen PA--->Guitar Amp, few questions

Quote:
Originally posted by Turbo7MN
IHow 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?

Quote:
Originally posted by SHiFTY
Mic inputs work well as guitar inputs, they are about the same level.
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.
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Old 8th February 2006, 01:22 PM   #4
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Email for a scan of the Sams Photofact folder. You shouldn't open it up without a schematic... tbavis(at)rochester(dot)rr(dot)com
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Old 8th February 2006, 03:01 PM   #5
rickl is offline rickl  United States
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Default Re: Bogen PA--->Guitar Amp, few questions

Quote:
Originally posted by Turbo7MN
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?
If possible, determine the plate and cathode resistors for the driver tubes and someone may be able to help. My HK PA amp uses both 12au7 and 12ax7 drivers. the au7 for line level and 12ax7 for mic level.

rick
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Old 8th February 2006, 03:58 PM   #6
anatech is offline anatech  Canada
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Hi Turbo7MN,
Yes, get a schematic first as Tom Bavis kindly offered. Most Bogen amps I've seen use 12AX7's for input tubes. Do check the tube wiring first.

-Chris
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Old 8th February 2006, 10:07 PM   #7
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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.
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Old 8th February 2006, 10:27 PM   #8
anatech is offline anatech  Canada
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Hi Turbo7MN,
First, get the schematic and see what you have.

-Chris
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Old 9th February 2006, 12:32 AM   #9
Enzo is offline Enzo  United States
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On some of them the input was lowZ only as an option, and that option was a can transformer that plugged into a socket. Otherwise jumper wires. Do find out what you have first. If yours has inout transformers, simply go around them for hi Z inputs.
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Old 9th February 2006, 01:22 AM   #10
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How could I miss that...

The amp has HiZ/LowZ select switches next to each input tube. So will that take care of the problem?
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