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Old 21st March 2012, 10:05 PM   #1
rdshear is offline rdshear  United States
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Default Mystery Amp.

I need some help and suggestions on what to do with an amplifier I just purchased.

I'm totally new to this so I'm hoping some of the people who've been around might recognize this and give me some advice.

First the photo's:

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The unit is in a steel box with a detachable speaker above it. The speaker (peering through the back slits) is a utah speaker.

Would the transformers on here be trustworthy? The coil? It appears to have been some sort of portable PA amp but the tubes are all marked JAN which if I have researched correctly means Joint Army Navy -- so is this possibly an old military amp?

Would it be worth rebuilding on this platform?... Just looking for info I guess right now.

Thanks,

Rick
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Old 21st March 2012, 11:20 PM   #2
Cassiel is online now Cassiel  Libya
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What do you want it for, to play guitar maybe? IMO that amp needs a LOT of work. Probably best idea would be to start anew with a new chassis. Dump the caps and resistors and keep the tubes and transformers.
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Old 21st March 2012, 11:31 PM   #3
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Do you own a boat? Are you near a large body of water? Do the winds and water current cause your boat to drift over time? If "yes" to all of the above, you now have a great anchor to fix its location.

Best of luck with your project. I'd probably start by replacing all the capacitors, and in particular the electrolytics.
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Old 21st March 2012, 11:49 PM   #4
Cassiel is online now Cassiel  Libya
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It's a SE amp with lots of preamp gain if that helps your cause.
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Old 21st March 2012, 11:54 PM   #5
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It'll make an excellent guitar amp, like a slightly bigger Champ.

Big fun,
Chris
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Old 22nd March 2012, 12:08 AM   #6
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If it's government issue, it would be pretty likely that there is a part number somewhere on it that could be used to help locate some public domain docs, including a schematic.
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Old 22nd March 2012, 12:15 AM   #7
rdshear is offline rdshear  United States
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Thanks for the info guys, I had figured I'd have to gut it after seeing it. Would the transformers be intact with their age? My thought was for a guitar amp. I haven't seen any schematics around those pre-amp pentodes-- would I be better off going with 12AX7's instead?
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Old 22nd March 2012, 12:22 AM   #8
rmyauck is offline rmyauck  Canada
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Don't plug it without a load (speaker or 12 -16 ohm 25-50W resistor as a load) or you risk the output transformer. Should use a variac and light bulb to bring it up and see if it works, but caps are likely dried up causing shorts. Handy to have after changing parts to make sure everything is okay.

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Best to replace all caps and check resistors as mentioned as you risk the transformers.

Clean tube pins and sockets, along with checking the tightness of tube sockets. Pins on tube sockets can be re-tightened with dental picks.

Pots can be cleaned.

See how it sounds, and it may be good, but small looking Output Transformer may mean it's lacking. Give it chance though! Output transformer is what makes a tube amp sound great if it is a good one. It can be changed as there are many good to great ones out there for sale.

Edcor makes affordable SE output trans that many like for a start.

With the 5881 tube you may get 6-10W depending how it's run.

Beware of high voltages held in caps & transformers after amp is first unplugged. Takes a while to bleed off. You can short caps to drain with 150K 2K resistor & wire.

Keep one hand behind your back when working on high voltages.

Good Luck,

Randy

Last edited by rmyauck; 22nd March 2012 at 12:44 AM.
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Old 22nd March 2012, 12:29 AM   #9
rmyauck is offline rmyauck  Canada
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I wouldn't gut it as it's good to learn on as is. Those old octal input tubes sound better than many of the later 9 pin! Replace one cap at a time so you don't mix a connection up. Take lots of pics as you don't have a schematic. Another bonus is you have the preamp too! You could also make a schematic by tracing wires and drawing it out for reference.

Those transformers should be okay if it wasn't stored in a wet place. It would be rusty if it was. If in doubt put it in the sun for several hours or 100 F oven if possible.

All those tube on it are very available and affordable too!

Just depends what you want to use the amp for and speaker as you need an efficient one.

If you don't need a guitar amp I would try it as is and update to a better output transformer if desired as SE Amps to have their + merits to many.

12AX7 is popular for guitar amps as you probably noticed, because of the distortion characteristics it has.



This might help make it easier!

Speedy Component Replacement

Randy

Last edited by rmyauck; 22nd March 2012 at 12:53 AM.
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Old 22nd March 2012, 01:04 AM   #10
rdshear is offline rdshear  United States
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That link was very interesting. I've messed with electronics for many years, I'm just new to audio and tubes. I've never heard of that method of component replacement. I'm planning on cleaning it up. I like your plan of attack. I'm going to pull the speaker that came with it out of the box and see how it looks. If it looks ok, I'll probably hook it up for testing.
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