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Old 18th January 2011, 07:58 PM   #1
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Default AMI "DD" Project: 6L6

I am planning to restore this amp to the best of my ability partly to play LPs, but mostly to learn old-school electrical engineering. So I definitely will be grateful for any knowledge/assistance along the way.

It uses
a 6SN7, a 6SA7, a 6N7GT, and two 6L6Gs; and it has a lot of interesting stuff on the bottom that I cannot possibly grok.

So far, I have found at least one wire that is not in the schematic. I am curious what kind of testing I should do before restoring insulation and crimping the loose metal in the mounts. There are two loose wires coming from the speaker-size transformer (that hang in the breeze), and I cannot imagine what they do as they don't seem to be on the schematic.

I am optimistic about this amp, as it should have a great sound if/when it gets up and running again. I decided to try to make it work after setting up a Sherwood 8000 that I found in the street near my parents' home in NYC. When I was warming up the Sherwood for the first time, I had a Modern Jazz Quartet record on a connected turntable; and when it finally warmed up it was like the long-dissolved quartet was in the room!

  1. I will need to test it first, to make sure that I don't fry it when I power it up
  2. Some of the frayed wires will have to be re-insulated
  3. A volume control will have to be obtained and added

I will need a few tubes, but I am not certain that this necessary to power it up.

After that I will try to grok what the different components do to make the sound, and develop some writing about building valve amps, as they are definitely worth the effort. I will put the writing on the Wikiversity.



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File Type: jpg AMI DD amp frayed wires.JPG (175.7 KB, 254 views)
File Type: jpg ami_dd_bottom.JPG (161.6 KB, 238 views)
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Old 19th January 2011, 12:37 AM   #2
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hello John,

There are a few things I do when first testing out an amplifier which has not been used for some time.

I check the schematic, as you have done and normally put the wiring back to the standard unless I understand what changes have been made and possibly why. Many valve amplifiers have had a number of modifications before we end up with them many decades later.

I always use a variac to slowly increase the voltage into the amplifier, usually over about five minutes. This prevents blowing up things because the capacitors are not operating correctly. By slowly bringing up the voltage electrolytic capacitors are reformed and will normally operate properly as long as they're not damaged.
If you do not have a variac, you can wire a light bulb holder in series with one side of the mains and start off with a 10 W lightbulb in the socket. The light should glow brightly and possibly start dimming a little after a short period of time. Once it is dimmed you can slowly increase the wattage of lightbulbs up to 100 or 200 W. Each lightbulb should slowly dim as the amplifier settles down and then you can go onto the next one. As long as the lightbulbs are not glowing brightly then the amplifier is not drawing too much power and could safely be connected directly to the mains. If by the time you get to somewhere around 50 W, if the light bulb is glowing full brightness then you've got a problem need to figure out why the mains input of the amp is behaving a bit like a short circuit.

I would always fit all of the tubes required into the amplifier because you can't really tell if things are working without everything in place.

Most pieces of electronics over 15 years old, especially if they've been left not operating for some time, require new electrolytic capacitors. I normally replace all of the electrolytics in any old piece of electronics, including semiconductor electronics. Sometimes just doing this sorts out whatever problems and noise that the item may have.

Hope this helps, regards, Marcus.
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Old 19th January 2011, 12:52 AM   #3
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Thanks, that is really helpful.
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Old 19th January 2011, 06:55 PM   #4
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Unfortunately I cannot edit my opening post as I need to correct a typo:

6SN7, a 6SA7, a 6N7GT, two 6L6Gs, and a 5U4G

Also, here is the link to the schematic: CLICK
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Old 19th January 2011, 10:46 PM   #5
SHiFTY is offline SHiFTY  New Zealand
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Hi

Please read the safety thread- there are lethal voltages present. To restore this mono beast is a fairly decent undertaking (although a neat project)

You will need a full complement of tubes for this to work; you will need to replace all the ancient capacitors and all out of spec resistors. All that frayed wiring needs to be insulated! The tubes will probably be ok to re-use. Those 5692 (?) are highly regarded 6SN7 replacements.

As for the "spare" wires on the output transformer, they are likely for different impedance speakers. You will probably need to connect the secondary windings in such a way that you get the correct loading- see if you can dig up any more info on how they should be connected.
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Old 19th January 2011, 11:27 PM   #6
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You will need a phono preamp ahead of this if you want to play LPs from a modern cartridge. This amp was made for hundreds of millivolts of input (and there is no RIAA equalisation either)
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Old 20th January 2011, 07:38 PM   #7
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Thanks, Do you think that this Moviola amp will work as a pre-amp?

(That may be the problem with my Sherwood stereo, I have to crank it for the phono, which brings out all the pops and static--radio is much more tolerable.)
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Old 21st January 2011, 10:56 AM   #8
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You need to review the tube compliment to get that sorted out. Your schematic shows an amplifier using a 5U4 rectifier, two 6L6s for power output, and two 6SN7s for AF amplification and phase inversion functions. You have mentioned that it uses a 6SA7 (a converter tube for AM radios), and a 6N7 (a class B driver or output tube). I don't see either of these tubes on the schematic, or in your pics, and they would be inappropriate for an amplifier of this type design anyway. I'd get this sorted out before going any further!

Dave
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Old 21st January 2011, 05:56 PM   #9
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Thanks,

I cut + pasted that list from another source. The tube types are painted on the deck, though one is obscured by spilled paint, and that can be verified from the schematic.

While I am collecting the pieces, what kind of volume control should I get? I guess I should mention that anything from modern sources would be helpful at this point.

The local library gave me the name of a local senior geek (who was the technology go-to during the 50s-60s). Maybe he can be a source.

I am uploading a "transformed" version of the schematic.

I am getting the impression that y'all generally think this is a worthwhile project. I want to see the tubes glow and hear the nice full sound that is not available from solid-state. But I have seen that restored ancient amps have a lot of value. Thanks everyone for the assistance.

I can't kick myself for not saving all the equipment I have owned; it was others who threw all that stuff out when I didn't have resources for storage (along a lot of good other material, such as a real neat bubble gum collection and endless negatives taken around the nation and Canada during the 70s).
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Old 21st January 2011, 05:59 PM   #10
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AMI DD SCHEM.
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