• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

AMI "DD" Project: 6L6

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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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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.
 
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.
 
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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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
 
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).
 
Determining what kind of effort and resources to put into a piece of equipment starts from accurately defining what it is. This amplifier was designed to be a "PA" type of record player amplifier. That is hardly to diminish what it is, as no doubt it did/does it's intended job quite well. But it is also to say that it was not particularly designed to be a high fidelity piece of equipment either, as that term has been established to mean.

As designed, this unit has very minimal tone control facilities, was designed for use with an economic crystal type pickup, has remote volume control capabilities, and has distortion, frequency response, and power bandwidth capabilities and a damping factor in keeping with PA type equipment.

In looking at it from a modification standpoint, you can look at two basic building blocks of the design: The AF amp/tone control section (1st 6SN7), and the power amp section (2nd 6SN7 and 6L6s).

You can certainly place a simple volume control at the input to control high level inputs into the unit. It's value is best determined by what source will be driving it. Use a 100K control for SS sources, and 500K for VT sources. However, a much better approach would be to bypass the AF amp/tone control section all together, use a good outboard mono preamp, and inject it's signal straight into the power amplifier section of this unit. This would bypass all the existing tone switches and crystal pickup equalization built into the unit now, allow for much more versatile use from a more flexible and capable preamplifier, and allow the unit to basically operate as a mono block power amplifier.

In doing that job, one of the biggest differences (i.e. most noticeable differences) between PA type power amplifiers and high fidelity power amplifiers, is that PA amplifiers offer very little electrical damping to the speaker. Therefore, PA type amplifiers often tend to have a larger bass sound than high fidelity amplifiers do. It may be enjoyable, but it is not accurate. However, used with a more flexible preamplifier, this could be addressed to some degree with the bass and/or loudness controls.

Finally, one of the greatest things about this unit is it's size, design, and construction. There's lots of room to work with, it's a straight forward, well proven circuit format, and is a great piece of equipment for you to learn and cut your teeth on. Basically, the sky's the limit as to how much you might want to modify it, from a simple restoration of the existing unit, to gutting the chassis for an entirely new design.

I offer all of this simply to help you channel your efforts with it. Good luck!

Dave
 
Thanks DCG, I will use this to initiate the engineering section. For the time being, I will be obtaining parts and cultivating the local contact.

I am unapologetic when I say I believe that proper bass response has been critically missing since the migration to solid-state; this I get from my Sherwood and high-end jazz recordings from that last stages of the tube era.

The parts quandary is that these units are all worth restoring, and, hence it makes no sense to "cannibalize" any of them (unless one intends to swap parts between them like race mechanics swap around engines and tires, and such).

Expansion into, say, tube-based reel-to-reel tape recording, will also be difficult as these units are museum pieces--and priced accordingly.
 
What kind of volume control will I need?

I am going to ebay for the new valves, and a volume control (and hence the above question.

I contacted the senior go-to only to find that he ditched his valves with the onset of solid-state, and went straight for computers. He is a retired math teacher, with a PhD, and still has active interested in tech education.

When I told him that I feel that tubes sound better, he immediately confirmed this. His own oscilloscope testing showed that transistors add distortion.

I asked him about the theory of tubes, such as why tubes require much simpler schematics, and his response when right over my head. I guess I should work on the physical side for a while, and then attack the theory.

(I am quietly hoping that this guy can help me form a computer club to look at L4, the most advanced, but ignored, microkernel)
 
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