New Tube Amp - Suggestions? Tubes?

rayslifecycle
rayslifecycle Posts: 511
edited October 2008 in Electronics
I just got a 15 watt Magnavox 6V6 Tube amp to power my SDA-CRS.
It is going to get some love - but I am new to the Tube world.

What have you found to be reliable tube sources?
Would you recommend new capacitors? I noticed a few underneath.
Power cord and switch recommendations?
Binding posts.......
What about that hum dial - does that stay or should something be done with that.

The goal is to make this a simple clean little amp.
I also need a pre-amp to switch 4 different sources - something real simple.
Should that be tube - or solid state?

Thanks Guys
http://rayslifecycle.blogspot.com/
Post edited by rayslifecycle on
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Comments

  • Face
    Face Posts: 14,340
    edited October 2008
    When was the last time it was used?

    On something that looks that old, the first thing I would do is replace the power supply filter caps and work from there.

    Here's a excellent place to start: http://www.tubesandmore.com/
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
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  • rayslifecycle
    rayslifecycle Posts: 511
    edited October 2008
    Thanks for replying.....

    It was probably last used a long time ago - I am pretty sure it came out of an old radio......thanks for the tip.......is there a way to test indvidual capacitors? I do have a VOA meter.
    http://rayslifecycle.blogspot.com/
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  • Face
    Face Posts: 14,340
    edited October 2008
    You need either a LCR meter or a high quality VOA meter that can measure capacitance. I picked up a VOA meter from Sears for around $60 that does a good job measuring capacitance. Amazon.com has a great selection of VOA and LCR meters, be sure to check the reviews though, there's plenty of junk out there.
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
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  • mhardy6647
    mhardy6647 Posts: 35,228
    edited October 2008
    There is a "Magnavoxfriends" group on Yahoo that is well worth joining.
    That little 6V6 push-pull stereo amp (of exceptionally modest construction) sounds far better than it looks like it should... or than it has a right to! It's about 10 wpc.

    This thread from another fourm may be of interest.
    http://www.audiokarma.org/forums/showthread.php?t=103122&highlight=magnavox
    You need a couple of preamp tubes. I think that one uses 6EU7s (but I could be wrong).

    Checking capacitance per se unfortunately won't tell you too much. An ESR (equivalent series resistance) meter is more useful. You can get a simple but inexpensive one from MAT Electronics (www.matelectronics.com ). Nothing wrong with having an LCR meter, too, of course! :-)

    The four coupling caps to the output tube input grids should be replaced if they haven't been. Note that there are probably two pairs of dissimilar value caps (assuming standard "Maggotbox" practice of using a "paraphase" splitter topology).

    The power supply (multisection) electrolytic "can" cap may or may not be OK. If the amp is known to be in running condition, is hum-free, and the can cap stays cold to the touch, it probably has some good life left in it. Otherwise, it too will need to be replaced. One can buy more or less exact replacement multisection can caps from www.tubesandmore.com (they're not cheap) or use single-value electrolytics from any supplier you like (which are cheap) and get clever with layout... plenty of space under the deck of that amp! www.mouser.com or www.digikey.com are popular (although their catalogs are so loaded with parts that it can be hard to find even simple things).

    EDIT: Get any binding posts you like. www.partsexpress.com is a good source.

    That amp may need to have 'balance' resistors installed if it had a connection to a balance pot on the preamp (check the schematic in the above-referenced thread).

    The 'hum' pot will null residual AC hum from the filaments. Yes, you should keep it in circuit, and adjust it (carefully) for minimal hum.

    Remember that the amp uses lethal AC and DC voltages and be very careful (especially below-decks)! Also, the output tubes will get very hot in use.

    Always operate the amp with a load (speaker or dummy load resistor) connected to each output channel.

    EDIT^2: IF the amp has NOT been run for a long time (let's say a year or more), it should either be "brought up on a Variac" to reform the P/S capacitor(s) or (as suggested earlier) simply replace the P/S cap before starting. That amp has a tube rectifier (5U4, IIRC). Ooops, judging from the photo the rectifier is a 6X4 or maybe a 6CA4...
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  • reeltrouble1
    reeltrouble1 Posts: 9,312
    edited October 2008
    Tubes Rule.

    Enjoy your project.

    RT1
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  • organ
    organ Posts: 4,969
    edited October 2008
    Great find. Looks like it started life as a console amp. You have gotton great adivce so far regarding the caps. If possible, you should replace all the caps just to be safe. Internal wiring should be looked at as well.

    The hum pot can be very useful if you're using very sensitive speakers. There are still amps made today that still include hum pots.

    There are a lot of Magnavox tube console amp collectors over at audiokarma.org
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  • rayslifecycle
    rayslifecycle Posts: 511
    edited October 2008
    Thanks all for your help and encouragement.
    I am so happy to have found such a helpful community of audio enthusiasts.
    ......It is such an exciting little project.........and what a great hobby this is turning into........I'll keep y'all posted

    ray
    http://rayslifecycle.blogspot.com/
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  • mhardy6647
    mhardy6647 Posts: 35,228
    edited October 2008
    just don't electrocute yourself, 'K?
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  • rayslifecycle
    rayslifecycle Posts: 511
    edited October 2008
    ___k
    http://rayslifecycle.blogspot.com/
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  • mhardy6647
    mhardy6647 Posts: 35,228
    edited October 2008
    excellent; I'm glad we had this little chat.

    ;-)
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  • Face
    Face Posts: 14,340
    edited October 2008
    Tubesandmore.com is having a capacitor sale, perfect timing for you.
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
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  • mhardy6647
    mhardy6647 Posts: 35,228
    edited October 2008
    a capacitor sale!

    whoo-hoo!!

    ;-)
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  • rayslifecycle
    rayslifecycle Posts: 511
    edited October 2008
    :eek::D;)

    :cool:
    http://rayslifecycle.blogspot.com/
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  • rayslifecycle
    rayslifecycle Posts: 511
    edited October 2008
    I Just got onto the Magnavox Friends Group at yahoo and found the specs, wiring diagram, and all kinds of fun info. There were about 5 or 6 of these same units with some aesthetic and functional mods.........but I thought none of their solutions for locating binding posts, on/off switches, or inputs were outstanding.......and only one had a perforated metal cover - Has anyone had any problems with this type of amp having all its parts exposed - too me, it would make sense to enclose the top and the bottom to prevent electrocution and protect the glass tubes.

    There was also a lot of chat over the past couple of days regarding the 6EU7 tube being out of production....that sounds like trouble.......
    http://rayslifecycle.blogspot.com/
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  • rayslifecycle
    rayslifecycle Posts: 511
    edited October 2008
    some picks of "finished" MAG 88 Amps
    http://rayslifecycle.blogspot.com/
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  • organ
    organ Posts: 4,969
    edited October 2008
    Some people like to have their tubes exposed for the cool factor.
    Other than the tubes, everything should be in the case because when the circuits are exposed, they can easily pick up RFI and EMI which results in constant noise from the speakers(and for safety, of course). Some amps however run perfectly fine with no chassis.

    If the 6EU7 are out of production, head over to ebay and buy some before prices go up.

    666,
    Nice pics of the amp. Thanks for sharing. Makes me want to start doing some serous work on my vintage amps.
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  • Face
    Face Posts: 14,340
    edited October 2008
    Besides getting burned by hot tubes, what else is exposed that could be dangerous?
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
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  • zingo
    zingo Posts: 11,258
    edited October 2008
    On his original picture, nothing really. Just keep your hands away from the circuits when it's running and away from the caps even when it's not.
    Jake
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  • mhardy6647
    mhardy6647 Posts: 35,228
    edited October 2008
    clarknova_666 wrote: »
    ...and only one had a perforated metal cover - Has anyone had any problems with this type of amp having all its parts exposed - too me, it would make sense to enclose the top and the bottom to prevent electrocution and protect the glass tubes.

    There was also a lot of chat over the past couple of days regarding the 6EU7 tube being out of production....that sounds like trouble.......

    I take absolutely no legal responsibility for this... but the only real hazard on the topside of the amp is (are) the hot output tubes. There should be no high voltage accessible above-deck at all. It's there (inside caps and tubes, and the transformer), of course, but if all is as it should be, you'd have to really work at it to shock yourself on the topside of the chassis.
    Of course, lots of 'fault' scenarios could put dangerous or lethal voltage on the chassis or elsewhere, so don't come running to me if you kill yourself! :-)

    That said, if you have children or pets, you'll have to think hard about safe ways to use the amp. Secondary containment is a good idea.

    Dunno if new-production 6EU7s are around or not. It is awfully similar to the "12 volt" twin triodes (12AX7 and its kin). The 12AX7 is the "high gain" member of the family; I think that the 6EU7 is virtually identical except for the 6V only filament and the pinout. Here's a little info thereupon.

    http://tdsl.duncanamps.com/show.php?des=6EU7
    http://gabevee.tripod.com/pptube.html
    http://music-electronics-forum.com/showthread.php?t=7078

    PartsExpress lists the Sovtek 6EU7 albeit at a stupid high price:
    http://www.parts-express.com/pe/pshowdetl.cfm?Partnumber=072-375&FTR=6EU7&CFID=26719852&CFTOKEN=90391265

    It should be readily available elsewhere. I think the paper PE catalog lists NOS 6EU7s, too (I could be wrong about that).

    I think NOS 6EU7s should still be floating around. Try the usual sources, including Dick Bergeron (Electron Tube Enterprises, http://members.aol.com/etetubes/ )

    HTH (as they say...)

    EDIT: I think I may have some used 6EU7s. If you get stuck, PM me and I'll check the basement.
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