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Thread: Speaking of neato little tube amps...

  1. #1
    Senior Member GordonW's Avatar
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    Speaking of neato little tube amps...

    Found an old Magnavox console, that had one of the AMP185, 6V6 PP tube amps in it, circa 1960. It should be about 15w/ch...

    There was very little wrong with it- in fact, the cathode resistors on all the 12AX7 input tubes, was the ONLY functional problem, preventing operation. Replaced those, along with the ceramic caps in the input stage, and WOW. I like it.

    It's a little "extra full" in the absolute lowest bass registers (I would imagine it kinda just "ramps up" a bit, starting at about 45 Hz going downward toward 20 Hz, where it's probably 2 dB louder at 20 Hz than 45 Hz), but BOY... does it EVER sound NATURAL on midbass transients. There's a tactile nature to pianos, plucked strings, even clarinets and flutes, that just sounds LIKE THE REAL THING. I'm amazed... this was NOT an exotic amp, in the first place... it just happened to be very elegantly and well designed, apparently.

    After getting it "souped up" and working, I "prettied it up" as well. Poplar wood planks on the sides and front of the metal chassis, with some planks on top to accomodate a power switch and output binding posts, with the wood stained cherry color. It's kinda cool looking now!
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    Senior Member GordonW's Avatar
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    I can't over-stress exactly HOW GOOD the subjective tonal balance and dynamic behaviour is on this amp. It just sounds natural... the same way a good Audio Research SP6/SP8 and D70/D75/D79 does. Smooth, but fast; full, but detailed. I'm amazed that this caliber of circuit spent most of its life buried in a console!!

    Another view:
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    Senior Member GordonW's Avatar
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    ... And then later, the next week, I found its little brother in another console. EL84 SE circuit, 3.5 watts per channel. Isn't it cute?
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    Dang. Amateur speakerdave's Avatar
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    Yep! those Magnavox consoles. When I was a kid I always looked for them when visiting relatives. My Great Aunt Edna in Rutland, Vermont--she had one!

  5. #5
    jtgyn
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    G'Day Gordon,
    What is you process for cleaning up the old chassis? They do look very clean.

    The power capacitors where O.K. too... some guys have all the luck.

    Regards Scott

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    Senior Member GordonW's Avatar
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    The majority of the cleanup, believe it or not, was with a toothbrush and Windex. These amps were inside consoles, so they usually don't get spilled on or anything... just lots of dust. Windex makes short work of that!

    Otherwise, I used a little buffing from a wire wheel brush on a Dremel, on a few small places. On the transformers of the AMP185, they were nasty looking, so I buffed them heavily with the Dremel wire brush, then treated them with aluminum black/gun-metal graying compound. Turned the transformer housings to a neat looking dark-gray... kind of the color of cast iron. A lot better looking, and kinda "period color" too...

    Regards,
    Gordon.

  7. #7
    Regis
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    Console tube amps, just how do you do it?

    Gordon, I am intriqued by your project. Allow me to ask some simple questions from someone who doesn't know a lot about tube amps (esp. those rescued from consoles, but would like to know more).

    1. How does a console amp work with the inputs and outputs?
    2. Does it take the input from any regular pre-amp or does there have to be a voltage/amperage/impedance match?

    3. Speaker output, what about impedance?

    4. While I know Tube amps are much less wattage, everybody seems to rave about how powerful they sound. How is this so?

    5. What kind of speakers are you looking to drive off've a tube amp? Pretty efficient ones, like most of the sealed cabinet designs, or will vintage large JBL's work?

    Thanks

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    Super Moderator Hofmannhp's Avatar
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    Speaking of heavy old tube amps...

    Originally posted by GordonW
    Found an old Magnavox console, that had one of the AMP185, 6V6 PP tube amps in it, circa 1960. It should be about 15w/ch...
    Hi Gordon,

    seeing your pics, I remembered two of my old studio amps from Telefunken type V69a, from early '60s.
    They weight about 20kg/40pds, output is 25W to 4,8 and 16Ohms, earthfree symmetrical input, tubes are F2A11 (very expensive today).
    Now they are a little dusty and need some work for a goodlookin.
    The sound is very fine.....like a good tube amp.
    With this 2 x 25W I fed my DIY 4333b for some years and it seems to be 100W.

    HP
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    Please help us save more info about the vintage systems. Let us register your speakers and drivers.

  9. #9
    Senior Member GordonW's Avatar
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    Re: Console tube amps, just how do you do it?

    Originally posted by Regis
    Gordon, I am intriqued by your project. Allow me to ask some simple questions from someone who doesn't know a lot about tube amps (esp. those rescued from consoles, but would like to know more).

    1. How does a console amp work with the inputs and outputs?
    2. Does it take the input from any regular pre-amp or does there have to be a voltage/amperage/impedance match?
    The Magnavox console amps had RCA inputs, and originally had a Molex plug for speaker output wiring. I rewired the outputs, to go to binding posts. On the larger of the two amps, I also installed some nice Audio Research-style RCA input jacks, to use in place of the original jacks (which, once I bypassed the rudimentary phono stage, were in the wrong place to get the signal into the 12AX7s... and there were actually pre-drilled holes in the chassis for another pair of jacks where they are, so I used them).


    3. Speaker output, what about impedance?
    I wouldn't recommend running either amp below 8 ohms, as a matter of general policy. The original speakers were 16 ohms, but there was a provision, to run another pair of 16 ohm speakers. 8 or 16 ohms speakers are the rule of the day, at least on the smaller amp. The 6V6 amp doesn't seem to mind lower impedence (i've run it on 6 ohm loads, no problem), but as can be expected, the little SET amp... homey don't play dat. The small SET amp will definitely "choke up" in the bass, when run into low impedence.


    4. While I know Tube amps are much less wattage, everybody seems to rave about how powerful they sound. How is this so?
    5. What kind of speakers are you looking to drive off've a tube amp? Pretty efficient ones, like most of the sealed cabinet designs, or will vintage large JBL's work?

    Thanks
    Tubes can be run much harder (as in, into the beginning of clipping) because the majority of the distortion products in tubes, are in the lower harmonics, which are much less "grating" to the ear, psycho-acoustically. Basically, it means you can "cheat" a bit on dynamic transients, and the amp isn't going to sound like fingers on a blackboard. Also, lower-order harmonics are less likely to overload tweeters and blow them, compared to the distortion from SS amps, for the most part. Yes, you can still blow drivers, it's just that the amp is more "forgiving"...

    As for speakers- if you find one of the push-pull amps (like the 6V6 AMP185 ), most any decently-efficient speaker -anything 90 dB or higher- should be able to work, to decent effect. I have enjoyed listening to a pair of Tannoy speakers, about 90 dB sensitivity and just over 6 ohm load, on the AMP185... it sounds fantastic, and has good dynamics. No, it's not going to be stadium-loud, but it's loud enough for 99.9 percent of what I would listen to, on a daily basis.

    For the smaller SET amps (the 8601 and its ilk), you definitely want a "vintage-esque", highly efficient speaker, with a relatively high impedence. However, stuff like JBL Barons, C50s, Viscounts, Altec VOT models, Klipschorns, Belles, and the like, should work fine on the SET amp, I'd think... many of those, have sensitivity ratings in excess of 95 dB, some in excess of 100 dB. Even with only 4 watts per channel a 100 dB speaker is going to top out over 106 dB... that's pretty darn loud!!

    Regards,
    Gordon.

  10. #10
    Regis
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    Thanks!

    I sincerely appreciate the time for the well-thought out answers to my questions. I bid on a ready-to-go vintage tube amp for under $50 on Ebay that uses 4 6BM8 tubes for the output and has all the jacks. For the money, I can at least get my feet wet and see what 7 wpc sounds like!

    The tube asylum on Audio Asylum is a fascinating source of info as well (though I can only get a gleaning of what I needed to know from there). I thought we were fanatics on this site, those tube guys arguing over Pentode and Triode modes are something else!

    Muchos Gracias!

    Regis

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