Monoprice 50w Hybrid Amp

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  • msg
    msg Posts: 6,402
    Sounds like a neat project, and welcome to the forum!
    I disabled signatures.
  • mhardy6647
    mhardy6647 Posts: 27,257
    edited April 2020
    We've been around the block with these a couple of times. :)

    the 25-watt morph
    https://forum.polkaudio.com/discussion/181379/karma-monoprice-fabulousness/p1

    Now, I'm too lazy to go upstairs and look, and it's risky to trust my memory :p -- but isn't the "power tube" in the 25 watt version a 6p15p? These were once sold in the US as Svetlana SV-83. They're kinda sorta EL84-esque, but not identical. I think the heater current might be higher(?) than an EL84.

    https://frank.pocnet.net/sheets/164/s/SV83spec.pdf

    The 50 watt morph has similar funny business going on with 6p15 pentodes :)
    https://forum.polkaudio.com/discussion/182501/monoprice-50w-hybrid-amp/p3

  • delkal
    delkal Posts: 755
    edited July 2020
    It is amazing they doubled the power, added a DAC and more inputs and they are still able to sell it for $160. That is the sale price I paid for my Monoprice 50 hybrid. I know the original 25 watt hybrid got a bad rep here (and flames) but I have been happy with my 50 hybrid for what it is. It is simply the best sounding amp I listened to for under $200. The tubes of the 50 hybrid are are wired in the audio path correctly and they do give a tubey sound (that is better with the Russian tubes). It would have been a game changing amp getting rave reviews 30 years ago.

    If this new Dayton model maintains the sound and build quality of the Monoprice 50 hybrid (and you get lucky and don't get the occasional dog) it should be a popular model.
  • mhardy6647
    mhardy6647 Posts: 27,257
    Based on the ad copy: the Dayton is still 50 wpc -- the '100' appears to be a stereo "rating".

    "wired correctly", eh? B)
  • delkal
    delkal Posts: 755
    edited August 2020
    mhardy6647 wrote: »
    "wired correctly", eh? B)

    There is a lot of confusion on the net comparing and mixing up the Monoprice 25 and 50 hybrids and many of the comments even reference totally different amps that just look like the Monoprice amps. It is clear that there are some Chinese models out there where the tubes are just for show but I can say with certainty the Monoprice 50 hybrids use the tubes in the audio path. The clearest proof is that I had some 6P1P-EV tubes that were microharmonic. You tap on the tube and you hear it in the speakers. Case closed.
    mhardy6647 wrote: »
    xneayjmyjo0m.png

    So -- grid one of the 6p15 (tube on the left in my flipped, annotated picture above) is connected to pin 2 according to RCA. It doesn't look like pin 2 is connected to anything on that PCB(?!) -- again, I am assuming it's a single layer board (which may not be true). Pin 3 (which is "grid 3", the suppressor grid, which is also tied to the tube's cathode, does look like it's hooked to something, though. It looks like it connects to pin 8, which is an "internal connection in the tube.

    I am also perplexed by that ring on the PCB that seems to connect most of the pins (pins 1, 2, 6, 8 and 9) of that 6p15 together (?!?)

    OK, my brain hurts, now, too.

    I also looked in my 50 hybrid and I was also wondering about them wiring so many pins together. This was an excellent analysis and it helped me understand how things are wired but unfortunately you looked up the datasheet for the wrong tube. The monoprice 50 uses 6P1P-EV and not 6P15 tubes.

    In the 6P1P-EV tube some of the pins are equivalent and connected internally and it has 2 grids. Pins 1 and 6 are the plate, 2 and 9 go to one grid, 3 and 8 are the cathode, and 4 ,5 is the heater. So Monoprice connected the the plate with one of the grids (1,6,2,9). They also connected the cathode pins even though they are equivalent (3,8). The audio signal goes into pin 7 and uses one of the grids.

    kbmk9iaxvziv.jpg

    russiantubes.com/prop.php?t=12&p=238

  • mhardy6647
    mhardy6647 Posts: 27,257
    edited August 2020
    It is still a power output tube (similar to a 6AQ5) -- why the heck would one use a power tube in a preamp?

    It is also possible that the filament as resistor hypothesis :) could explain microphonics as described in @delkal's post. Heck, a regular old power resistor can be microphonic when tapped on with a stick.

    Sorry I had the wrong tube type -- I don't have a '50 watter', so I was going by what I could find on the webz as to what the power pentode was in that morph. I had a 50/50 chance of getting it right. :/

  • delkal
    delkal Posts: 755
    edited August 2020
    mhardy6647 wrote: »
    It is still a power output tube (similar to a 6AQ5) -- why the heck would one use a power tube in a preamp?

    I have no clue why they used a power tube but I doubt it is acting as one. I am pretty sure they don't use a plate voltage even close to 250V. Maybe because they are $2 tubes? You can even upgrade the the better Russian military tubes for $5.

    SimTech wrote: »
    But then I started digging into the tube section and according to Monoprice it uses 2 6N1s and 2 6P15s. All my research shows a 6P15 to be an EL84. That's great because I have a couple but when I looked closer I noticed that this amp has pins 1, 6, 7, &9 connected together. That struck me as odd since on EL84s AND 6P15s pin 7 is the PLATE. That's where the high voltage (if any) should be. I'm not an expert on tubes but I'm 99% certain that there is no possible way a tube can operate with the same voltage on the plate that is on grids 2 and 3.

    I do not know exactly how pentode (EL84) and beam tetrode (6P1P-EV) are supposed to be wired but if you look at the datasheets it seems like grid 2 voltage (Vg2) of both tubes are supposed to be the same voltage as the plate voltage (Va). I am not an electrical engineer and don't understand how these tubes work but it appears both are wired correctly in the Monoprice amps. In your EL84/6P15 case pins 1 and 6 do nothing and pins 7 (Plate) and 9 (grid 2) are supposed to be at the same voltage (250 volts). Same with the 6P1P-EV tubes. Pins 1,6 (plate) is supposed to be the same voltage as pins 2,9 (grid 2). And that is how they are wired.

    If you are using a real Monoprice 25 and not another brand that looks like one give the tube a tap with your finger. If you hear anything from the speakers they are in the audio path.

    r-type.org/pdfs/el84.pdf



    Post edited by delkal on
  • delkal
    delkal Posts: 755
    OK......final post on tube electronics but I was doing a little more reading to try and understand a little more about how pentodes work. I think I have it figured out. Looking at the EL84 datasheet (first page) they have one table titled "pentode connection" where they have G2 and the plate at the same voltage but with a different current going to both of them. Next to the table is "Triode connection" and they describe that is when "G2 is connected to A (plate)". So the pentode tubes in the Monoprice 25 and 50 are just wired to run in triode mode. Can someone above my pay grade confirm or correct this?

    And the previous post were correct. Your brain can hurt...........
  • mhardy6647
    mhardy6647 Posts: 27,257
    The EL84 (or 6AQ5 or KT88 or etc.) operated in "triode mode" is still a power output tube with no particular business in most normal preamp designs. It is also perfectly possible to use a small signal pentode as a preamp tube (and many, many commercial preamps have done so -- not to mention the current cheap Chinese preamps* that use small signal pentodes as active components (again, because they are hyper-cheap).

    It is also possible to use power tubes as voltage regulators (the 6L6 was commonly so employed) in a power supply -- but that's not what we're dealing with in these "hybrid" amplifiers.

    I will say this (based on the 25 watt morph) there is some current flowing through the power pentodes, as they get hot. Lighting the filaments won't generate that kind of heat in these tubes. Again, though, that's consistent with some folks' resistor hypothesis (if the resistors in question are dissipating some power).

    It's possible that the tubes do something (i.e., are used as active components -- as opposed to just part of the signal path), and it is also completely possible to operate "normal" tubes at low B+ voltages in a "plate starved" ("space charge") mode, but there's not much good reason to do so.

    There's just nothing about the preamps in these amps that makes any sense.

    It would be really really interesting to have, or to trace out, the circuit(s) in these.
    Failing that, it would be nearly as interesting to examine the voltages on the tubes, pin by pin, and hypothesize their operating points (if any).

    I am not keen to spend 160 bucks to do so -- but I am getting incrementally more tempted :(


    __________________
    * To wit (and for example) the "31 dollar wonder" aka FX-audio 6J1 preamp, which uses two pentodes for something.

    https://audiokarma.org/forums/index.php?threads/fx-audio-6j1-tube-preamp-a-31-wonder.848535/


  • invalid
    invalid Posts: 529
    Why can't power tubes be used in a preamp?
  • mhardy6647
    mhardy6647 Posts: 27,257
    edited August 2020
    invalid wrote: »
    Why can't power tubes be used in a preamp?

    Well... it's not that they can't be -- and there are cases where they are, e.g., in very high power (relatively speaking), high voltage SE triode amplifiers, a 300B is often used as a driver... for a big power triode.

    This kind of thing, where the 8 watt output of the 300B is driving the input of an 845!

    s7zfwdn92fwx.png

    EDIT: or this, which is what I was looking for in the first place. :)
    https://www.stereophile.com/content/cary-audio-cad-805rs-monoblock-power-amplifier

    pbgh66bgc43j.png


    It's just that there's no good reason to use one (in the case of these little hybrid amps, that is). They're not typically designed to provide gain, which is what you typically want from a preamplifier (gain or buffering). They're horrifically inefficient to use for anything other than what they're designed for.

    In a cheap low end product like these amps, there's just no good reason for them to be there.
  • invalid
    invalid Posts: 529
    I understand the low end product point of view, but 300b tubes are used in some preamps, and they can also be used for rectifiers or the pass tube in a regulator.
  • mhardy6647
    mhardy6647 Posts: 27,257
    They won't make very good rectifiers.
  • audioluvr
    audioluvr Posts: 3,247
    edited August 2020
    95s70ft7tq2x.gif
    Cambridge Audio 851N Streamer/DAC
    Belles 21A Pre modded with Mundorf Supreme caps
    B&K M200 Sonata monoblocks refreshed and upgraded
    Belles 350A Reference modded with Mundorf Supreme caps
    Polk SDA 1C's modded
    Wireworld Silver Eclipse IC's and speaker cables


    There is about a 5% genetic difference between apes and men …but that difference is the difference between throwing your own poo when you are annoyed …and Einstein, Shakespeare and Miss January.

    by Dr. Sardonicus
  • delkal
    delkal Posts: 755
    audioluvr wrote: »
    95s70ft7tq2x.gif

    I don't think we are at the popcorn stage.........or even at a disagreement stage.

    Many earlier posts in this thread say these amps respond to tube rolling and upgrading them to military Russian tubes improve the sound. So applying the "just trust your ears" dogma the answer is clear. I think we are just trying to figure out how and why. My personal opinion is that the tubes are wired in the audio path but are run at such a low voltage they do very little amplifying. They just "color" the music adding a tube sound but send well under a watt (total guess) to the solid state amplifier. But I have no problem hearing different explanations.


    Getting back on track I looked at the Dayton model again and I would wait for a lot more reviews before I would even think about it. The original monoprice 25 had some introduction problems and many were DOA or soon to be DOA. Looking at the reviews for the 50 hybrid the dependability seems to be better. There are very few 1 and 2 star reviews and some of them appear to be operator error.
  • invalid
    invalid Posts: 529
    mhardy6647 wrote: »
    They won't make very good rectifiers.

    I don't disagree with that, but I was just saying it does actually work as a rectifier, I prefer a couple of 1616's or a couple of 866a tubes. They sound much better than any of the mainstream rectifier tubes at way cheaper prices.
  • invalid
    invalid Posts: 529
    delkal wrote: »
    audioluvr wrote: »
    95s70ft7tq2x.gif

    I don't think we are at the popcorn stage.........or even at a disagreement stage.

    Many earlier posts in this thread say these amps respond to tube rolling and upgrading them to military Russian tubes improve the sound. So applying the "just trust your ears" dogma the answer is clear. I think we are just trying to figure out how and why. My personal opinion is that the tubes are wired in the audio path but are run at such a low voltage they do very little amplifying. They just "color" the music adding a tube sound but send well under a watt (total guess) to the solid state amplifier. But I have no problem hearing different explanations.


    Getting back on track I looked at the Dayton model again and I would wait for a lot more reviews before I would even think about it. The original monoprice 25 had some introduction problems and many were DOA or soon to be DOA. Looking at the reviews for the 50 hybrid the dependability seems to be better. There are very few 1 and 2 star reviews and some of them appear to be operator error.

    I think you might be on the right track about that particular tube being in there to color the sound.
  • oldrocker
    oldrocker Posts: 2,389
    I had to.
    It was less than this month's utility bill.
    Including the fire extinguisher cost :D

    6thah2wpoui7.jpg
  • Clipdat
    Clipdat Posts: 9,536
    Good luck with that! lol :D
  • mhardy6647
    mhardy6647 Posts: 27,257
    edited August 2020
    oldrocker wrote: »
    I had to.
    It was less than this month's utility bill.
    Including the fire extinguisher cost :D

    6thah2wpoui7.jpg

    I wanna see the guts!

    B)


    PS I doubt that the Pioneer and the Marantz in the background are feeling any anxiety at all... ;)


    EDIT: In all seriousness... wicked cool! Really curious to read what you think of it.
    I am, also in all seriousness, really curious about the guts,too. Not just the preamp circuit, but the identity of the DAC that it apparently houses (i.e., at least the chip that's at the heart of it). Not really expecting you to perform exploratory surgery -- but I certainly wouldn't dissuade you, either ;)
  • oldrocker
    oldrocker Posts: 2,389
    But but, If I opener up, the lil hamster on the wheel will escape... :o
  • mhardy6647
    mhardy6647 Posts: 27,257
    edited August 2020
    Imma start a fresh post; I've gunked the one above up enough already! ;)

    so... this PE "Dayton" amp has two tubes per channel: 1 x 6U1 and 1 x 6F2, per the manual online: https://www.parts-express.com/pedocs/manuals/300-3842--dayton-audio-hta100bt-user-manual.pdf

    I just today :) learned that the 6F2 is (at least reputedly) a Chinese "version" of the 6U8.
    https://audiokarma.org/forums/index.php?threads/6n3-tube-on-ecf82-aka-6f2-amplifier.924898/#post-14017140

    The 6U8 is a combination of a medium mu triode and a sharp cutoff pentode (more or less interchangeable with the 6GH8) that found some hifi service back in the olden days (HH Scott, e.g., used 'em... and the 7199 used in the Dynaco ST-70 can be replaced with a 6U8 or 6GH8 with rewiring of the socket -- and maybe some minor circuit mods).

    xne69k98daj8.png


    Now... about that 6U1... :p If this is to be believed (http://analogmetric.com/download/Vacuum tube cross reference table.pdf) the 6U1 is 'equivalent to' a 6AJ8. The 6JA8 is a weird radio tube I haven't encountered before -- a triode and a heptode in one bottle.
    https://frank.pocnet.net/sheets/133/6/6AJ8.pdf

    zr8urnr5qldw.png


    The only problem is - the 6AJ8 looks nothing like either of the tubes in the Dayton amp. The "tall" tube in the Dayton amp looks like a power tube... like the above-mentioned 6p1p-ev tubes (Chinese "6P1") reportedly used in the Monoprice "50 watt"* or the (EDIT) Chinese "6P15" tubes used in the Monoprice "25 watt"** amp?!?

    @oldrocker, can you read what kind of tubes the tall jobbies in your new amp are?
    (and/or the small ones, for that matter?)

    :smiley:

    _______________
    * https://downloads.monoprice.com/files/manuals/16153_Manual_160816.pdf
    ** https://downloads.monoprice.com/files/manuals/13194_Manual_160322.pdf



  • oldrocker
    oldrocker Posts: 2,389
    No markings on the tubes.

    Found this:
    • Power output: 50 WPC RMS @ 4 ohms
    • THD: 0.5% @ 50 WPC
    • Minimum speaker impedance: 4 ohms
    • Channel separation: 50 dB
    • Frequency response: 20-20,000 Hz
    • Preamplifier tube: 6U1 x 2, 6F2 x 2
    • Transistors: Toshiba A1941 x 2, Toshiba C5198 x 2
    • Bluetooth: Version 4.2
    • Input impedance: 47 kohms (minimum)
    • Input sensitivity: 350 mV
    • Switching voltage: 110/220V
    • Power connection: Standard IEC
    • Dimensions: 6.6" x 11.5" x 10.6"
  • oldrocker
    oldrocker Posts: 2,389
    I also found this, not sure what it all means :D

    https://fccid.io/2AV8X-HTA100BT
  • oldrocker
    oldrocker Posts: 2,389
    1 hour in, no fire yet.....
  • oldrocker
    oldrocker Posts: 2,389
    Looked again this time with my glasses on D'oh, can see the 6U1 and 6F2 on the tubes. Quite hard to see due to cage.

    Some imperfections in the chrome on the cages, nothing major, just cheap metal.

    Base finish seems fine for what it is.

    The bass knob set flat does not show mark on knob at the true 12 o'clock position, is off a tad.

    Treble knob is fine, meters quite small, fan quiet when it comes on, I hear nothing.

    Seems like when you increase the volume, the fan activates.

    Left 6U1 tube not seating perfect, slightly tilted to the left.

    Running it with a Denon CD player, bose 301's older Polk sub. Doesn't sound that bad not as harsh or muddy as I expected. Not a power house but I didn't expect either.

    Bass & treble controls set flat seems ok, once these were increased the sound perks up with the bass slightly flubbing a bit and the treble can get a bit harsh for my old ears.

    Volume control at about 7:30 on the dial, sound starts, went to about 11:00 on the dial, meters jump and sounds gets fuller with tad more upper frequencies but not bad for what it is.

    Played some Boston, Fleetwood Mac and now some Eagles. At the 9:00 position and the Eagles sound quite nice.

    Nothing to right home about but for the price, the lil bugger is something fun to play with.

    I do like the small foot print.

  • mhardy6647
    mhardy6647 Posts: 27,257
    oldrocker wrote: »
    I also found this, not sure what it all means :D

    https://fccid.io/2AV8X-HTA100BT

    Duuuude.
    That is freaking brilliant!
    Well done!

    https://fccid.io/2AV8X-HTA100BT/Internal-Photos/Internal-Photos-4789123.pdf

    zikbhgzy332h.png
    54tlpamxzq2g.jpg
    ri4gk10chclp.jpg
  • mhardy6647
    mhardy6647 Posts: 27,257
    Chase -- is on the case!

    ocx1u86mf2av.png

    B)

    That 6U1...
    9m6mw927532o.png
    per https://www.radiomuseum.org/tubes/tube_6u1.html

    yz80qld7lsaw.png

    maybe almost (?) the same as a Russian 6И1П -- which is 'the same as' a 6AS8/ECH81? -- which may or may not :p be the same as the ECH83???

    https://www.radiomuseum.org/tubes/tube_6i1p.html
    https://www.radiomuseum.org/forum/ech83_qru.html

    IF SO... and IF I am interpreting the tube PCB photo correctly and if I've got the spatial aspects right (which is not a given for a spatially dyslexic left hander like me!)... it looks like (only) pins 2, 3, 4, 5, 8, 9 of the 6U1 are hooked to something/anything.

    lhwp7hf1ug3p.png

    4 and 5 are the filament (good)
    2 is a grid in the heptode section
    3 is the cathode of the heptode section
    8 is the plate of the heptode section
    9 is the grid of the triode section (?!?)

    The plate and cathode of the triode section are not connected (?!?!?!)

    curioser and curiouser...

    :|






  • oldrocker
    oldrocker Posts: 2,389
    Re-seated the crooked tube, popped in a 5 disc player, loaded it up. Letting in run.

    No heat issues yet, runs cool.

    If it doesn't catch fire :o then to the wife's office it goes.

    For me, the on button light is too bright, meters are really small and the volume knob has no light, has a black dot on it.

    A balance knob and a remote would be nice.

    It did arrive double boxed with some decent foam packing. Arrived in one day.

    For $172.00 shipped express, it is what it is.







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