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Monoprice "Pure Tube" 15 wpc stereo vacuum tube amp - worth review?

mhardy6647

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So -- I've been thinking about the Carver "Crimson" 275 amplifier and its (apparent, as of now) performance criteria and price.
As of the intel available on 27 December, the aforementioned Carver appears to be a 15 watt per channel stereo power amplifier (albeit with a specification of "at least 75 watts" per channel) that uses KT120 "kinkless tetrode" output tubes, weighs 19 lbs, and costs $2795 (if memory serves). A 15 watt per channel stereo vacuum tube amp is tantalizingly close to the 'sweet spot' of output power for push-pull EL84 (6BQ5, in the US) pentode outputs (i.e., 'by the book' and allowing for modest quality of output iron. IMO/IME (and of course all y'all may differ in your opinions from mine!) it is very difficult to build a bad-sounding pp EL84 amplifier, even if one scrimps on the output transformers. There have been many, many, many decent pp EL84 powered power amps, integrateds, and receivers over the decades -- some plain/cheap, some fancy/not so cheap. It's a pretty mature and well-understood product space since ca. the late 1950s. :)

... and that is what reminded me of the Monoprice "Pure Tube" amplifier. Monoprice (and many, many others) sell a really execrable little vacuum tube "hybrid" amplifier that is very cheap... but the actual function of its four vacuum tube payload is... controversial.

BUT -- Monoprice has an actual vacuum tube stereo amp that uses, guess what?!, pp EL84 outputs: the above-mentioned "Pure Tube" amplifier.
https://www.monoprice.com/product?p_id=27222

272221.jpg


Currently listed price (it's on sale!) $329.99
It's actually rated at a seemingly conservative 10 wpc into 4 ohms at 1 kHz (THD not specified) -- i.e., realistically reflecting either the operating points of the tubes and/or the capability of fairly meager output iron.
It weighs 16.1 lb

Here are the full specs from the link above.
1640658423447.png


It might be really interesting to test this little amplifier as a comparator for the perfrmance of a Carver Crimson 275 of fully established provenance and see which is the better investment (measurable performance per USD spent).

Relative to the Carver amp's review, I know this is, at best, scope creep, and I am not really advocating this as a new task for either @paulbottlehead or @amirm ;) -- but I felt it worthy of mention. I, at any rate, would be curious to see one of these little amps put through its paces! Maybe it's just me...

PS The MP "Pure Tube" amp also has built in BT and some manner of phono preamp/EQ built into it, too! ;)
 
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MakeMineVinyl

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I have a Dynaco SCA-35 which I took the power amplifier out of and put in a new chassis; it has the typical 15 watts output although they spec 17. Good little amp but its not going to break my measurement gear with astounding measured performance. :)

I'd be interested in seeing a review of that Monoprice amp but I can't imagine its going to be spectacular given the price.
 
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mhardy6647

mhardy6647

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I don't know. Too afraid to listen to it. :) I have it boxed up to ship to SIY. All questions should be put to him from here on. :D
Wait... did this ever happen? I.e., did @SIY ever get to snoop around inside the "hybrid" version mentioned above?
Inquiring minds want to know! ;)

I -- ahem -- I actually have one of these (the "25 watt" morph).

pjty7p5ibg0a.jpg


Here's the back story, if anyone's interested. ;)

I have a Dynaco SCA-35 which I took the power amplifier out of and put in a new chassis; it has the typical 15 watts output although they spec 17. Good little amp but its not going to break my measurement gear with astounding measured performance. :)

I'd be interested in seeing a review of that Monoprice amp but I can't imagine its going to be spectacular given the price.

Slightly modified and rehabbed SCA-35 here, as well and FWIW.

I don't expect too much of the MP "Pure Tube" either -- but would it not be interesting to see it compared, ampo a ampo, so to speak ;) with the Crimson 275?
 
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ThatM1key

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If Monoprice can't make a good monitor, I don't trust them with amplification.
 

ThatM1key

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MakeMineVinyl

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If Monoprice can't make a good monitor, I don't trust them with amplification.
I've heard that the Monoprice Monolith power amps made by ATI are very good. :cool:
 

SIY

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So -- I've been thinking about the Carver "Crimson" 275 amplifier and its (apparent, as of now) performance criteria and price.
As of the intel available on 27 December, the aforementioned Carver appears to be a 15 watt per channel stereo power amplifier (albeit with a specification of "at least 75 watts" per channel) that uses KT120 "kinkless tetrode" output tubes, weighs 19 lbs, and costs $2795 (if memory serves). A 15 watt per channel stereo vacuum tube amp is tantalizingly close to the 'sweet spot' of output power for push-pull EL84 (6BQ5, in the US) pentode outputs (i.e., 'by the book' and allowing for modest quality of output iron. IMO/IME (and of course all y'all may differ in your opinions from mine!) it is very difficult to build a bad-sounding pp EL84 amplifier, even if one scrimps on the output transformers. There have been many, many, many decent pp EL84 powered power amps, integrateds, and receivers over the decades -- some plain/cheap, some fancy/not so cheap. It's a pretty mature and well-understood product space since ca. the late 1950s. :)

... and that is what reminded me of the Monoprice "Pure Tube" amplifier. Monoprice (and many, many others) sell a really execrable little vacuum tube "hybrid" amplifier that is very cheap... but the actual function of its four vacuum tube payload is... controversial.

BUT -- Monoprice has an actual vacuum tube stereo amp that uses, guess what?!, pp EL84 outputs: the above-mentioned "Pure Tube" amplifier.
https://www.monoprice.com/product?p_id=27222

272221.jpg


Currently listed price (it's on sale!) $329.99
It's actually rated at a seemingly conservative 10 wpc into 4 ohms at 1 kHz (THD not specified) -- i.e., realistically reflecting either the operating points of the tubes and/or the capability of fairly meager output iron.
It weighs 16.1 lb

Here are the full specs from the link above.
View attachment 175021

It might be really interesting to test this little amplifier as a comparator for the perfrmance of a Carver Crimson 275 of fully established provenance and see which is the better investment (measurable performance per USD spent).

Relative to the Carver amp's review, I know this is, at best, scope creep, and I am not really advocating this as a new task for either @paulbottlehead or @amirm ;) -- but I felt it worthy of mention. I, at any rate, would be curious to see one of these little amps put through its paces! Maybe it's just me...

PS The MP "Pure Tube" amp also has built in BT and some manner of phono preamp/EQ built into it, too! ;)
It's an LM1875 output stage with a bad tube input. I got it to stop thermaling out, but couldn't fix the noise issues from their cheapo heater supply. It's been sitting on a shelf since (in case Amir decides to go slumming again).
 
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mhardy6647

mhardy6647

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It's an LM1875 output stage with a bad tube input. I got it to stop thermaling out, but couldn't fix the noise issues from their cheapo heater supply. It's been sitting on a shelf since (in case Amir decides to go slumming again).
so -- the tubes -- they actually do something? Besides the tubes, they glow in the dark*? :)
What are the power output tubes doing being used for?
The... umm... design (for lack of a better word) of these little boxes just flummoxes me.
Although I will cop to having a fairly low flummoxation threshold.

Mine's packed in its box someplace. MP amp, that is -- not my flummoxation threshold. That's always front and center. :cool:

Thanks for the update, sir!

________________________
* with apologies to one of Warren Zevon's best songs, IMO and FWIW...

 

Vict0r

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I know these won't measure well, but they sure look attractive!
 
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mhardy6647

mhardy6647

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I know these won't measure well, but they sure look attractive!
Definitely not unattractive, and this particular amp may be perfectly pleasant to listen to... especially for $300-ish smackers. That said, the same 300 Samoleums "invested in" a modern "chip amp" may result in much better quantitative and qualitative performance... but, of course, no glow.

And the glow's worth... at least... 75 bucks, right? ;)

(full disclosure -- I am a tube guy, but I like to think that my Weltanschauung is realistic.)
 

Vict0r

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Definitely not unattractive, and this particular amp may be perfectly pleasant to listen to... especially for $300-ish smackers. That said, the same 300 Samoleums "invested in" a modern "chip amp" may result in much better quantitative and qualitative performance... but, of course, no glow.

And the glow's worth... at least... 75 bucks, right? ;)

(full disclosure -- I am a tube guy, but I like to think that my Weltanschauung is realistic.)

I think the glow is worth something. :) Not sure if it's $75, but there's nothing wrong with shelling out a little for something that just looks nice.
 

sergeauckland

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Definitely not unattractive, and this particular amp may be perfectly pleasant to listen to... especially for $300-ish smackers. That said, the same 300 Samoleums "invested in" a modern "chip amp" may result in much better quantitative and qualitative performance... but, of course, no glow.

And the glow's worth... at least... 75 bucks, right? ;)

(full disclosure -- I am a tube guy, but I like to think that my Weltanschauung is realistic.)
When I was at school, I built a number of Mullard 5-10 amps, for myself and friends, using a variety of cheap output transformers, as I couldn't afford Gardner's, Parmeko or Partridge transformers. Results were always audibly satisfactory, although at the time I had no means of measuring performance. Suffice to say that the 5-10s were considerably better than the Sinclair Z-12 I had been using, and surprisingly not audibly lacking when I built my first 'high-power' SS amp, (30 watts p.c.) with around 0.01% distortion rather than 0.3% at best.

Consequently, I expect this amp to do OK, certainly a bit of fun, for not much money. I would like to see some numbers, and see whether it's as good as what was available in the 1950s!

S.
 

Thermionics

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I have a Dynaco SCA-35 which I took the power amplifier out of and put in a new chassis; it has the typical 15 watts output although they spec 17. Good little amp but its not going to break my measurement gear with astounding measured performance. :)

I'd be interested in seeing a review of that Monoprice amp but I can't imagine its going to be spectacular given the price.
My first amp (which I still have) was an SCA-35 converted to a P2P Stereo 35 build. I will say that Dave Gillespie's EFB (Enhanced Fixed Bias) modification helps the performance of the amplifier, especially around measured distortion and power output (gets you to an honest 17WPC). A well-sorted Stereo 35 would probably out-perform the Crimson 275. :D
 

bethslave

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I wondered about the Monoprice Pure Tube amp myself. I didn't pursue it because I couldn't find any objective measurements. I then planned to buy a TubeCube 7 but didn't because I WAS able to find objective measurements indicating that the frequency response dips dramatically above 2 KHz. I wound up purchasing a Gemtune PA1501A-APPJ based on this review and the measured flat frequency response: https://retrovoltage.com/2016/08/17/reviewing-the-gemtune-appj-pa1501a-6ad10-stereo-tube-amplifier/

I wasn't expecting magic, tube euphoria or any of that. I kind of wanted to recreate a system built around a Zenith suitcase record player I had as a child that utilized 50EH5s just for nostalgia. Also, my employer [wisely] decided we could no longer use space heaters in our offices. I now have a hand warmer that also sounds quite nice for desktop listening with some Advent Baby II's I had left from my college days. It's only 3.5 Watts per channel [ish] but that's more than enough for this application. THD for the 6AD10 according to GE specs is 10% at 4.5 Watts output but that would be completely audible and I'm not hearing it. At some point I'll have to figure out how to measure it for myself.

From what I've read, some of the Chinese manufacturers are building their tube amps based on old GE reference designs. Some seem to be correctly implemented. I'm also curious about this one: https://www.amazon.com/Nobsound-Amp...ne/dp/B073ZXZ3Z5/ref=sr_1_5?crid=W5VG1PO9QHZX More tubes=warmer office. ;)
 

MakeMineVinyl

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I wondered about the Monoprice Pure Tube amp myself. I didn't pursue it because I couldn't find any objective measurements. I then planned to buy a TubeCube 7 but didn't because I WAS able to find objective measurements indicating that the frequency response dips dramatically above 2 KHz. I wound up purchasing a Gemtune PA1501A-APPJ based on this review and the measured flat frequency response: https://retrovoltage.com/2016/08/17/reviewing-the-gemtune-appj-pa1501a-6ad10-stereo-tube-amplifier/

I wasn't expecting magic, tube euphoria or any of that. I kind of wanted to recreate a system built around a Zenith suitcase record player I had as a child that utilized 50EH5s just for nostalgia. Also, my employer [wisely] decided we could no longer use space heaters in our offices. I now have a hand warmer that also sounds quite nice for desktop listening with some Advent Baby II's I had left from my college days. It's only 3.5 Watts per channel [ish] but that's more than enough for this application. THD for the 6AD10 according to GE specs is 10% at 4.5 Watts output but that would be completely audible and I'm not hearing it. At some point I'll have to figure out how to measure it for myself.

From what I've read, some of the Chinese manufacturers are building their tube amps based on old GE reference designs. Some seem to be correctly implemented. I'm also curious about this one: https://www.amazon.com/Nobsound-Amp...ne/dp/B073ZXZ3Z5/ref=sr_1_5?crid=W5VG1PO9QHZX More tubes=warmer office. ;)
I have to laugh at the verbage on that amplifier's page on Amazon.

  • This Class A single-ended vacuum tube amplifier adopts hand welding to ensure the true reality of the sound quality (with headphone amp function ASIN: B076CC92BX).
  • This amplifier adopts transformer parallel output to reduce its internal resistance by half and achieve doubled improvement on its power.
  • Using antimagnetic transformer covers to shield current interference and achieve much lower static noise.
  • Its sound output is full of analog style. The bass part is deep and powerful and the vocal part is sweet and intoxicating.
  • It has a classic and graceful appearance. The VU meters and vacuum tubes makes it more elegant and attractive.
 

bethslave

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I have to laugh at the verbage on that amplifier's page on Amazon.

  • This Class A single-ended vacuum tube amplifier adopts hand welding to ensure the true reality of the sound quality (with headphone amp function ASIN: B076CC92BX).
  • This amplifier adopts transformer parallel output to reduce its internal resistance by half and achieve doubled improvement on its power.
  • Using antimagnetic transformer covers to shield current interference and achieve much lower static noise.
  • Its sound output is full of analog style. The bass part is deep and powerful and the vocal part is sweet and intoxicating.
  • It has a classic and graceful appearance. The VU meters and vacuum tubes makes it more elegant and attractive.
Yes, especially:

  • This amplifier adopts transformer parallel output to reduce its internal resistance by half and achieve doubled improvement on its power.

I've got just enough basic electronics knowledge to be completely amused and confused by that one. One possibly shouldn't depend on Google Translate for marketing materials.
 

AudioTodd

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So -- I've been thinking about the Carver "Crimson" 275 amplifier and its (apparent, as of now) performance criteria and price.
As of the intel available on 27 December, the aforementioned Carver appears to be a 15 watt per channel stereo power amplifier (albeit with a specification of "at least 75 watts" per channel) that uses KT120 "kinkless tetrode" output tubes, weighs 19 lbs, and costs $2795 (if memory serves). A 15 watt per channel stereo vacuum tube amp is tantalizingly close to the 'sweet spot' of output power for push-pull EL84 (6BQ5, in the US) pentode outputs (i.e., 'by the book' and allowing for modest quality of output iron. IMO/IME (and of course all y'all may differ in your opinions from mine!) it is very difficult to build a bad-sounding pp EL84 amplifier, even if one scrimps on the output transformers. There have been many, many, many decent pp EL84 powered power amps, integrateds, and receivers over the decades -- some plain/cheap, some fancy/not so cheap. It's a pretty mature and well-understood product space since ca. the late 1950s. :)

... and that is what reminded me of the Monoprice "Pure Tube" amplifier. Monoprice (and many, many others) sell a really execrable little vacuum tube "hybrid" amplifier that is very cheap... but the actual function of its four vacuum tube payload is... controversial.

BUT -- Monoprice has an actual vacuum tube stereo amp that uses, guess what?!, pp EL84 outputs: the above-mentioned "Pure Tube" amplifier.
https://www.monoprice.com/product?p_id=27222

272221.jpg


Currently listed price (it's on sale!) $329.99
It's actually rated at a seemingly conservative 10 wpc into 4 ohms at 1 kHz (THD not specified) -- i.e., realistically reflecting either the operating points of the tubes and/or the capability of fairly meager output iron.
It weighs 16.1 lb

Here are the full specs from the link above.
View attachment 175021

It might be really interesting to test this little amplifier as a comparator for the perfrmance of a Carver Crimson 275 of fully established provenance and see which is the better investment (measurable performance per USD spent).

Relative to the Carver amp's review, I know this is, at best, scope creep, and I am not really advocating this as a new task for either @paulbottlehead or @amirm ;) -- but I felt it worthy of mention. I, at any rate, would be curious to see one of these little amps put through its paces! Maybe it's just me...

PS The MP "Pure Tube" amp also has built in BT and some manner of phono preamp/EQ built into it, too! ;)
Have a Monoprice one that failed early. A module, input or volume control but I don’t remember which now, failed. It was not within warranty and there was no process or recommendation for repair and a request for replacement was rejected.

With an inspection it appeared a crude design modification was done during production by hand soldering opposite polarity parallel pairs of small signal diodes on the back of the PC board for the failed module, presumably for some over voltage or discharge protection. One set was correct, presuming that was their function, but the other were not connected with opposite polarity but the same instead. I can’t remember for sure, but I think that was the channel that failed.

Anyway, it appeared to be a neat idea with MOSTLY good execution, but the one or two things that are not right ruin the whole…
 
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mhardy6647

mhardy6647

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Have a Monoprice one that failed early. A module, input or volume control but I don’t remember which now, failed. It was not within warranty and there was no process or recommendation for repair and a request for replacement was rejected.

With an inspection it appeared a crude design modification was done during production by hand soldering opposite polarity parallel pairs of small signal diodes on the back of the PC board for the failed module, presumably for some over voltage or discharge protection. One set was correct, presuming that was their function, but the other were not connected with opposite polarity but the same instead. I can’t remember for sure, but I think that was the channel that failed.

Anyway, it appeared to be a neat idea with MOSTLY good execution, but the one or two things that are not right ruin the whole…
Was that this "all vacuum tube" model or one of the hybrids?
 
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