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

[AudioTodd]

Was that this "all vacuum tube" model or one of the hybrids?

The all tube model.

The power amp section is tubed, but it is solid state prior to that. As best I can tell, and it is in storage out of state so unfortunately I cannot check further at the moment, the input/phase splitter and output circuitry is normal tube stuff starting with a DC blocking capacitor. I toyed with getting the power amp section working and ditching the rest but didn’t get beyond some initial experimentation. One of these days…

 

[mhardy6647]

Interesting. I wondered about the preamp -- it might have enough triode-equivalents to do the phono section with tubes (IIRC) -- not that I am surprised if it does not.

Not to ask more than this offhanded thread deserves, @AudioTodd -- but if you still have that amp and if you get bored enough, a photo or two of the innards (ahem, an upskirt photo, if you will, of the board(s)) would be most interesting.

EDIT: Not tryin' to be an enabler or anything, but bypassing the preamp and taking line level signal straight to the phase splitter (or to the vacuum tube voltage amp, if there is one upstream of the splitter) should be fairly straightforward.

 

[AudioTodd]

Interesting. I wondered about the preamp -- it might have enough triode-equivalents to do the phono section with tubes (IIRC) -- not that I am surprised if it does not.

Not to ask more than this offhanded thread deserves, @AudioTodd -- but if you still have that amp and if you get bored enough, a photo or two of the innards (ahem, an upskirt photo, if you will, of the board(s)) would be most interesting.

EDIT: Not tryin' to be an enabler or anything, but bypassing the preamp and taking line level signal straight to the phase splitter (or to the vacuum tube voltage amp, if there is one upstream of the splitter) should be fairly straightforward.

I'll go look through old photos in the cloud - I think I took some while I was messing around with it.

What I did discover is that the output of the line level part of the preamp section (it has a phono preamp) went to the main power amp board and to a set of "line out" connectors. I assumed that any needed voltage amplification beyond line level was in the tube power amp circuitry. There were DC blocking caps on those line outs. I wasn't thinking of those and used a spare old iPhone as a signal source and some disposable speakers to try to find a good place to connect to a real volume pot (the thing uses a chip, which I think is what blew up), but had issues that, after I shelved it all of course, I realized could be from DC on the input from within the power amp section, presumably to bias the input/phase splitter section. Even though it is effectively junk, I had some mental issue with just de-soldering those caps and putting them before the board input and hooking up the iPhone again.

I probably had a true crime show or an episode of "Curse of Oak Island" to watch...

 

[mhardy6647]

FWIW, I suffer similar reservations when confronted with similar opportunities -- I'll refrain from psychoanalyzing why in either my or your case, though!

Yeah... again -- if you find time hangin' heavy on your hands and if you dig up those photos -- I'd be interested to see 'em.

Thanks for the reply (replies)!

 

[AudioTodd]

I’m having some issues getting the pix to upload, but this is an overview during the first disassembly stage.

 

[mhardy6647]

I’m having some issues getting the pix to upload, but this is an overview during the first disassembly stage.

oooh, I see what you mean. Lots of stuff in there.
Not really bad looking, though -- from a distance, at any rate.
I mean, these things were/are quite inexpensive.

 

[Jim Matthews]

execrable. I think you mean excrable

Excrable??

Ex-scrabble, maybe.

 

[AudioTodd]

oooh, I see what you mean. Lots of stuff in there.
Not really bad looking, though -- from a distance, at any rate.
I mean, these things were/are quite inexpensive.

An AMAZING amount of stuff in there considering the price. It is currently out of state in storage, but when I get a chance to mess with it again, I am going to try to resurrect the power amp section and see how it actually sounds on its own without the tone controls and SS volume control. I'll have to figure out again how all of the modules fit together since there are several boards that are interconnected and I have forgotten the details.

 

[mhardy6647]

To the credit of... whomever designed/built these... the analog section(s) are remarkably old-fashioned (i.e., modification-friendly) with good ol' DIP I/Cs and through-hole components. I am actually surprised -- although that may be (may have been) the cheapest way to go at the time.
Thanks again for sharing those photos!

 

[AudioTodd]

To the credit of... whomever designed/built these... the analog section(s) are remarkably old-fashioned (i.e., modification-friendly) with good ol' DIP I/Cs and through-hole components. I am actually surprised -- although that may be (may have been) the cheapest way to go at the time.
Thanks again for sharing those photos!

You're welcome - I was kinda excited to see this come up! At the price, these things are cool toys to play with.

It worked well and sounded pretty decent on my wife's "party system" for when she had friends over - UNTIL one channel went out, which I discovered was from the volume control chip or related component failing, which I presume was from the improperly installed added-on protection diodes...

 

[thunderchicken]

You know, I have a fatman iTube that I still use. It needed some repairs a few years back and I did them myself. I was surprised to find that it was just a chipamp and that the tubes were just for show. The monoprice stuff reminds me of that. Don't get me wrong, I still love the iTube, it's just not really a "tube" amp and the monoprice hybrids aren't either.

 

[hansponz]

ed whaSo -- I've been thinking about the Carver "Crimson" 275 amplifier and its (apparent, as of now) performance criteria and price.

Carver Crimson 275 Measurements

I tried posting this information to a different forum, and it caused a HUGE stir and got taken down very quickly, but I think this information is important to put out for public consumption and thankfully I saved a copy before it was removed. OK, here we go! So a friend of mine brought one of...

www.audiosciencereview.com

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.

Review and Measurements of Monoprice Hybrid Tube Amp

This is a review and detailed measurements of the Monoprice Hybrid Tube Power and Headphone Amplifier. It is on kind loan from a member who sent it to Monoprice to be fixed twice. It costs US $199 from Monoprice direct. I can't find it on Amazon but there is a "Rockville" amp that looks...

www.audiosciencereview.com

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

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!

I just bought a 50wpc Dayton Audio hybrid amp, and I see that the glow is from yellow LEDs. I'm wondering about the supposedly tube preamp stage being fake. I like the amp, which has a bunch of features and input methods. It's just what I needed to replace an older, broken amp (an Arcam integrated amp from about 2000). I'd be disappointed if the tubes were just for show. I am no audiophile and am just someone who's been shopping fpr a new amp. I too have been looking at the Monoprice, and I didn't get it partially because I wanted more power.

Has anyone tried out a Dayton Audio hybrid? What makes the hybrids you refer to "execrable"?

 

[mhardy6647]

I haven't had the gumption to trace out the circuit completely in my 25 watt morph. I did do a little assessment of the 50 watt monoprice morph based on published photos of the PCB.
The tubes need to be physically installed (in circuit) for these amps to work.
The filaments do light, although the glow is somewhat overshadowed by the LEDs.
There was early speculation (based ont the "Jaycar" branded version of the 25 watt morph) that the primary role of the tubes might be as resistors -- i.e., that the filaments are used as resistors in the circuit. This is a "trick" that was used more than occasionally in bona fide vacuum tube amplifier designs BITD.
I will say that the "power tubes" in the 25 watt Monoprice morph do get quite warm with use -- but they are not used as power output tubes per se (the power amplifier section is ss -- a 'chip amp'), so one wonders, at the least, why they were chosen for preamp (or resistor) use, other than to give a visual impression of a single-ended vacuum tube amplifier.

One is left with the indelible imression that the 'tube' part of these amplifiers is a deliberately engineered diversionary tactic*.
This being said, I cannot vouch for the PE morphs of any of these amps, because I have not encountered one in the flesh! Full disclosure.

The 25 watt Monoprice morph doesn't sound all that great compared to... well... anything else I compared it to.

___________________
* In the glory days of radio as home entertainment centerpiece, the number of tubes in a radio console was correlated to owner bragging rights. There were numerous (faily good quality and expensive) radios that added fairly superfluous tubes to circuits (typically paralleling other tubes, IIRC) to inflate the tube count. I.e., this gambit has a long history!

 

[hansponz]

I haven't had the gumption to trace out the circuit completely in my 25 watt morph. I did do a little assessment of the 50 watt monoprice morph based on published photos of the PCB.
The tubes need to be physically installed (in circuit) for these amps to work.
The filaments do light, although the glow is somewhat overshadowed by the LEDs.
There was early speculation (based ont the "Jaycar" branded version of the 25 watt morph) that the primary role of the tubes might be as resistors -- i.e., that the filaments are used as resistors in the circuit. This is a "trick" that was used more than occasionally in bona fide vacuum tube amplifier designs BITD.
I will say that the "power tubes" in the 25 watt Monoprice morph do get quite warm with use -- but they are not used as power output tubes per se (the power amplifier section is ss -- a 'chip amp'), so one wonders, at the least, why they were chosen for preamp (or resistor) use, other than to give a visual impression of a single-ended vacuum tube amplifier.

One is left with the indelible imression that the 'tube' part of these amplifiers is a deliberately engineered diversionary tactic*.
This being said, I cannot vouch for the PE morphs of any of these amps, because I have not encountered one in the flesh! Full disclosure.

The 25 watt Monoprice morph doesn't sound all that great compared to... well... anything else I compared it to.

___________________
* In the glory days of radio as home entertainment centerpiece, the number of tubes in a radio console was correlated to owner bragging rights. There were numerous (faily good quality and expensive) radios that added fairly superfluous tubes to circuits (typically paralleling other tubes, IIRC) to inflate the tube count. I.e., this gambit has a long history!

That is some fun I've just had reading your post just now. The deviousness of some people when it comes to obtaining money cannot be forgotten! I like the sound of my Dayton Audio amp. I don't have much experience listening to different amps and speakers etc. The bass is strong, and the cymbals on a 24/192kHz Kind of Blue, especially "So What," played through a Surfans F20, sound great. Blossom Dearie sounds as lovely as I've heard her. I'm playing through a tower pair of PSB Stratus Silveris (91 dB) a friend gave me.

 

[hansponz]

I haven't had the gumption to trace out the circuit completely in my 25 watt morph. I did do a little assessment of the 50 watt monoprice morph based on published photos of the PCB.
The tubes need to be physically installed (in circuit) for these amps to work.
The filaments do light, although the glow is somewhat overshadowed by the LEDs.
There was early speculation (based ont the "Jaycar" branded version of the 25 watt morph) that the primary role of the tubes might be as resistors -- i.e., that the filaments are used as resistors in the circuit. This is a "trick" that was used more than occasionally in bona fide vacuum tube amplifier designs BITD.
I will say that the "power tubes" in the 25 watt Monoprice morph do get quite warm with use -- but they are not used as power output tubes per se (the power amplifier section is ss -- a 'chip amp'), so one wonders, at the least, why they were chosen for preamp (or resistor) use, other than to give a visual impression of a single-ended vacuum tube amplifier.

One is left with the indelible imression that the 'tube' part of these amplifiers is a deliberately engineered diversionary tactic*.
This being said, I cannot vouch for the PE morphs of any of these amps, because I have not encountered one in the flesh! Full disclosure.

The 25 watt Monoprice morph doesn't sound all that great compared to... well... anything else I compared it to.

___________________
* In the glory days of radio as home entertainment centerpiece, the number of tubes in a radio console was correlated to owner bragging rights. There were numerous (faily good quality and expensive) radios that added fairly superfluous tubes to circuits (typically paralleling other tubes, IIRC) to inflate the tube count. I.e., this gambit has a long history!

I did try running the Dayton Audio hybrid without a tube, and it didn't work. I hadn't thtof the resistor trick. I'd thought they might have used a microcontroller or something to sense whether all tubes were plugged in. Kind of like that ridiculous juicer that wouldn't squeeze juice out of a bag if it wasn't connected to WiFi!

 

[Seventhrest]

I own all three mp amps. And a couple other tube amps. All w upgraded tubes. The pure tube amp def sounds the best but it's the only ss rectified tube amp I have. It excites speakers below 10hz via Bluetooth aptxhd. I put Tesla el84s and Jan 5670s? In it. It sounds like magic with any speakers you connect to it but if you get the power tube signal to a sub yeah it's sonic euphoria. It seems to use the third preamp to create a virtual hafler channel that adds depth beyond belief. This is not going to rock a block party but the 10wpc pure tube plays louder than the 25w hybrid.

 

[fpitas]

__________________
* In the glory days of radio as home entertainment centerpiece, the number of tubes in a radio console was correlated to owner bragging rights. There were numerous (faily good quality and expensive) radios that added fairly superfluous tubes to circuits (typically paralleling other tubes, IIRC) to inflate the tube count. I.e., this gambit has a long history!

I'm old enough to remember when transistorized stuff had "extra" transistors tossed in to increase the count. I had a radio (with schematic printed inside) that featured a 5 stage Darlington

 

[fpitas]

As for the Monoprice Pure Tube amp...I say, measure it, then make it work properly by redesigning it For that price, you get a nice looking chassis with everything in place.

 

[mhardy6647]

I'm old enough to remember when transistorized stuff had "extra" transistors tossed in to increase the count. I had a radio (with schematic printed inside) that featured a 5 stage Darlington

In the heyday of "centerpiece" console radios for home entertainment (1930s), gratuitous increase in tube count was a marketing ploy, as well.
In fairness, though -- some of the best radios of the era (e.g., E.H. Scott*) had lots of tubes... and used 'em wisely.

Do High Tube Count Radios have an advantage? - Antique Radio Forums

________________
* Not to be confused with H.H. Scott. E.H. Scott was the Duesenberg of high-end radios.

 

[fpitas]

In the heyday of "centerpiece" console radios for home entertainment (1930s), gratuitous increase in tube count was a marketing ploy, as well.
In fairness, though -- some of the best radios of the era (e.g., EH Scott*) had lots of tubes... and used 'em wisely.

Do High Tube Count Radios have an advantage? - Antique Radio Forums

________________
* Not to be confused with HH Scott. E.H. Scott was the Duesenberg of high-end radios.

There was no doubt the venerable 5-tube radio design was a big compromise. But, tubes and their power supply were expensive. And big!