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Monoprice 5 watt tube combo

Discussion in 'Glowing Bottle Tube Amp Forum' started by shuggoth, Oct 15, 2015.

  1. Keefsdad

    Keefsdad Tele-Holic

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    No, it's nothing like a Blues Junior, basically a Marshall circuit and tubes.
     
  2. gobi_grey

    gobi_grey Tele-Afflicted

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    Been using the 15w for a few years now and like it a lot. Been thinking about picking up the 5w with the intention of putting it in a different cab, like an old radio cab or something like that. Would it be a good candidate for that?
     
  3. Chuck_D_Bones

    Chuck_D_Bones TDPRI Member

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    Yeah, that schematic is pretty rough. A few missing / wrong connections, but looks to be about 90% correct. I'm getting mine delivered in a day or two, I'll verify the circuit and post something more schematicy.
     
  4. Chuck_D_Bones

    Chuck_D_Bones TDPRI Member

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    Here is the schematic, traced from my new Monoprice 5W amp (p/n 611705). Tubes are unmarked; the user's manual identifies them. The tubes in mine measured better than spec on a TV-2B/U Military tube tester. The 12AX7 was pretty well balanced, not that it matters in this circuit. Voltages marked are measured with nominal 120VAC line, no signal, tubes warmed up. There is bound to be some variation from unit-to-unit. With the 6V6GT removed, the B+ floats from +305V up to +349V. The 1W / 5W switch is a gain switch and has nothing to do with how much power gets to the speaker, provided the guitar or last pedal in the chain has enough signal to drive the amp to full power. The Tone pot acts like a treble cut from 0 to 9 and a presence control from 9 to 10. First thing I would do to improve this amp is replace the linear taper Tone pot with a reverse audio taper so there is more usable adjustment range at the top end. The fuse is obscured inside some blue plastic. The fuse rating on the schematic is a guess based on the max power consumption (50W). The MOV (RV1) ensures gradual filament warm-up, which promotes longer tube life. Build quality looks pretty good. I have not yet measured the full-power output capability. Based on the 6V6GT's operating point and the size of the transformers, I believe the 5W claim.
     

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    Last edited: Jun 17, 2019
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  5. Chuck_D_Bones

    Chuck_D_Bones TDPRI Member

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    I made a correction to the schematic, there was an error in the C13 - C15 connections. It was updated here this morning. I have also confirmed that the fuse is 0.5A fast-blow. I know because I had to change it. And C15. Not sure if the problem was an inferior capacitor, or a power glitch from SCE. While I was in there, I changed the Tone pot to C250K. Much better.
     
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  6. warrent

    warrent Friend of Leo's

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    Definitely more schematicy thanks for posting it.
     
  7. Chuck_D_Bones

    Chuck_D_Bones TDPRI Member

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    One more correction...
    R40 is marked 22R on the board but is actually 470R.
     
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  8. JWH7

    JWH7 Tele-Meister

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    I bought 2 monoprice 30 watt amp heads. I hear the 5watt, 15 watt 30 watt are all good. Got both for $700. it was hard to find an all tube amp for less than $900 until i found these. first tube amps I've owned. I see or hear now what all the hype over tube amps is all about now. i'll never use my SS amp again. oh they are very customizable you can make them look cool with a famous name brand logo.
     

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  9. skydog6653

    skydog6653 Tele-Meister

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    Many thanks to @Chuck_D_Bones for bringing some worth to this thread! I just read all 18 pages and prior to his posts, about the only useful info was some speaker and tube recommendations and a shot of the PCB boards. The shot of the shipping box was particularly entertaining and informative! Has anyone successfully removed the NFB feature? If so, was it worth it? Using @CASD57 post #114, can you describe which connector it is? Thanks
     
  10. skydog6653

    skydog6653 Tele-Meister

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    FB9FE90D-8A27-47CC-8197-0D84918156D9.jpeg I have a bunch of tubes laying around. I loaded in the big bottle Mullard EL37 along with a Tung-Sol 12AX7. This really brought it to life. Slightly louder, but very much warmer tone.
     
  11. warrent

    warrent Friend of Leo's

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    That sounds like an neat substitution other than the fact that the tube is worth twice as much as the amp.:)
     
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  12. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Thanks for tracing this out! Whoever designed it didn't just clone a particular Fender amp, they took inspiration from a couple different sources.

    I'm kinda shocked that the 1W/5W switch isn't a pentode/triode switch on the 6V6. Seems like that would have been the obvious thing to do.
     
  13. skydog6653

    skydog6653 Tele-Meister

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    Well, like I said, I had it already previously installed in my Siegmund Midnight Special. I have an RCA 6V6 GT gray plate I can try too, plus a variety of 12XXX’s to try.
     
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  14. AndrewG

    AndrewG Friend of Leo's

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    Yes, probably all built in the same factory. Monoprice are marketed over here as 'Sub Zero', previously badged 'Belcat'. This is the Belcat/Sub Zero BJ clone; £280 vs. £580 for the equivalent Fender:

    https://www.gear4music.com/Guitar-and-Bass/SubZero-Valve-20-Guitar-Amp/S7P
     
    Last edited: Nov 27, 2020
  15. skydog6653

    skydog6653 Tele-Meister

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    In the U.S. a Sub Zero is a refrigerator!
     
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