Monoprice 15 watt amp upgrades

Discussion in 'Amp Tech Center' started by Stewtech, Aug 22, 2019.

  1. cheepnis

    cheepnis TDPRI Member

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    Before my Mono 15w went dead (replacement on the way) I swapped the stock speaker for a Jensen C12N8 and without being a trained ear ... it sounded better, especially after breaking in a little bit. Certainly warmer/fuller yet no loss of the sparkle/spank from single coils. Not sure if I'll even power the replacement up, before putting the Jensen back in it.

    I did the speaker swap iindependent of a JJ EL844s and a 12AY7 in v1, which I did a little bit later. Playing mostly at "1W", I was interested at getting the breakup at even lower volumes if I wanted it ... the EL844s seemed to do the trick, but I put the stock 12AX7 back in.
     
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  2. Stewtech

    Stewtech TDPRI Member

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    Thanks for replying! I was thinking about going with Jensen also cause the Blues Jr III has a Jensen that comes stock and that thing rips! I am also a fan of the 1 watt feature. I hope the new amp works out for you!
     
  3. hoglundtw

    hoglundtw TDPRI Member

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    Hey V52S - see you swapped out the reverb - did you have the old Monoprice 15W with the input jacks and terrible reverb or the newer model (came out in late 2017/early 2018)? I tried the MOD reverb in my newer model and didn't really liked it. The stock 2 spring had a nice longer tail to it that I like. Kept wanting to like the 3 spring MOD, and A/B'd it over and over with plugging in one vs the other and in the end, sent the MOD back to Amazon.
     
  4. bsman

    bsman Friend of Leo's

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    I must have a tin ear. I'm still loving my totally stock post 17 model... :)
     
  5. hoglundtw

    hoglundtw TDPRI Member

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    Me too - had to replace the power tubes because they were defective on arrival (Amazon paid to replace) and after I put a tweed covering on it, I'm loving it, too.
     
  6. Pontiaddict

    Pontiaddict TDPRI Member

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    I just wanted to drop in to follow-up on my previous posts... after obsessing over speaker replacement I eventually decided upon the WGS Reaper 55Hz.
    With that, I am done.

    This amp is IMO phenomenal and that speaker was absolutely perfect for the tone I sought; everyone wants their individual tone and for me WGS was the undeniably right choice.

    Net effect is a larger sound than the cabinet footprint might suggest w. rich deep tones, crisp non biting highs and sweetness in between.

    JJs in all valves: V1=5751, two AX7-S-JJ, EL84-MP
    Warehouse Guitar Speaker: Reaper 55Hz LE
     
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  7. Stewtech

    Stewtech TDPRI Member

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    Congratulations on the new speaker! Are you using single coils? In the video I could hear the deep thick almost Supro sound and was wondering if it still comes through using single coils?
     
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  8. Pontiaddict

    Pontiaddict TDPRI Member

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    Thank you.

    Swapping guitars quite a bit, but a new (bone stock) Squier CVC is getting the most play atm... so that plays exactly to your question and I’m seriously loving the tone, articulate at the bridge but not sharp, the neck and mid position is where it really shines, experimenting with lots of jazzy tones and, JMHO truly sweet.
     
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  9. LightningPhil

    LightningPhil Tele-Meister

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    Nope, the most dramatic change in tone would come from letting me (or anyone else really) lose to play component kerplunk on the amp after 2 pints of coffee.

    More sensibly, yes the speaker has a huge involvement in tone, but small component changes can have a massive effect on tone - as long as the frequencies the amp is asking for are within the range of the attached speaker.
     
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  10. Stewtech

    Stewtech TDPRI Member

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    You have surely sparked my attention. So, if I were to drop off a brand new Monoprice 15 watt amp to you and you drank some nice strong coffee. Where would you begin and what would you aim to do. Everyone I know always goes for speaker and tubes, what adjustments am I missing out on?
     
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  11. LightningPhil

    LightningPhil Tele-Meister

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    Challenge accepted.

    I don't own one as they're not sold in Europe yet. And as my interest in them is an inexpensive victim for tinkering, won't be buying the original Laney...

    The following is based on the Laney diagram for thre Cub 12R.

    Input:
    The less than 1w option is not power scaling. It's a single set point master volume operated by a relay. Would rewire it as a standard VOX high/low and add a proper master volume in its place, then bin the relay and supporting circuit and relay. That would make the input more functional as the low would present a lower impedance to pickups and roll off the brightness as we're used to.

    First gain stage:
    C11 is quite small and causes more gain in the upper mids and treble than the bass by about 6dB. While i usually mod amps in this direction, a 22uf cap on a switch could be fun to beef up the lows.

    If it's a double pole switch, all the better. C17 could be replaced with 3.3 or 4.7nf. Then C29 can be shorted out with the switch to allow more bass to pass to the tone stack.

    That should give a bit more flexibility while still allowing stock tone.

    Power rails:
    Very simply, replace all the 22uf caps with 47uf or a little bigger if they fit on the HT rails. It lowers the effective filter frequency and so will almost certainly reduce hum a bit.

    Grid stoppers:
    It looks like some of the grids don't have local stoppers. Would aim for 10k ohm on the 12ax7 valves. Big enough to keep RF out and small enough to retain all the tone. Heading to 33k could ok, much higher and some highs may be lost.

    On the EL84 valves, something reasonable on the screen connections.

    In all cases, when I add stoppers, I cut the trace to the socket and replace the connection with the resistor. Easier when the sockets are on leads...

    Op-amps:
    Not keen on TL072 amps. Would replace all of them with sockets, add a 1uf film cap across the back of the board on the power pins and head to the parts bin to find some upgrades. Very possibly a sprinkling of opa2134 amps. Would have to check with a scope that none ends up oscillating. This will improve the noise floor and remove any slight colouration from the original amps.

    That should be a good start. Less noise and more flexibility.
     
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  12. LightningPhil

    LightningPhil Tele-Meister

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    Perhaps rather than adding a master volume, an l-pad could be mounted on the back.

    Ok, done now.
     
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  13. Pontiaddict

    Pontiaddict TDPRI Member

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    Wow! I’m amazed at the description, to me personally it’s like trying to understand a fluid transmission, how thin oil traveling through wormholes can make my car move?

    Your description touched every minor complaint I’ve read to date regards the SR 15w, addressed each as a fault inherent to the build process and most disheartening offered practical fixes.

    Unfortunate truth, I lack the skill set, therefore I read the comment as a practical guide into the world of Boutique amplifiers, what makes them better and why they demand such high prices.

    I hope you get the chance to modify one, the end result... WOW!
     
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  14. LightningPhil

    LightningPhil Tele-Meister

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    Thanks Pontiaddict. Had a look at the thread to see what all the excitement is about. It's probably quite a nice little amp and I rather like the look of the gain control.

    Looks like great value and probably doesn't need modding.

    Thought of another couple of tweaks though...

    For a little more gain, decrease R18 from 220k to 180k or 150k. Probably best not to go much below that.

    To make our a little brighter, decrease R5 from 330k to allow more signal through the bright cap. 150k might be a good place to start. Many amps don't have a resistor here and I often add one to tame them at low gain settings. And for a darker sound, either remove the bright cap or the resistor.

    For a bit more headroom (not much really, but it would give a bit), decrease the voltage dropping resistors on the HT bus (R23 and R8) from 10k to 4k7. May raise the voltage on the first gain stage from 220V to 250V ish. Best to do only if the bus caps have been increased in size otherwise it'll hum more.
     
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  15. Stewtech

    Stewtech TDPRI Member

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    Thanks for blessing this thread with your wisdom! I have a lot to aim for, appreciate that headroom suggestion!
     
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  16. LightningPhil

    LightningPhil Tele-Meister

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    Waiting for lunch to cook so did a little circuit modelling.

    For few dB of top boost, bypass the 10k cathode resistor on the 2nd gain stage with a 330nF capacitor. For more mids and top, use 470nF.

    To keep the mod consistent throughout the whole gain knob range, disconnect the capacitor that bleeds some signal from the cathode resistor through the connected pot.

    With 330nF, there should be 10dB more gain at the top of the guitars range compared with the lowest notes. So the lows should remain reasonably tame and not fart out, and the highs should be quite harmonically rich with more sustain. If you like the VOX sound with a treble boost pedal, or the sound of Celestion blues (or both) this may be for you. Certainly won't sound anything like a rounded Fender amp. Maybe put a switch on it...
     
  17. LightningPhil

    LightningPhil Tele-Meister

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    To brighten a dark sounding speaker and give more emphasis on chime, a 100nF cap could be used to bypass the above mentioned 10k cathode resistor.

    And if the effect is a bit too much, any if the cathode mods mentioned can be taimed by adding a resistor in series with the new bypass cap. Suggest 3k3 or 4k7 as sensible starting points.
     
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  18. Stewtech

    Stewtech TDPRI Member

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    This is great, I would like 2 of these amps running in parallel. 1 Vox and 1 Fender sounds incredible!! Thanks Again!!
     
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  19. Gambitpawn1

    Gambitpawn1 NEW MEMBER!

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    R22 is the feed back resistor for the power tubes? This helped tame the overdrive? How much of a volume drop did this cause? I have been looking a "clean mod" for a while now and have been studying the schematic. Now granted, I'm more familiar with ICs and solid state mods then tube mods... thanx for any clarity in advance.

    Also, did you use a logarithmic pot or a regular one?

     
    Last edited: Jan 1, 2020
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  20. Rick330man

    Rick330man Tele-Afflicted

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    Having purchased and experimented with three of these, my ears hear noticeable improvements when you change out V3 with a 1980s production GE 5751. If you make no other investment in upgrading, let this be it.

    The next significant change is the speaker. The stock Celestion G12P-80 sounds tolerable once broken in, but you have to give it a few hours. That said, I am not a fan. A speaker swap helps tremendously. Those maligned Fender Gold label speakers that came stock in Hot Rod Deluxes and Blues Juniors were essentially Eminence Legend 125s. They are every good speakers and sound excellent in these amps - a cheap way to improve your tone. The G12M-25 Greenback also sounds top notch in these.

    If you're going for it all, change the V1 tube. I'm using a GE 12AX7 in V1 of one Monoprice and a JAN Philips 12AX7 in another.

    Power tubes: they do make a difference you can hear. I really liked a pair of old 6P14P-EVs in one of mine. Another got a pair of Ei EL84s. The first give the amp's tone a more "in your face" character. The Ei tone is a little softer but more rounded. JJs sound pretty decent, too.
     
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