Announcement
Collapse
No announcement yet.

Monoprice Class A tube amps

Collapse



X
  • Time
  • Show
Clear All
new posts

  • Monoprice Class A tube amps

    I'm surprised there isn't a thread around here dedicated to these little amps. They're not half bad sounding and are an extremely good bargain when you step back and look at the price. From what I can gather, these are supposed to be rebranded Laney Cub8 amps. They seem to always be going out of stock on their site (like right now) if that tells you anything. Anyway, I've been messing with one for over month now and it certainly doesn't disappoint. So far I've added a kill switch for the NFB (doesn't do too much since the amp isn't super clean to begin with) and also added a double throw switch with an eight ohm resistor to make the speaker load 16ohms (drives the power tube a bit more). Both of those mods together give the amp a little more growl. Only issue I have with it (may be a good thing for some folks) is that turning the amp up more than half way makes it a bit muddy (sounds great for early Black Sabbath type tones, but is a little disappointing for singing leads). Seems like the issue is most likely in the preamp stage so I tried swapping out the cheap Chinese 12ax7 preamp tube with a JJ one and it didn't make much of a difference. Also swapped out the factory Ruby 6J6GT with a JJ one too for ****************s and giggles, but that didn't make much difference either (didn't think that it would anyway, just bought one for a spare and wanted to test it to make sure it was good). Overall this is a fun little practice amp and spits out some great blues tones. And it's certainly cheap enough to mod a little. Anyone with one of these care to chime in on what they think about these? http://www.monoprice.com/product?p_id=611705
    Attached Files
    Get superior guitar tone in a small package using this 5-Watt 1x8 Guitar Combo Tube Amplifier with Celestion Speaker from Monoprice!As the title indicates, this guitar amplifier features a 5-watt tube
    Last edited by RyanL; 05-11-2016, 05:22 PM.

  • #2
    I had to do a little digging on their site to find it. Good pricing on small tibe and SS amps there.

    There are a couple of things you mentioned I can make suggestions on. I suspect the amp since its 5 watts may be very similar to the Epiphone Valve Jr. You can find a bunch of mods for those amps that are out there. You may want to look them up to see if any are useful.

    For a preamp tube you may want to try using an Electro Harmonix. They have probably the highest amount of clean headroom out of all the new tubes being made and will also brighten the tone. I'd likely use an EH for the Power tube too. I'm not a big fan of Ruby tubes. They are just rebranded Chinese tubes and the set I tried for my Bassman were so bad I sent them back.

    The Resistor mod on the speaker you mentioned makes no sense technically. The output transformer self adjusts its power to the load so its not going to increase the power in any way. A fixed resistor Resistance is not the same thing as a speaker coils impedance. A coil reacts magnetically and provides more resistance as the frequency increases.

    A fixed resistor has no reactance to AC waves so its only going to provide the correct impedance at one specific frequency. If anything this may provide a change to the dynamic response of the speaker but its not something most would consider to be an improvement in any way.

    You might want to look at buying a higher quality output transformer. http://www.mercurymagnetics.com/page...ejr/EVJ-01.htm I doubt the amp has a very good one in there for its price and since it seems to suffer from saturation issues I'd think that would have the most dramatic effect on it. You could also bias the preamp tubes lower is you have extra unused driven headroom. If you do it right the maximum volume on the knob can also be the maximum saturation you'd ever need.

    Here's some others. http://www.ratvalveamps.com/epiphone...r-sp-879237018
    This one might be your best bet for cleaning the drive up. You'd have to adapt the mod to that particular amp of course. http://www.premierguitar.com/article...the-final-mojo

    This schematic which shows various mods may be similar to your amp. http://www.sewatt.com/files/sewatt/StockVJ_1-3.pdf


    This also lead to a question about your pickups though. If you have hot humbuckers over say 8K, its likely the pickup impedance is causing the drive issue and the amp may be working just fine for normal or vintage pickups.

    The other item might be the speaker. The add says it uses a Celestion speaker. I'm not sure which Celestion is used. None of the manufacturers make a wide range of 8's so and improvement there with something that's cleaner or has a higher SPL (louder) speaker might not be an option.
    In any case you can always plug into a higher wattage cab and see if the saturation decreases. I suspect it might be the same and the saturation is coming from the amp. A 5W amp is rated in clean watts. That may be with the volume cranked 1/2 way up. Above that it may get louder but its all clipped.
    Last edited by WRGKMC; 04-28-2016, 08:55 AM.

    Comment


    • #3
      Thanks for the input. You're probably right about the 8ohm resistor not being the same thing as using a 16ohm speaker (forgot about how impedance changes with frequency - if I remember correctly, the speaker has its lowest impedance right around its resonance frequency). In other words, that was probably a useless mod. It could be that the speaker doesn't handle gain well, but I don't think that's the problem because it happens so quickly as you turn it up. Wish I still had access to a bunch of gear like I did back in the day, I could just plug it into a cab and see what it does. I do have a Johnson practice amp which has a 12" speaker which I could disconnect and hook up to this thing and see what happens, but I'm leaning more towards a power supply issue like you said.

      There's also 15w tube amps on Monoprice's site too that look very tempting. They have more tone controls and reverb too. I may bite on one of those eventually too. Only issue with that one seems that the reverb stinks, but I would rather have it than not and reverb tanks are cheap.

      Comment


      • #4
        Yea, that gain mod is likely your best bet for getting a more linear gain curve from the volume knob. I doubt the speaker itself has anything to do with it. Chances are that speaker is well above 5 watts and doesn't distort at all. All the drive is coming from the electronics.

        A better Transformer can go a long ways towards cleaning it up. A higher SPL speaker will make the amp louder and you can run the amp volume lower which means cleaner. It may be the pot itself too. Some pots ramp up in resistance non linearly. Its it's say a 10K pot you may go from a very low resistance of a couple of hundred ohms up to thousands in a very small turn of the pot. It again can be the way the tubes are biased. Tubes in general use a small signal to produce a much larger copy of itself.

        In a class A amp, the tube is biased 50% on so the wave can go both positive and negative without the negative side hitting the floor or the positive side hitting the ceiling because that would obviously clip the wave. If you bias the power tube hotter then it should be, The positive wave clips too soon as you increase volume. If the bias is too low, the negative cycle bottoms out first. The wave expands on both the negative and positive sides equally, as you increase volume its just a matter of the wave being centered on the gain slop of the tube so both sides clip about the same time. When you have the floor or ceiling clipping before the other side does it creates and unsymmetrical wave and doesn't sound as good. you also loose allot of power too.

        The way you know for sure requires you to use an oscilloscope on the heads output. you then feed it a nice clean sine wave of around 1000 Hz which is about the center of the guitar frequency range and then look at the wave as you crank the level up. If the top wave flattens before the bottom, you have to decrease the tubes bias. If the bottom of the wave flattens first, you need to increase the bias.

        These amps are likely assembly line built with best guess circuits and probably little testing besides making sure they turn on and run. With a small investment you can probably make it run as good as any boutique amp out there. Its just a matter of whether you want to go that route.

        Oh and since the output transformer will influence tube bias, I'd likely focus on that first. Using a Hammond 125ESE for a single ended amp like that should give you a better response. You'd have a choice of impedances from 4~32 ohms too. You could easily wire in some extra jacks so you'd have an impedance choice.

        Comment


        • #5
          Interesting amps

          I record with a small Gibson Goldtone Les Paul Jr 5 watter. I just turn up the volume and add what ever I want with a pedalboard. There's not even a tone control knob on the Goldtone.

          I was wondering at the time how much I would really use it. For recording it's the bomb and between that and a Fender PRRI those two are my main recording amps now. I dropped a JJ Tesla Power tube in to my Goldtone.


          Thumbs up for small recording amps.

          _____________________________________
          Faith is taking the first step even when you don't see the whole staircase.

          Join Date: Aug 2001
          Location: N. Adams, MA USA
          Posts as of Jan 10th 2013: 82,617

          Comment


          • #6
            Originally posted by WRGKMC View Post
            Yea, that gain mod is likely your best bet for getting a more linear gain curve from the volume knob. I doubt the speaker itself has anything to do with it. Chances are that speaker is well above 5 watts and doesn't distort at all. All the drive is coming from the electronics.

            A better Transformer can go a long ways towards cleaning it up. A higher SPL speaker will make the amp louder and you can run the amp volume lower which means cleaner. It may be the pot itself too. Some pots ramp up in resistance non linearly. Its it's say a 10K pot you may go from a very low resistance of a couple of hundred ohms up to thousands in a very small turn of the pot. It again can be the way the tubes are biased. Tubes in general use a small signal to produce a much larger copy of itself.

            In a class A amp, the tube is biased 50% on so the wave can go both positive and negative without the negative side hitting the floor or the positive side hitting the ceiling because that would obviously clip the wave. If you bias the power tube hotter then it should be, The positive wave clips too soon as you increase volume. If the bias is too low, the negative cycle bottoms out first. The wave expands on both the negative and positive sides equally, as you increase volume its just a matter of the wave being centered on the gain slop of the tube so both sides clip about the same time. When you have the floor or ceiling clipping before the other side does it creates and unsymmetrical wave and doesn't sound as good. you also loose allot of power too.

            The way you know for sure requires you to use an oscilloscope on the heads output. you then feed it a nice clean sine wave of around 1000 Hz which is about the center of the guitar frequency range and then look at the wave as you crank the level up. If the top wave flattens before the bottom, you have to decrease the tubes bias. If the bottom of the wave flattens first, you need to increase the bias.

            These amps are likely assembly line built with best guess circuits and probably little testing besides making sure they turn on and run. With a small investment you can probably make it run as good as any boutique amp out there. Its just a matter of whether you want to go that route.

            Oh and since the output transformer will influence tube bias, I'd likely focus on that first. Using a Hammond 125ESE for a single ended amp like that should give you a better response. You'd have a choice of impedances from 4~32 ohms too. You could easily wire in some extra jacks so you'd have an impedance choice.
            Well, I had the chance today to connect this little tube amp to that Johnson modeling amp I mentioned in a previous post. Put it this way, I never heard anything like that ever come out of that Johnson amp. Always loved class a amps, just never owned one. It really made me want to put a larger speaker into this amp though. I believe I can squeeze a 10 incher into there as long as it's not too deep and/or doesn't have a large magnet. Anyway, the tone issue is definitely present with another speaker though. And the more I mess with it the more I believe it's an issue in the preamp stage, possibly right in the tone/volume circuit. I'm not hitting any sort of limit because for one, it's a very gradual decrease in tone as the volume is increased, plus playing softer or adjusting the volume on a guitar still yields the same test results. Very strange they would have it designed this way (maybe it's a halfassed attempt to decrease noise as the volume is cranked). I expect there's a capacitor who's value needs to be changed or completely eliminated altogether. I'll have to look into that next time I crack open the thing. Hopefully it's something I can actually work on (many of the components are really small in this thing).

            Good info about biasing class A amps; didn't realize that this would need to be done with a single tube (shows you how much I know about tube amps). Don't know how I would even go about biasing an amp like this with no controls to speak of. All there really is, is a micro pot marked hum on the main board which I'm almost certain is low voltage anyway. And before you suggest adjusting it, already tried turning it both ways for testing before I returned it to it's original position. Thanks again for all your input.

            Comment


            • #7
              Had a chance to map out and create a schematic for this amp (since I couldn't find one for this amp anywhere on the web). Attached a picture to the original post. A lot of small components and tracers which were hard to follow (had to ring some of it out with a multimeter), but I believe I got it all. Been doing a lot of research on tube amps in general and thought I was onto something with resizing cathode bypass caps to help with my tone issue and tried a few things. Seems to do the trick for a lot of small Fender amps with dull tone, but didn't seem to do much in my case. I have to dig into this a bit more. Maybe someone a bit more knowledgeable can take a gander at the wiring diagram above and throw out a few ideas. I ended up getting rid of the 8 to 16 ohm switch since it didn't seem to do a whole lot and instead got a DPDT switch and used the resistor I had along with another to create an attenuator switch. Very sweet mod. I have a few other things I want to take a stab at before I throw it all back together, but I'll post back if I figure out anything else or other simple mods.

              Comment


              • #8
                Have you made any progress on this? I have ordered one and will be looking at starting with some new tubes, probably ehx and maybe swapping the speaker for a Weber alnico. Other than that I was wondering if swapping out some of the components for better quality parts might have a positive effect. EG: orange drop caps etc...

                As a base for a project this looks like it could be a lot more fun to play around with than assembling a diy amp as it is already running, and cheaper than most kits.

                Thanks a lot for the information you and the others have posted in this.

                Comment


                • #9
                  Originally posted by canajien View Post
                  Have you made any progress on this? I have ordered one and will be looking at starting with some new tubes, probably ehx and maybe swapping the speaker for a Weber alnico. Other than that I was wondering if swapping out some of the components for better quality parts might have a positive effect. EG: orange drop caps etc...

                  As a base for a project this looks like it could be a lot more fun to play around with than assembling a diy amp as it is already running, and cheaper than most kits.

                  Thanks a lot for the information you and the others have posted in this.
                  Sorry I haven't posted back in a while, I didn't really think of this thread I made until now since I'm about to undertake a few more mods. To answer your question, yes I did make a little progress with this amp. I believe that I jotted a lot of things down, but I'll need to dig it all up later. Off the top of my head I did a few things to help eliminate the muddines. I eliminated the plate bypass capacitor (c21 looking at my drawing above) to help brighten it up. Plus I believe I may have lowered some of the values of the resistors in the preamp stage and may have reduced some of the values of the coupling capacitors (higher values seem to block a lot of higher frequencies). Also reworked the wiring to eliminate the wonky cathode follower design they had in this amp. Didn't know what I was looking at at first, but once I figured out I was really confused since it's not needed when you have one tone pot and only one gain stage. Anyway, it sounds slightly better wired up the conventional way. Also added some cathode bypass caps to the mix too for both sides of the 12ax7. Played around with the values a bit on those but think I ended up going with smaller ones like .22uf after some trial and error (need to get back to you on that value but I do remember 3,2,&1uF ended up being too high). I also took a look at the tone circuit while I was reworking the cathode follower design and had a couple ideas. The tone control is really no different than the one in a guitar so it seemed rather redundant. I almost made it into a master volume control, but opted for a big muff tone control instead (one way is a bass cut and the other way is a treble cut). Most of the soldering can be done right at the tone control pot (had to scrape away one of the tracers and add a couple capacitors and resistor to the pins there). The Amp is much more versatile now. I don't believe I mentioned it in one of my posts above, but I also added a switch for the NFB and changed the value of the resistor too while I was at it (believe I put in a lower value one to let more of the negative feedback in). Now I essentially have a clean/dirty switch. Kind of a drag having a clean/dirty switch on the back of the amp so I had plans to add a 1/4" jack in place of the switch and make it foot switchable. I believe that was about all I've done in a nutshell.

                  You have any luck with this amp and end up replacing the speaker? I'm sure that a better speaker would clear things up a lot. I was leaning towards the ten inch warehouse veteran speaker since it looks small, but let me know if the one you got ended up fitting and sounding good.

                  Edit: Here's a good tool I found to figure out resistor & capacitor values: https://www.ampbooks.com/mobile/ampl...ass-capacitor/
                  There's also a big muff tone calculator floating around but I'll need to find the link.
                  Last edited by RyanL; 02-02-2017, 10:34 PM.

                  Comment


                  • #10
                    No idea why I did not get notified that you had replied ryanl. I was just taking a look to see if I should be keeping a bookmark on this page.

                    I went for the simple stuff, got an 8" Weber classic series speaker (I will have to double check the model), which did a lot to get rid of some of the fuzziness. I may change this yet again for one of their blue pups.

                    Next was changing the tubes, I had a fomoco 6v6, can't recall who actually made it right now, but I just grabbed it from the pile simply because it has ford logo on it, and then I swapped the 12ax7 for a tube store premiere unit. Both of these helped to deepen it a bit while removing the harshness on low volume but helping it sound a bit more clean when cranking it up.

                    As for the electromical type stuffs, I kept it simple, at this point I only installed the bitmo voice mod which now gives me 3 different sound types, clean, fenderish & marshally, not much different from your clean dirty switch.

                    At this point I am still using it once a week, its good amp with a baby in the house, i switch between this and a modded valve jr. My next idea for the monoprice is to maybe replace the transformers with some better quality units.

                    I would be interested in seeing how you reworked the wiring if you don't mind sharing that, being constantly tired from having a baby makes the other work you've done way over my head these days.

                    Thanks

                    Comment













                    Working...
                    X