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Amp Modding Questions

Discussion in 'Amps and Cabs [BG]' started by PennyroyalWe, Sep 13, 2020.


  1. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    Hello all,
    Some of you may be familiar with the Monoprice Stage Right 1x12 15w tube combo. I’ve had one for a few years now, and it’s a great little guitar amp. I picked up another one locally for $100, and I’m going to mod it to be more bass geared. I know it will never be a great bass amp, and that’s not the point. It’s just a fun project with (hopefully) cool results. It comes stock with a Celestion 70/80, a speaker that doesn’t garner much love, and a partial open back. My first thoughts are 1. Swap the speaker 2. Close the back, but playing my p bass through it today it didn’t sound that awful. It has me thinking maybe my money would be better spent first taking it to the local amp tech to get the output transformer upgraded and the capacitors changed, and the bass roll off (that these allegedly have) removed. As I understand it there’s no drop in replacement for the transformer, so what am I looking for in a better one other than it being larger? Also, I’ve heard a few people say that bass frequencies ruin reverb tanks, is that true? I kinda like the option of putting some reverb on the bass, but if it’s going to cause problems I’d rather have the tech bypass it while they’re in there. Maybe put a bright/dark switch in it’s place?*I’m not an amp whizz, so this is secondhand understanding, and hopefully the amp tech will know what I’m talking about when I explain my objectives*. I like how efficient the speaker is, and it is an 80w speaker being driven by 15w, but with it’s stated low end of 80Hz, will it be damaged by the low E’s frequencies? Eventually I’d like to build a larger pine cab for this combo, and have been considering this speaker as an eventual replacement "Goldwood GW-1238/PA 12" PA / Musical Instrument Speaker" from www.parts-express.com!, would that speaker actually do any better than the 70/80 at producing bass frequencies? Lastly, and the easiest to do, I’m considering swapping the V1 12AX7 with either a 5751 or 12AT7 to get more clean headroom. I’d lose some volume, but once it breaks up too much any volume I would get from the 12ax7 is useless. The lower gain tube will let me turn up the gain higher than I could otherwise...but which one is better suited for the purpose? Anyway, thanks for any input on the matter!
    9E933C3D-C73A-4801-AF6B-654321AB798D.jpeg
    TLDR: When making a guitar combo into a bass combo, what electronics upgrade makes the biggest differences?
     
  2. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    My thoughts are for the cabinet portion and driver. For this to be a win, those have to be addressed too. For sure, shop sensitivity ratings. 3dB is considered an audible difference in loudness. See what US Speaker has to say for your box dimensions.

    You might find the speaker enclosure better suits a smaller bass driver than the guitar driver currently in it. And you really should consider completely closing it up, possibly needing a port too. That may not be hard to rear port using what's (not) there.

    Anyway, good luck!
     
    smogg and PennyroyalWe like this.
  3. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    I’ve used a few cab calculators, still haven’t settled on the exact dimensions I want, as storing it is also a consideration. My hesitation on completely closing the back is that the tubes then won’t have any way to vent, and I don’t want to accidentally fry the amp or catch it on fire. I have considered making a 2x8” baffle when I build the new cab...but I also like the 1x12. Maybe I’ll make a baffle for each and try both.
     
  4. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    I see. I didnt realize you were actually building an enclosure. I thought just working with what is there.
    Yeah if the combo was designed with no other ventilation than the open back it does complicate things.
    Since your building a new combo cab, add vents to it?

    Dunno about the driver linked. Guitar combos are usually not so deep. It wants a sealed cab for bass use I'd bet. Sensitivity is average, not too low. Affordable. Probably do better in a bigger box than the combo has.

    2x8 might need to be even bigger, depending.
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Why in the world would you need to "upgrade" the transformers and caps? Really, is this something that your tech suggested or was it something that you read on the internet.

    You would probably improve the amp's performance more by changing your socks ;)
     
    Dabndug, MCF, Geri O and 13 others like this.
  6. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    For the time being, I’m running with the factory combo housing, but I plan to build a new housing for it once the community center woodshops reopen again (I’m staying optimistic that will happen soonish?).
     
  7. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    Just from what I’ve read. The transformer on this is small, it’s the same one used on the 5w amp too. I’ve read a larger one will allow for better bass response. The caps, as I understand it, would be changed so the EQ better adjusts the bass’s frequency range.
     
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Now would be a good time to learn about how amplifiers actually work, and why proposing to change something without actually measuring how they perform and discovering what the confirmed limiting factors are.

    It would also be a good time to think about possibly a better designed used amp might be a better VALUE for your application rather than pouring money into a budget amp with questionable modifications IMO.
     
    iiipopes, AudioTaper, Frank77 and 9 others like this.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I see it as a good learning opportunity whether or not you come away with a good practice amp or write the adventure off.

    My advice for modding the amp is to adopt what works. Don’t try to design, find one that you like and copy it. Do your homework. There are many examples of low wattage amps out there. There’s nothing wrong with a 12AX7 in the first stage. It’s the components attached to it and the power supply voltage.

    If you don’t have a schematic for your amp, start by tracing it out and draw one, then measure the voltages at each stage. That will tell you what you have. Then decide what to adopt and how to adapt it. You can get that far without causing any issues with your current amp.
     
    BassmanPaul, abarson and PennyroyalWe like this.
  10. Redbrangus

    Redbrangus Supporting Member

    Nov 19, 2018
    Under The X In Texas
    I'd be willing to go along with the concept of closing up the back to see if you can coax a bit more bass response out of it, but investing any more money or effort than that seems pointless. Yes, if one wants to use it as a vehicle to learn more about amps, then there might be something to that, but I can't imagine paying a tech's bench rate to do stuff like replacing the (functioning) power transformer, trying to bypass the (suspected) LF roll-off, or adding a bright/dark switch. To me, that seems analogous to paying a mechanic to bolt a bunch of 'speed' parts on an old Buick you inherited from your grandma. In fact, I'd be somewhat surprised if you found a tech that would even be willing to bother with it, it's such a pointless concept.

    And I wouldn't consider $100 much of a bargain for that amp -- I've bought several used, brand-name, small bass amps for that and they were usable 'as is' for a lot more than just a bedroom practice amp.

    I'm sorry to be such a Negative Nellie about it, but this is one of the stranger concepts I've seen on TB... and that's saying something. :D

    Edit: To provide further rain on this little parade, I'll suggest that the Goldwood driver you referenced is likely only marginally better than the speaker you have. If that.
     
    Last edited: Sep 14, 2020
  11. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    Oh I’m aware it’ll be the lifted Miata of amps. 8343C53A-D138-4D7D-9624-F933D8421212.jpeg
    completely pointless, other than it can be done. The tech I know in town isn’t too expensive, only cost me $60 to have an old Gibson amp completely recapped last time I went to him, so I’d expect a similar expense for recapping this. I’ll admit the notion for changing the caps is one I picked up from other TB threads about alter guitar amps for bass, it seems a very common suggestion. Not sure why it wouldn’t be needed on this one? As for more practical amps...that’s not the point. I have my Rumble 500 for an actual bass amp, but there’s very little to nothing for small bass tube combos. IMO the musicmaster bass amp is/was one of the coolest amps ever, so I’m trying to roughly approximate that...but hopefully with more bass response. Alas, if the overwhelming opinion is that a larger transformer won’t change anything, I’ll save the $30 on that, and if the speaker isn’t any improvement, there’s another $50 I’ll hang onto. I do find it odd how there’s multiple articles that recommend changing the OT to get better tonal range, though. Is it all just a sales pitch?
    The "Overlooked Upgrade": Guitar Amplifier Output Transformers
    Why and How to Replace Your Amplifier’s Output Transformer
     
    mikewalker likes this.
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    All myths unless analysis and measurements say otherwise.

    There is a LOT of very poor information on the internet. Without understanding the particular circuit and analysis, it’s all just mythology. This may be yet another example of such.
     
  13. Redbrangus

    Redbrangus Supporting Member

    Nov 19, 2018
    Under The X In Texas
    I suppose a case could be made for replacing an output transformer in a Marshall, Fender or Vox, but not in an off-brand, Far-East designed and manufactured generic amp. If you want to hot-rod something, I think you'd be better served to start with something more normal and widely distributed. And for which there is a lot of documentation and on-line support. And I think you could buy a tube amp kit for not much more than you're considering spending on this project. IMO, anyway.
     
    PennyroyalWe likes this.
  14. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    My max budget for this is under $250 at the end of it all. that’s what they sell for new, and I won’t go over that. Most amp kits I’ve seen are more in the $500-800 range, and I have to trust myself not to lose all those tiny parts haha. This amp is the same as a Laney Cub 12R, pretty sure from the same factory. The older gen MP15 with the TV front had the same faceplate even. I wouldn’t mod my AC15, as it’ll affect the resale value. This amp has no resale value, so nothing to lose by modding it. From what I’ve been told so far, it sounds like I might be best off just closing up the back and calling it good. I’ve got some
    spare plywood in the garage, total cost of mods $0 this far...though I do still want to make a bigger housing for it eventually.
     
    Redbrangus likes this.
  15. Redbrangus

    Redbrangus Supporting Member

    Nov 19, 2018
    Under The X In Texas
    If you haven't discovered Rob Robinette's tube amp site yet, I recommend it highly.

    Amp Stuff
     
    ddnidd1 and PennyroyalWe like this.
  16. abarson

    abarson

    Nov 6, 2003
    Santa Cruz
     
  17. PennyroyalWe

    PennyroyalWe

    Sep 2, 2018
    Oregon
    Thanks for that, hasn’t heard of it before!
     
  18. Redbrangus

    Redbrangus Supporting Member

    Nov 19, 2018
    Under The X In Texas
    I hadn't really thought about it, but it could be that your Asian amp employs circuits that have the same familiar heritage as do many of the more popular American/British amps. It would be an informative project to draw out the circuit diagram just as @beans-on-toast recommended -- it may be more 'normal' than I had originally thought. There may well be a lot to recommend it as a mod test-bed for your own enlightenment, but I wouldn't pay a tech to mess with it. And I don't think the output transformer is necessarily a place to start -- I doubt that's the limiting factor in the amp's performance. Nor is the 12AX7 preamp tube, IMO. But as I say, it might be a decent vehicle to start learning some more about how tube amps work.
     
  19. TonH

    TonH

    Jan 26, 2011
    The Netherlands
    On another forum I found this thread:
    $165 tube amp new??? - Stage Right - Gearslutz
    In the discussion they mention that the amp has a PCB and surface mounted devices (SMD). That makes the amp even more difficult to mod if one doesn't have that much experience.
     
    PennyroyalWe likes this.
  20. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    The creamy Tolex is cool. Good luck with the innards. Just don't let the blue smoke out.
     
    PennyroyalWe and mikewalker like this.
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Apr 15, 2021

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