Monoprice Stage Right 15 Watt Tube Amp - Schematic Review & Mods

I review the schematic for the Monoprice Stage Right 15 watt tube amp. I'll explain what the components are doing and suggest / brainstorm some mods. This amp is similar to a Harley Benton Tube15 and also a Laney Cub 12R. The schematic is for the Laney and I don't know yet how close it is to the amp I have, but will find out in the next video.
Time Stamps:
0:00 - Intro
2:35 - Power Supply
3:58 - Preamp
23:00 - Output Section
28:40 - Suggested Mods
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[@jcugnoni]

Last comment: the 220k VR3 pot is wired as variable resistor and is in series with the 4n7 cap to form a variable high pass filter. At high frequencies the cap conducts and the direct and inverted signals are bridged which reduces the input to the power tubes. At low freq, the cap is not 'active' and the full signal goes to the power tubes. This circuit is the traditonnal 'Cut' control (high cut) from the AC30. The VR3 pot acts like the opposite of a presence knob by selecting the cutoff frequency. It is good to tame the usually bright EL84 'chime'. Overall this looks like a nice circuit to play with and I look forward listening to what you come up to. Wish you a happy 2024 with lots of amp tinckering 😊

[@KleyDeJong]

Yes, very helpful! It is a Cut control through and through! So the preamp is very close to a JCM800 into a Vox AC15 type output section?

[@Wooburnmusic]

Very very clever man to have knowledge and to understand things like this, well done and thankyou. 🙂👍🇬🇧

[@freakkyt]

Great video as usual 🤘 Possible explanations of those few doubts you expressed: 1. Those HD1 ... HDX symbols appear to be PCB connectors for the offboard wiring, rather than relay switches. 2. The 1M being labelled VR1A suggests it varies together with VR1B ("GAIN"). Assuming those 3 pin connectors are bridged, the cold clipper resistor would get "more bypassed" by the 2u2F and 220n bypass capacitors as the gain pot goes up, so probably a boost to increase gain not just at the grid input. 3. The relay connected to the input socket attenuates the signal by switching the source of the power amp tubes to a point before or after the voltage dividers (R39, R42, R43, R44). VR3 is actually always in circuit, regardless of the relay position, affecting the signals being in phase or not. Maybe a tone control? Really like these videos, keep 'em coming!

[@KleyDeJong]

Very helpful! So on the Gain pot, I think I see what you're saying. Its a dual gang pot. So if you turn the Gain up you move the wiper up (as you're looking at the schematic). On VR1B (Gain) the 100k pot resistance is in series with the 2.2k to ground, so most of the signal moves forward towards the grid of the next gain stage. That also changes VR1A. The 10k cold clipper biases on the cathode. But there is a 2.2uf bypass cap feeding into VR1A. If you increase the gain pot it moves the wiper up so the 220nf cap is basically in series with the 2.2uf bypass cap - fully bypassing the 10k cold clipper. Lots of gain boost because you're introducing the bypass caps. Conversely if you turn the gain down you take out the 220nf cap and the 2.2uf cap runs through 1M of resistance - making it essentially have no effect on bypassing the 10k? So in total the Gain control both controls the voltage divider between gain stages, and also the amount of the cathode bypass capacitor engaged on the cold clipper?

[@KleyDeJong]

Regarding your point 3, I think I see that now! So the relay either sends the signal from the phase inverter to the power tubes OR it introduces the four resistors (R39, R42, R43, R44). R39 and R42 make one voltage divider dumping some signal to ground on that half of the inverted signal. R43 and R44 do the same thing for the other half. Like a preset post phase inverter master. VR3 has to be the Vox Cut control - on this amp called 'TONE'. robrobinette.com/Voicing_an_Amp.htm#Vox_Cut_Control

[@freakkyt]

​@@KleyDeJong[Gain] Correct, that's what it looks like, it would be interesting to test how much it changes the overdriven sound, worth testing it on a JCM800 preamp or similar

[@freakkyt]

@@KleyDeJong [Tone] Good catch! It's the same, down the the cap and res values

[@jcugnoni]

Interesting analysis. Seem a nice circuit. By the way, the weird symbol that you thought were relays are in fact board connectors. So you can simply consider than the corresponding pins are always connected.

[@KleyDeJong]

Helpful, thanks!

[@musterionsurly]

The cub12r has a lot of these board connectors which annoyingly get oxidised and need cleaning to keep the amp working well.

[@WeaponsRemorse]

Amazing breakdown I’ll do some mods myself thanks

[@musterionsurly]

Thanks so much, I've been trying to see what makes the cub12r tick for ages and feel like i can do a handwired with mods now if i ever get the time.

[@jcugnoni]

As you say, the relays on the phase inverter actually select if you send the full signal to the power tube or if you go through a fixed post phase inverter master volume. But the voltage divider of the PPIMV are actually the resistors R39/R42 for the top side and R44/R43 foe the lower side. The relay select if you take the signal before or in the voltage divider. A PPIMV pot would fit well in there instead of the relay.

[@KleyDeJong]

Yes, I'd like to put a PPIMV in there for total control. This isn't a big deal, but to me - calling a preset PPIMV a '1 Watt' mode is pretty inaccurate. I get that amp manufacturers need to market products to guitar players who don't really know, but it kinda bugs me. Isn't wattage a function of how many watts of power the amp can make with the output section set to max clean headroom? So to reduce wattage you'd have to mess with the power supply rail or the output tubes? I have some loose awareness of a VVR circuit.

[@chrisgmurray3622]

I'd be curious to see if a similar circuit to this be used, but haing 6V6's at output, rather than EL84's. Obviously some major substitutions of smaller component values would be needed, for one thing, but I'm curious to see if that type of circuit would have less of the 84's tightness and brightness, but more of the slow bloom and sweetness of a 6V6 chirp.😏🎸

[@tommckeown6970]

What you say are relays; are they not just headers to interconnect boards? Anything labeled HD would be a header.

[@YinFuBG]

Replaced R27 with 3.3k and cut the cold clipper. This changed the distortion curve quite a bit resulting in stronger low mids with more grind and less fizz. Replaced the tone stack caps but this had minor improvement. Best mod that I made was a larger multi-tap output transformer with a jensen Jet 40 watt speaker. Noticeably larger sound stage and better bass response which is a weakness in this amp. Still don't like the bottom end but i run in parallel with a Quilter 25 watt to fix the bass response.

[@jcugnoni]

Also, the 1Meg pot on the V2 cathode side is connected to the cathode via the 2u2. When the pot is on one side, the cathode 10k resistor is actually bypassed by the 2u2 and 220N in series (which gives something close to 200nF bypass). Together with the 10k resistor, that gives gain boost of the high frequencies above 200Hz. When the 1Meg resistor is on the other side, it effectivelly disables the bypass cap and so gives a lower gain with no emphasis of mid frequencies. At least it is how I see it...

[@KleyDeJong]

I reached the same conclusion on a second look. Very cool idea for a dual gang GAIN pot.

[@murrayatuptown353]

Maybe you address this later (I'll listen). More internet opinion says the differences are Laney 12R has a proprietary (their own choice) speaker and a digital reverb module, vs. the Celestion Seventy 80 & spring reverb tank in the MP SR15.

[@user-hy6yq4uk8e]

Dear Mr DeJong. When you went to cover the preamp, why did you trace the HI input to the top of the 3 pin terminal (Header 1 to Header 10)? I own one of these Monoprice15 watters, and all I have is a single instrument input. Can't wait for the speaker to be broken in. Good for the price, it doesn't have an overdrive. It has an adjustable distortion. I have yet to open it up. I will have to see if there's a set of diodes shunt to ground or if it really uses the preamp circuit to push the amp stage. It's funny , I have a little 8 watt solid-state HiWatt someone gave me and it's "overdrive" is a pair of diodes to ground from the 4558 opamp and not in its feedback loop all before the output transistor, hence it's hard clipping or distortion. On the Monoprice, it's sort of nice having it dialable, but I would rather it being switchable and preset the overdrive, er,...distortion.

[@JamesMcCutcheon]

When these amplifiers were first available I purchased 4 of these. 2 still in the box. To me is a 1x12 vox-like amp. I liked the TV front, but then people complained about the reverb, and then the company changed the whole amplifier. I am glad I have the TV fronts. These sound very good and I am sure the MONO group makes these for Laney.

[@TheCameronGlenn]

Great video. I've been trying to track down an issue I'm having with my Stage Right 15R (single input jack). After a year of using it, a tube went bad. For some reason, I had all of the tubes out at the time I put the new one in. At that time, I accidentally put the tubes in wrong -- the EL84s went into slots V1 & V2 instead of V4 & V5. There was a loud bang and the buzzing started. The sound of the buzz gets louder when I turn the volume up. Based on your video, would I be correct to assume the issue is in the pre-amp stage? Also, in your opinion, did I blow a capacitor, or would this be a tube issue?

[@KleyDeJong]

Does it buzz with fresh tubes? I'd be most concerned about putting a 12ax7 in a power tube slot. A bang then buzzing makes sense that you popped a cap. I'd try new tubes. If not fixed then I'd open it up, discharge the caps, and look for residue of a blown cap spilling it's guts all over.

[@TheCameronGlenn]

​@@KleyDeJong I'll give all that a shot, thanks.

[@Turtlehands]

I just put a new set of JJ's in my amp and I had to admit it I had to change the original power tubes a lot quicker than I thought. I think the bias is too high. I never checked the bias I probably should.

[@TheCameronGlenn]

@@KleyDeJong I replaced the tubes and the buzzing is still there. Any ideas on what I should look at next?

[@lasvegasira]

Here's a video on modding the 5-watt version of the amp. kzbin.info/www/bejne/ppqVeoKpq9Jni7csi=0eN0GCrVqtSPrAtE

[@russlora]

think its a Laney Lc15

[@neilslade]

Just add an graphic eq pedal in front or in the effects send and you've got most any tone you like- no need to swap out the speaker. I did swap the 3 12AX7's for 3 lower gain 5750s, much happier with a tamed overdrive. kzbin.info