Monoprice Stage Right SB20 50 watt tube amp

[Pentoad]

I just ordered a 3rd one at the new sale price! See first post.

I have issues.

LoL at $233 hard to pass up...even for parts. Wonder if the price will drop further?

 

[backporchmusic]

They had a $25 off coupon yesterday so I ordered one for $208. How could I not?

I will be keeping an eye on this thread to learn more about it, see what sorts of mods people might get in to.

 

[Ronquest]

They had a $25 off coupon yesterday so I ordered one for $208. How could I not?

I will be keeping an eye on this thread to learn more about it, see what sorts of mods people might get in to.

Nice! That's what I paid for my last one. Check the bias as soon as you get it. Both Pentoad's and one of mine was way too hot. I'll be making a video ASAP for biasing this amp and will post it here.
Price went back up to $399. Now I'm waiting for the Tube 15 to go on sale!!!

 

[Pentoad]

Yeah their prices fluctuate like crazy. Saw their 30W and cab for $325 Sunday night and now it is +$600 and out of stock? BTW I too bought another one for $233 (missed the coupon) and got it Friday! Tonally seems to sound better than the first one (have to check where bias is set at but same set and brand of tubes) I bought and doesn't have the weird volume drop when mid pot is dimed. Going to gut the first one and try to sketch the layout.

 

[Pentoad]

BTW I found out my coworker has the 15w laney knock-off. He is going to bring it to work tomorrow as he is thinking of selling it. Found the schematic of it and from the looks of it the dang thing is a copy of the 2204 but with effects loop, reverb, and EL84 tubes. I see they go for ~$250 now but I am hoping to do some horse trading with my co-worker who apparently bought it for half that price. Jeebus, I don't need more cheap amps but whatever you guys have seems to be more contagious than covid...must be a variant of covid too because I seem to have lost my sense of common.

 

[solarburn]

BTW I found out my coworker has the 15w laney knock-off. He is going to bring it to work tomorrow as he is thinking of selling it. Found the schematic of it and from the looks of it the dang thing is a copy of the 2204 but with effects loop, reverb, and EL84 tubes. I see they go for ~$250 now but I am hoping to do some horse trading with my co-worker who apparently bought it for half that price. Jeebus, I don't need more cheap amps but whatever you guys have seems to be more contagious than covid...must be a variant of covid too because I seem to have lost my sense of common.

I have the 15 watter. I can't stop playing it. It's my living room combo. Going straight in or thru my new Guv'nor pedal(instant Marshall)it rocks. I did put a Anniversary H30 In it but it sounded fine with the 70/80. Only shortcoming that's obvious is it's boxy sounding due to the small cab and speaker baffle is barely useable.lol It has a cab extension tap.

I've had it since 2019. Still p!ay it all the time. Despite cheap cab build the circuit sounds excellent. Fun grab and go jam amp!

 

[backporchmusic]

Mine is arriving today, looking forward to checking it out. Will definitely keep that bias issue in mind.

I've had the 15 watt (original, top controls design) for many years with no issue. I put some better tubes in it and a WGS ET65 and have always been impressed with the sound. It was my primary amp in our practice space, but our first outdoor gig tested its headroom, so I've moved on to an Ampeg GVT 52. Still have the MP 15 though, I see absolutely no reason to get rid of a very lightweight great sounding amp.

 

[BmfToolco]

After seeing this thread last week I ordered the sc20 on sale with the coupon for $208. I received it yesterday.

At first it wasn’t so good but after inspecting it I found one of the power tubes wasn’t making a good connection. After pulling it and putting it back in it seems to work fine. The amp even sounds good. Mine doesn’t seem to have the problem with the overdrive channels lack of volume either.

Thanks for the tip MF’ers

ETA mine came with 3 ruby 12ax7, 1 Phillips 12at7 and 2 psvane 5881 tubes

 

[Ronquest]

Issues and Resolutions
As we find problems I will post and list them, then copy to the first post to keep thinks organized. Maybe we can help others with this amp.

1. FX Loop Jacks, loss of contact when cables removed (cheap jacks). Result in loss of volume, cutting in and out. Fix - re-tension Jack contacts (springy part), or replace jack with quality jack. Had this problem on 2 of the 3 amps I just got so far. I would imagine a lot of amps have been returned for this alone!

2. Big volume difference of channels when levels set the same. All 3 of my amps. I think the stock amp is dropping to much gain in one of the stages.

3. Reverb could be stronger. I don't care that much, but may look into it.

4. Bias on some amps is way too hot and should be in the 30-35 mA range.

5. Foot Switch (2 button) doesn't work with pedals that have LEDs. Fix - snip the LEDs or buy a non LED pedal.

6. Monoprice list the amp as using 6L6 tubes, but now seem to be shipping with 5881s. No Issue other than their listing. The Bias will still be in the 30-35 mA range with either tube. The 5881s are good tubes, I will test with some 6L6s, but I don't expect to be surprised.

 

[BmfToolco]

Issues and Resolutions
As we find problems I will post and list them, then copy to the first post to keep thinks organized. Maybe we can help others with this amp.

1. FX Loop Jacks, loss of contact when cables removed (cheap jacks). Result in loss of volume, cutting in and out. Fix - re-tension Jack contacts (springy part), or replace jack with quality jack. Had this problem on 2 of the 3 amps I just got so far. I would imagine a lot of amps have been returned for this alone!

2. Big volume difference of channels when levels set the same. All 3 of my amps. I think the stock amp is dropping to much gain in one of the stages.

3. Reverb could be stronger. I don't care that much, but may look into it.

4. Bias on some amps is way too hot and should be in the 30-35 mA range.

5. Foot Switch (2 button) doesn't work with pedals that have LEDs. Fix - snip the LEDs or buy a non LED pedal.

6. Monoprice list the amp as using 6L6 tubes, but now seem to be shipping with 5881s. No Issue other than their listing. The Bias will still be in the 30-35 mA range with either tube. The 5881s are good tubes, I will test with some 6L6s, but I don't expect to be surprised.

Great info. Glad we have you on our side.

On another note I find it interesting that these come with better tubes than my “unnamed” boutique amp, that I can’t get to even imitate a clean tone.

 

[Ronquest]

I had to, just for fun!

 

[Ronquest]

SB20 BIAS Adjustment Points and Pots.
- If you don't know what you are doing, then don't do it, take it to a tech. 480V inside!
- Measurements (mV) taken from each test point to ground.
- I set one side to desired mA and then listen to the speaker hum and adjust the other one to get the quietest position.
- You may have a few mA difference and that is ok as long as your bias is not too hot.


-

 

[Pentoad]

So on my amps I notice the primary OT leads are flipped and don't correspond to the colors written on the board. I flipped them on one of the amps to what the board stated but that was a bad idea as the amp quickly oscillated like crazy...I am guessing due to polarity being flipped? @Ronquest do you notice that on your amps?

On a side note, I ended up getting the 15W monoprice combo from my co-worker (paid him what he paid). That little sucker rocks. I have a ~8.5k hammond OT from another project that I plan on installing with a 4/8/16 ohm switch along with a V30 or WGS green beret. I checked the bias per what I found in thread and the test point by the trim pot is WRONG location. You need to test it at different location. At the trim pot you should see about 7.5V not 12.5V.

Ok now back to the regularly scheduled SB20 amp channel.

 

[Ronquest]

So on my amps I notice the primary OT leads are flipped and don't correspond to the colors written on the board. I flipped them on one of the amps to what the board stated but that was a bad idea as the amp quickly oscillated like crazy...I am guessing due to polarity being flipped? @Ronquest do you notice that on your amps?

Ya, the amp will let you know if your wires are crossed, LOL Don't be concerned about the color, just that it works. It's a crap shoot sometimes, so I don't cut my OT wire lengths until I know for sure.
I'm waiting to see if that 15W gets below $200 and then I may grab one. On a side note I have about 17 amps to build or I can just look at them parts on the shelves and buy another amp......////

 

[BmfToolco]

Update.

After playing my sb20 for a couple of evenings. Which I was enjoying. I love the clean channel. I’m also impressed with the reverb. Not the best but it’s far from the worst built in reverb. I can deal with the overdrive channel. Sounds a bit flat like a solid state to me.

To the point my channel switch for the clean/overdrive crapped the bed. It randomly would switch back forth on its own then quit switching entirely. I can still get it in to overdrive with foot switch. Problem is I don’t have a non led dual latching switch laying around. I thought that was odd. Everyone should have one of those right?

Anyone else have problems with the switches?

 

[Ronquest]

The foot switch usually overrides the front switch. And since the jacks are crap, the jack may not be closing it's contacts. So your front switch may be ok and the issue is the jack.

I've found the Drive Channel sounds best with the gain at 3 or less.

 

[BmfToolco]

The foot switch usually overrides the front switch. And since the jacks are crap, the jack may not be closing it's contacts. So your front switch may be ok and the issue is the jack.

I've found the Drive Channel sounds best with the gain at 3 or less.

It’s the opposite of what you said. Switch doesn’t work at all and the jack does.

 

[Ronquest]

It’s the opposite of what you said. Switch doesn’t work at all and the jack does.

You may be correct, but reread my post. It's possible that the jack is messing up your front switch. If the foot controller jack contacts are not closing (when unpluged) and making a complete circuit, it won't let the front switch work.

 

[Workerunit]

SB20 BIAS Adjustment Points and Pots.
- If you don't know what you are doing, then don't do it, take it to a tech. 480V inside!
- Measurements (mV) taken from each test point to ground.
- I set one side to desired mA and then listen to the speaker hum and adjust the other one to get the quietest position.
- You may have a few mA difference and that is ok as long as your bias is not too hot.

View attachment 127528
-

Just so I am clear here, with speaker connected, amp power on and standby off, you are grounding your DVOM negative lead to the chassis and using the positive lead to probe the Bias Test Points as shown in your photo and adjusting each Bias Test Point to read in the 30-35 mA range?

 

[Ronquest]

Just so I am clear here, with speaker connected, amp power on and standby off, you are grounding your DVOM negative lead to the chassis and using the positive lead to probe the Bias Test Points as shown in your photo and adjusting each Bias Test Point to read in the 30-35 mA range?

Close.
Speaker connected (always if the amp is on)
Power and Standby - ON (playing positions)
Volumes all at 0
Ground DVM neg to the chassis. Better if you can alligator clip the ground to chassis so you don't have both hands in there.
Set DVM to mV to measure voltage.
V / R = I ( Voltage Divided by Resistance = Amps ) The bias resistor is 1 ohm. If you measure 35mV, then it's .035V / 1 = .035A or 35mA
Bias Test Points set to 30-35mA range
No need to have reverb tank connected.
Recheck after ~10min, (amp warmed up) to make sure the BIAS has stabilized.