Monoprice Stage Right SB20 50 watt tube amp

[Workerunit]

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.

Thank you, when I typed "standby off" I meant standby in playing mode.
I just received one of these amps this week. I look forward to any advise you can give on modding this interesting amplifier.
I would like to give mine a bit more gain, if possible.

 

[Ronquest]

Thank you, when I typed "standby off" I meant standby in playing mode.
I just received one of these amps this week. I look forward to any advise you can give on modding this interesting amplifier.
I would like to give mine a bit more gain, if possible.

Ah, ok, the whole Standby on/off is confusing, especially when manufactures label them differently. Maybe label them REST / ROCK. I don't put a standby switch on any of my built amps.

As far as more gain, I think the 2nd gain stage of the Drive Channel is a 10K cathode, so it could be lowered and bypassed and can be done without removing the board. The 3rd stage is 2.7k cathode and not bypassed, so there is gain to be had. I'll try a few things this week and let you know.

 

[BmfToolco]

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.

Copy that. I misunderstood your previous post. I will check that out.

Thanks

 

[Workerunit]

I check my BIAS it was about 34.8 - 35.2 mv, I adjusted it to about 35.5-35.7 mv.
Mine has the 5881 tubes in it. I would be very interested in seeing if this thing can be made to have more gain.

 

[PlayinOutTonight]

What kind of reverb tank does the SB20 Head have in it...? I just ordered one up, and I'm wondering if an upgrade to a TAD or MOD tank might produce a better sounding reverb...? Anyone tried doing that yet...? Any idea which model reverbs might be equivalent replacements....?

 

[solarburn]

What kind of reverb tank does the SB20 Head have in it...? I just ordered one up, and I'm wondering if an upgrade to a TAD or MOD tank might produce a better sounding reverb...? Anyone tried doing that yet...? Any idea which model reverbs might be equivalent replacements....?

I used the Mod tank in my Monoprice 15 watt combo. 100% better than the stock reverb the 2019 amp had.

 

[Ronquest]

What kind of reverb tank does the SB20 Head have in it...? I just ordered one up, and I'm wondering if an upgrade to a TAD or MOD tank might produce a better sounding reverb...? Anyone tried doing that yet...? Any idea which model reverbs might be equivalent replacements....?

The specs I found are for tank part # tpsb2eb1c1b This is the stock tank in my SB20.

Impedance:
INPUT - 600 Ohm
OUTPUT - 2250 Ohm

LENGTH - 9.25"
POSITION - Horizontal

 

[Marcomel79]

I find it interesting you mentioned getting this amp as a donor for parts to build something else. I'm new to building and built my first amp last year but I'm always thinking about how to build another with minimal investment.

Welcome to the club!

 

[gumball]

(deleted)

 

[orange.kangaroo]

The specs I found are for tank part # tpsb2eb1c1b This is the stock tank in my SB20.

Impedance:
INPUT - 600 Ohm
OUTPUT - 2250 Ohm

LENGTH - 9.25"
POSITION - Horizontal

Interesting. For fun I swapped in a 9AB3C1B three spring. It's a bit quiet, but it would seem the little 4458D opamp that drives the reverb isn't up to the job as-is. I'm tempted to boost the signal using an external amp.

 

[pbecker314]

Hi Guys,

I have been gigging with a Stage Right SB20 head running into a Mesa compact rectifier 2x12” V30 closed back cab for about 3 years now and I really love this rig as a clean pedal platform. It’s the best (and most consistent) tone I have ever gotten on stage over many years of gigging. In fact, like it so much that I bought a second SB20 head to keep in the car as a backup. I actually had to use the backup head recently when a power tube in my main head went bad at a show. I swapped in 2 new power tubes when I got it back home and it sounds great again. However this thread is making me wonder if I should be checking the bias on the two heads. I have done lots and lots of electronic work inside guitars, but relatively little inside amps. If someone could perhaps please kindly post the detailed procedure for performing the bias on the SB20 I would be truly grateful! And actual photos of the PC board as a guide would be even better! Thanks in advance!

 

[Ronquest]

Hi Guys,

I have been gigging with a Stage Right SB20 head running into a Mesa compact rectifier 2x12” V30 closed back cab for about 3 years now and I really love this rig as a clean pedal platform. It’s the best (and most consistent) tone I have ever gotten on stage over many years of gigging. In fact, like it so much that I bought a second SB20 head to keep in the car as a backup. I actually had to use the backup head recently when a power tube in my main head went bad at a show. I swapped in 2 new power tubes when I got it back home and it sounds great again. However this thread is making me wonder if I should be checking the bias on the two heads. I have done lots and lots of electronic work inside guitars, but relatively little inside amps. If someone could perhaps please kindly post the detailed procedure for performing the bias on the SB20 I would be truly grateful! And actual photos of the PC board as a guide would be even better! Thanks in advance!

Hi. Page 2 has the bias info and a detailed picture. You should know amp safety as far as the "high voltages inside". Amp voltages can kill you. This amp runs 485 Volts.
Read this post if you haven't there are a few things to look out for.

 

[pbecker314]

Hi. Page 2 has the bias info and a detailed picture. You should know amp safety as far as the "high voltages inside". Amp voltages can kill you. This amp runs 485 Volts.
Read this post if you haven't there are a few things to look out for.

Thanks man! Much appreciated!

 

[sixstringslinger]

I just picked one of these guys up and apparently it's got a known issue that the 100w version (SB100) suffers from too. When switching channels via footswitch, the volume drops and fades in. Has anyone encountered this problem and figured out a fix? Low quality relay?

Also, any solution to the low output on the OD channel when compared to the clean channel?

Last, did anyone find or trace out the circuit and upload a schematic?

 

[LoTech]

Still looking for a schematic. Any luck?

 

[sixstringslinger]

I e-mailed Monoprice tech support for a copy of a schematic or an authorized repair tech and they acted like I was asking for plans for military weapons guidance systems. They've very anti-right to repair. They refused to send me a schematic and would only take it back and give me a credit to wait for them to come back in stock to buy a new one. This thing is probably a straight copy of another manufacturers amp and they don't want to sued for knocking it off.

 

[Ronquest]

This thing is probably a straight copy of another manufacturers amp and they don't want to sued for knocking it off

Id say nope. Some similarities, but not a direct copy, or it would be a better amp.

 

[sixstringslinger]

Id say nope. Some similarities, but not a direct copy, or it would be a better amp.

I agree. I'm referring to copying another production amp circuit but with cheaper components (transformers, caps, pots etc.). I'm in no way implying the amp is junk even with the cheaper components. Just the fact that they don't support repairing their amps by 3rd party techs so their "trade secrets" don't get discovered. I'm 99% happy with this amp, other than the volume lag when switching channels. That makes the Drive channel unusable in a gigging setting. I'm perfectly happy to use this as a pedal platform but I'd still prefer to get my hands on the schematic to get the channel switching issue fixed.

 

[TDE]

Hi Guys,

I have been gigging with a Stage Right SB20 head running into a Mesa compact rectifier 2x12” V30 closed back cab for about 3 years now and I really love this rig as a clean pedal platform. It’s the best (and most consistent) tone I have ever gotten on stage over many years of gigging. In fact, like it so much that I bought a second SB20 head to keep in the car as a backup. I actually had to use the backup head recently when a power tube in my main head went bad at a show. I swapped in 2 new power tubes when I got it back home and it sounds great again. However this thread is making me wonder if I should be checking the bias on the two heads. I have done lots and lots of electronic work inside guitars, but relatively little inside amps. If someone could perhaps please kindly post the detailed procedure for performing the bias on the SB20 I would be truly grateful! And actual photos of the PC board as a guide would be even better! Thanks in advance!

I've been using an SB20 for a couple years now. I have the SR 2x12 speaker cab with vintage 30's in it. I use only the clean channel. It's a really good pedal platform. I'm running 3 gain stages, 2 compressor stages into the front of the amp and volume, verb, chorus, and shimmery stuff in the loop. I have absolutely gone overboard on my pedalboard... But you know.
Oh yeah, I switch electric and acoustic guitars out throughout my set. Both sound really good. Bang for the buck on this thing is really good.