I expected this amp to be built like junk, it is not rather it is quite nice and the YouTube reviews are excellent:
Complain about a power cord, come on guys, really?
$238 on Monoprice now, free shipping, some have found coupons for about $200! $279 normally.
https://www.monoprice.com/Product?p_id=611815
$249 on Amazon.
This amp looks very much like the Fender Blues Junior (FBJ) but with some interesting advantages:
EL84 s like the FBJ
Effects loop, none on the FBJ
Switch for 1W practice mode
Adjustable fixed bias design, the Blues Junior has no adjustment and comes with the tubes biased above the max rating
Celestion speaker, cheap one of course
Disadvantages:
Hum, passable but should be better. Edit: Hum is not bad at normal volume settings.
Reverb and effects loop are buffered with TLO72 op amps as is done with most "modern" Fenders
Fuse is internal
No screen grid stoppers resistors on the output tubes
Cheap caps
I bought a very early one used for $125 thinking that it was worth it for the box, chassis reverb tank and speaker.
It is Tweed like in that the controls point up, rather than forward as on the latest production - I believe that the
design is the same.
I'd rebuild it and use it for parts if we don't like it.
This was claimed to have new tubes, a MOD reverb tank and it has an Eminence Legend 12" speaker.
Opened it up and the layout is completely different from the FBJ and not good IMO but should not
be too hard to work on for basics but if any of the surface mount components has to be changed then
it is probably not worth having a shop fix it, and a DIYer will need surface mount skills to do it.
Tubes are JJ s with V1 being a Tung Sol Russian reissue (this was an upgrade by the previous owner)
It has some mods one being a "Neg Feedback" control
Found a schematic not sure if it is early or late, tempted to remove the mods.
Edit: I've decided that it will not be easy to work on so I'll leave the mod in place and let my son try it.
The price was certainly worth it for the case, chassis, speaker, tubes and reverb tank so no loss.
Previous owner told me that the work was done by an amp tech in MA who sold it to him.
If anyone knows of documented mods I'd like to see if those are the ones in this unit, please post a link.
Complain about a power cord, come on guys, really?
$238 on Monoprice now, free shipping, some have found coupons for about $200! $279 normally.
https://www.monoprice.com/Product?p_id=611815
$249 on Amazon.
This amp looks very much like the Fender Blues Junior (FBJ) but with some interesting advantages:
EL84 s like the FBJ
Effects loop, none on the FBJ
Switch for 1W practice mode
Adjustable fixed bias design, the Blues Junior has no adjustment and comes with the tubes biased above the max rating
Celestion speaker, cheap one of course
Disadvantages:
Hum, passable but should be better. Edit: Hum is not bad at normal volume settings.
Reverb and effects loop are buffered with TLO72 op amps as is done with most "modern" Fenders
Fuse is internal
No screen grid stoppers resistors on the output tubes
Cheap caps
I bought a very early one used for $125 thinking that it was worth it for the box, chassis reverb tank and speaker.
It is Tweed like in that the controls point up, rather than forward as on the latest production - I believe that the
design is the same.
I'd rebuild it and use it for parts if we don't like it.
This was claimed to have new tubes, a MOD reverb tank and it has an Eminence Legend 12" speaker.
Opened it up and the layout is completely different from the FBJ and not good IMO but should not
be too hard to work on for basics but if any of the surface mount components has to be changed then
it is probably not worth having a shop fix it, and a DIYer will need surface mount skills to do it.
Tubes are JJ s with V1 being a Tung Sol Russian reissue (this was an upgrade by the previous owner)
It has some mods one being a "Neg Feedback" control
Found a schematic not sure if it is early or late, tempted to remove the mods.
Edit: I've decided that it will not be easy to work on so I'll leave the mod in place and let my son try it.
The price was certainly worth it for the case, chassis, speaker, tubes and reverb tank so no loss.
Previous owner told me that the work was done by an amp tech in MA who sold it to him.
If anyone knows of documented mods I'd like to see if those are the ones in this unit, please post a link.
Last edited:
This is a discussion about the early model (roughly 2016) like ours with the controls pointing up rather
than forward as they are on the latest version:
http://markweinguitarlessons.com/forums/threads/monoprice-15w-tube-amp-laney-12r-review.73085/
than forward as they are on the latest version:
http://markweinguitarlessons.com/forums/threads/monoprice-15w-tube-amp-laney-12r-review.73085/
My son finally came to get this amp and tried it out. It works great as a clean amp, but as he turns the gain,
up past 5 and plays it hard it sounds great for about 10 seconds and then the volume drops to almost
nothing and it crackles when he plays loudly. The very weird thing is that it seems to latch into this mode
and cycling power for say 20 seconds doesn't clear it. I don't know if leaving it off for say 5 minutes would
clear it, that's an experiment that we have to do. He seemed to think that playing it softly for a minute or
two also clears it. But who knows perhaps silence for a few minutes would also clear it, another experiment to do.
He liked the clean sound so much that he decided to take it, he mainly plans to use it for clean.
Also, it did it with his very nice PRS guitar with hum buckers,
but it didn't seem to do it with my very cheap Peavy guitar with single coils.
I'll need to test it out on the bench and see if I can get it to latch up, then probe voltages etc.
I had a passing thought that it might be an oscillation but you'd think that turning it off would clear that.
Any tips and suggestions would be appreciated.
The plan from the start was to use the box, chassis, tubes transformers, etc. and toss the circuit board if it was
no good, this was further reinforced when I saw that the circuit board had a good amount of surface mount
parts. I'll fix it if I trace it down to a cold solder joint, dirty relay contacts or just a bad through hole part but
if it becomes a time sink to fix I'll just build a hard wired Vox, Tiny Terror, or Blues Jr design in it. Since he likes
it so much he'll probably buy a new one if we decide to gut this one.
up past 5 and plays it hard it sounds great for about 10 seconds and then the volume drops to almost
nothing and it crackles when he plays loudly. The very weird thing is that it seems to latch into this mode
and cycling power for say 20 seconds doesn't clear it. I don't know if leaving it off for say 5 minutes would
clear it, that's an experiment that we have to do. He seemed to think that playing it softly for a minute or
two also clears it. But who knows perhaps silence for a few minutes would also clear it, another experiment to do.
He liked the clean sound so much that he decided to take it, he mainly plans to use it for clean.
Also, it did it with his very nice PRS guitar with hum buckers,
but it didn't seem to do it with my very cheap Peavy guitar with single coils.
I'll need to test it out on the bench and see if I can get it to latch up, then probe voltages etc.
I had a passing thought that it might be an oscillation but you'd think that turning it off would clear that.
Any tips and suggestions would be appreciated.
The plan from the start was to use the box, chassis, tubes transformers, etc. and toss the circuit board if it was
no good, this was further reinforced when I saw that the circuit board had a good amount of surface mount
parts. I'll fix it if I trace it down to a cold solder joint, dirty relay contacts or just a bad through hole part but
if it becomes a time sink to fix I'll just build a hard wired Vox, Tiny Terror, or Blues Jr design in it. Since he likes
it so much he'll probably buy a new one if we decide to gut this one.
Last edited:
Video showing some mods:
Forum discussion about the mod:
https://www.tdpri.com/threads/monoprice-15-watt-tube-amp.609524/page-35#post-8522889
Forum discussion about the mod:
https://www.tdpri.com/threads/monoprice-15-watt-tube-amp.609524/page-35#post-8522889
I just bought one of those amps. The Monoprice 15-watt with front-facing knobs. I bought it brand new from Monoprice. It snapped, crackled and popped like crazy. I was able to dial in some good tones. It's plenty loud. But, I couldn't get past all the noise, I talked to Monoprice and they told me to send it back. I brought it to FedEx today. I hope the replacement is better. I've read a lot of good things about this amp.
Try tapping on the tubes with a pen or whatever next time you might have just gotten a bad tube.
Yes, most of the reviews by some really top notch people are positive.
Ours has problems but it has been modded, I'm going to look at it next time my son drops it off.
Yes, most of the reviews by some really top notch people are positive.
Ours has problems but it has been modded, I'm going to look at it next time my son drops it off.
I found, thanks to this video, that the Laney Cub 12R is the same except that it has digital reverb:
Clean schematic here: https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-12R-Schematic.pdf
Clean schematic here: https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-12R-Schematic.pdf
The Laney schematic is accurate down to component label numbers and I'd guess that the main circuit board
is the same, as already mentioned the reverb board is completely different:
https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-12R-Schematic.pdf
I've used this to dig into ours again and wanted to point out an error in the schematic. The gain control is a dual
with 100K and 1M sections which are reversed in the schematic. I used an ohm meter to verify this. The 1M is in
the signal path and the 100K is in the cathode bypass position. I've shown this corrected here in red with red
arrows pointing to them:
is the same, as already mentioned the reverb board is completely different:
https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-12R-Schematic.pdf
I've used this to dig into ours again and wanted to point out an error in the schematic. The gain control is a dual
with 100K and 1M sections which are reversed in the schematic. I used an ohm meter to verify this. The 1M is in
the signal path and the 100K is in the cathode bypass position. I've shown this corrected here in red with red
arrows pointing to them:
Last edited:
I've also figured out what mods were done to our unit.
ELEVATED HEATERS
The wiper on the heater balance pot (TR2) was lifted and tied to a +67V supply produced from a voltage
divider 220K from +290V and 100K to ground. The 67V is bypassed by 2.2uF 100V.
It is considered good practice to have the heaters close to the nominal cathode voltage and most of the triode
and output stages have the cathodes close to zero volts. But some designs, like this one, have cathode followers
that typically have the cathode elevated over 100V. In this design the cathode follower (V2A) is at 145V. Also,
the driver pair in the power amp have their cathodes at about 35V, all the others are close to 0V. They elevated
the heater supply to 67V making the cathode follower have a small difference but now all the other stages have
roughly a 67V difference. This fixed one problem and potentially caused another.
The cathode follower feeding the tone stack, and the tone stack position is right out of the Fender Bassman design
and indeed the cathode follower is at about 180V in the Bassman due to a higher voltage B+ to the preamp.
12AX7 has a rating where the heater can be +/-100V from the cathode, and therefore this design, in stock form
exceeds this by 45V and the Bassman by 80V. This mod does bring the design into spec making none of the
heaters +/-100V from the cathodes. I don't know of any other guitar amp that elevates the cathodes.
FEEDBACK CONTROL
A pot was added to the rear chassis panel with a clear "FEEDBACK" label. The power amp section has a rather
strange feedback arrangement where a 47K resistor R22 goes back to an RC network to ground R20 and C27
which is typically how it is done in a solid state amp to provide more feedback at DC to minimize DC offset. But
this is a tube amp with no DC on the output, it seems to me that C27 is not needed. Anyway, the mod removes
R20 a 2.2K resistor and replaces it with a 160K pot. All the way down shorts the feedback to ground through
C27, the time constant will change with the setting of the pot, probably not a good idea. All the way up provides
more feedback which might cause instability. I don't like this mod, there are probably better ways to do this. I
would allow the stock amount of feedback or less down to zero, not more. Zero feedback is what Vox does in
their power amps, but they use much better output transformers, this one is tiny.
Also worth noting, the power amp driver is biased as in a Vox amp with R35 being 47K, not the typical Fender and
Marshall value of 10-15K. The "Tone" pot (VR3) is in the Vox position at the power tube inputs. This is very interesting
because the amp has a Bass, Mid, Treb (BMT) tone stack AND a tone pot. Also, the 1W mode is an attenuator in front
of the power tube inputs as is done with a Vox Master Volume.
ELEVATED HEATERS
The wiper on the heater balance pot (TR2) was lifted and tied to a +67V supply produced from a voltage
divider 220K from +290V and 100K to ground. The 67V is bypassed by 2.2uF 100V.
It is considered good practice to have the heaters close to the nominal cathode voltage and most of the triode
and output stages have the cathodes close to zero volts. But some designs, like this one, have cathode followers
that typically have the cathode elevated over 100V. In this design the cathode follower (V2A) is at 145V. Also,
the driver pair in the power amp have their cathodes at about 35V, all the others are close to 0V. They elevated
the heater supply to 67V making the cathode follower have a small difference but now all the other stages have
roughly a 67V difference. This fixed one problem and potentially caused another.
The cathode follower feeding the tone stack, and the tone stack position is right out of the Fender Bassman design
and indeed the cathode follower is at about 180V in the Bassman due to a higher voltage B+ to the preamp.
12AX7 has a rating where the heater can be +/-100V from the cathode, and therefore this design, in stock form
exceeds this by 45V and the Bassman by 80V. This mod does bring the design into spec making none of the
heaters +/-100V from the cathodes. I don't know of any other guitar amp that elevates the cathodes.
FEEDBACK CONTROL
A pot was added to the rear chassis panel with a clear "FEEDBACK" label. The power amp section has a rather
strange feedback arrangement where a 47K resistor R22 goes back to an RC network to ground R20 and C27
which is typically how it is done in a solid state amp to provide more feedback at DC to minimize DC offset. But
this is a tube amp with no DC on the output, it seems to me that C27 is not needed. Anyway, the mod removes
R20 a 2.2K resistor and replaces it with a 160K pot. All the way down shorts the feedback to ground through
C27, the time constant will change with the setting of the pot, probably not a good idea. All the way up provides
more feedback which might cause instability. I don't like this mod, there are probably better ways to do this. I
would allow the stock amount of feedback or less down to zero, not more. Zero feedback is what Vox does in
their power amps, but they use much better output transformers, this one is tiny.
Also worth noting, the power amp driver is biased as in a Vox amp with R35 being 47K, not the typical Fender and
Marshall value of 10-15K. The "Tone" pot (VR3) is in the Vox position at the power tube inputs. This is very interesting
because the amp has a Bass, Mid, Treb (BMT) tone stack AND a tone pot. Also, the 1W mode is an attenuator in front
of the power tube inputs as is done with a Vox Master Volume.
I intended to delete " This fixed one problem and potentially caused another." from the previous post but
I'm not able to edit it. All the heater to cathode voltages are now in spec of +/-100v.
I'm not able to edit it. All the heater to cathode voltages are now in spec of +/-100v.
I want to add that at least one very competent guitar tech says that the cathode follower in, for
example the Fender Bassman does have a hard life with that voltage and does not often last
long with newer production tubes.
I don't know if there is a disadvantage in another area to having the heaters elevated.
Anyone?
Older gear would have two heater windings one to operate the tubes that require elevated
voltage and another for the ones at ground potential.
example the Fender Bassman does have a hard life with that voltage and does not often last
long with newer production tubes.
I don't know if there is a disadvantage in another area to having the heaters elevated.
Anyone?
Older gear would have two heater windings one to operate the tubes that require elevated
voltage and another for the ones at ground potential.
Here's a thread discussing the elevated heater subject with them at 75V:
Correction, the 12AX7 cathode positive with respect to heater spec is 200V not 100 as I previously
stated, and older tubes seem to meet this spec but not all modern production part do. RCA spec:
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6681_12AX7.PDF
It's specifically for stress on the cathode follower - a 12ax7 that sits on the JCM design at B+ (350V) on the plate with the cathode at +196V with a directly connected grid to the previous stage. This time honoured design is known to be harsh on V3 which houses the previous preamp 3 and the cathode follower.
The 12ax7 itself has a limit for heater to cathode of ±200V which means the cathode at 196V is very close. Probably OK for old 12ax7 but new construction 12ax7 I would be concerned about. Elevating the 12ax7 heaters by 75V is within spec to all the heaters and reduces...
The 12ax7 itself has a limit for heater to cathode of ±200V which means the cathode at 196V is very close. Probably OK for old 12ax7 but new construction 12ax7 I would be concerned about. Elevating the 12ax7 heaters by 75V is within spec to all the heaters and reduces...
Correction, the 12AX7 cathode positive with respect to heater spec is 200V not 100 as I previously
stated, and older tubes seem to meet this spec but not all modern production part do. RCA spec:
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6681_12AX7.PDF