Monoprice 25 watt hybrid tube amp

M

mikerm

Active Member
I picked up a 25 watt monoprice tube amp from goodwill today, it looks great but the sound output is no good. Loud humming, weird cracking noises, and very low output of the actual music. The tubes light up, but I have no idea if they are good.

Is this type of behavior typical when the tubes are bad? I sort of wanted this for my office, figured it would be a cool conversation piece and good for some background music. Not sure how to test if the tubes are the problem or something else without actually buying a new set of tubes. I would just offload it if there is something else wrong.

Thanks!
 
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Is that the unit that has the tubes just for show and not function?
Maybe you can send it back to Monoprice for replacement.
 
It says "hybrid". This usually indicates half tube, half transistor. It also usually indicates that the tube part of it is the preamp stage while the transistor part is the output stage.

I would bet on something wrong with the transistor or power supply portion before I would suspect the preamp tubes. Preamp tubes don't get driven very hard.

After all, you do take a chance when you get stuff from the goodwill. :oops:

Not saying that it's not repairable, you just have to know what you are looking for and know basic electronics if you want to tackle a repair job for yourself. :smoke:
 
I agree the SS output stage is probably where the fault is. Maybe someone shorted the speaker wires while turned on or ran the volume so high the amp and speakers were clipping and fried the outputs. Depending on what all failed, it may or may not be worth repairing if you have to pay someone to fix it. How difficult is it to remove the bottom to get a good look at the inside?
 
Those two diodes are probably fine especially if you tested them with a meter. They just get hot and cook the board a little.
 
Is that capacitor next to those two diodes bulging a bit on the top along with the one in the upper far right in the picture? Those diodes did get hot. Other than that you can't see anything else. I wonder if the output transistors are damaged. Only one output device per channel to test.
 
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Lavane said:
Is that capacitor next to those two diodes bulging a bit on the top along with the one in the upper far right in the picture? Those diodes did get hot. Other than that you can't see anything else. I wonder if the output transistors are damaged. Only one output device per channel to test.
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Definitely look bulged to me. I'd go after those after checking the outputs for a short. Probably a fairly easy fix with easily available parts.
OP, if you do replace the caps, get the highest temp rating you can find. Those diodes seem to be throwing a lot of heat.
 
I agree, if you do have those caps replaced, use 105° instead of 85°. I see 5 legs on the output transistors. So I'm guessing these are small Push pull output devices, not a single ended amplifier? If it were mine, I think I would replace those two diodes and mount the new ones vertically to have more space between them and that capacitor and PCB. Maybe a higher rated pair of diodes as well.
 
Any success resolving this issue? I picked up the same unit with a loud 60 Hz humming at power up which does not vary with volume level. The diodes look fine on this unit but the adjacent capacitor heats up quickly after unit is powered. Any ideaa or input will be appreciated.
 
You can't go by looks unless they actually look physically burnt. Check the diodes with a meter. It seems that if the capacitor is getting hot, it is most likely getting AC due to shorted diodes.

Replace the diodes and capacitor where necessary.
 
Just got this for cheap. Looks great! Haven't tried powering it up yet...as all the threads and reviews are horrible for this model...now I find them, sigh. Anyways, has anyone modded this or got it to good working condition? I'm not afraid of some DIY and I figured the case alone was worth what I paid for it. Wouldn't be afraid to throw a few Benjamins at it, if it will give me some tubey goodness.
 
There isn't much you can really mod on these I think. Maybe replace the tubes if they're Chinese? There is that one capacitor very close to those two diodes that seems to get pretty warm. It would be a good Idea to replace it with a better 105° Nichicon, Panasonic, etc., capacitor. The original is probably a cheap 85° Chinese made cap like Chong brand.
It really isn't worth throwing a lot of money at, and being PCB based, more difficult to do so, even with a schematic.
 
Lavane said:
There isn't much you can really mod on these I think. Maybe replace the tubes if they're Chinese? There is that one capacitor very close to those two diodes that seems to get pretty warm. It would be a good Idea to replace it with a better 105° Nichicon, Panasonic, etc., capacitor. The original is probably a cheap 85° Chinese made cap like Chong brand.
It really isn't worth throwing a lot of money at, and being PCB based, more difficult to do so, even with a schematic.
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Was thinking of replacing all caps, resistors and tubes. Possibly removing PS and relocating it.
Or buying a amp tube kit and using this as a case.
The latter probably being best, as I'm starting to add up the costs of both.

I'm wondering if I could just use it as a tube preamp/ buffer?

I just found some cheap sanyo speakers in the trash. Going to fire it up and see what happens?
 
Fun update. It works!!! Didn't even let the tubes really warm up. Hooked it up to the sanyo 4 ohm speakers without binding post....no hum, all tubes light up. Just a slight sticky/crackle in a certain spot on volume pot. Pretty sure some deoxit will fix that.
How do I proceed with better speakers? I only have rare keepers. Not trying to ruin any of them. What are the possibilities of messing up my speakers if this amp goes south? Or decides to make some magic smoke?
 
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I guess it could maybe if the output transistors fail and short out. But it should blow a fuse first. You could fuse your speakers as a precaution if you want. I've only ever had one amp fry a pair of speakers and that was a Phase Linear amp years ago.
 
Lavane said:
I guess it could maybe if the output transistors fail and short out. But it should blow a fuse first. You could fuse your speakers as a precaution if you want. I've only ever had one amp fry a pair of speakers and that was a Phase Linear amp years ago.
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What size fuse should I use?

I love this idea, and have a big box of assorted fuses already.
 
For your model speaker, I don't really know. There are formulas for this, but it can get confusing. The power output of the amp, the impedance and power handling of the speaker, etc. You might contact the speaker manufacturer. I've seen anywhere from .5A to 3A on various speakers.
 
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