Mojotone Bassman very low output
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Mojotone Bassman very low output
Hello all. I've read a BUNCH of similar problems, but I haven't been lucky enough to run into my answer. I've watched YouTube videos as well. This is my 3rd amp build, I have a Tweed Deluxe and Tweed Super I have already built. I'm no electrician. I vaguely get the schematics, so I rely on the wiring diagrams to get the build. So I hopefully reordered a OT and choke to replace the first 2 thinking the OT had to be the problem and the choke was only $18.00 it wasn't. So I swapped out the power and preamp tubes with my working Super. They work great.The reason I replaced the OT was when I originally had wired up, I was checking voltage to the power tubes , my hand slid and I heard a nice buzz and seen a blue spark at the pin and my probe. I thought I might have fried something. So I have removed the board to replace the OT and choke, according to Mojotones diagram all is where it is supposed to be unless I'm just missing something. I have checked continuity all across. I get volume at 12 but it sounds like it should be 1. Please help. Lol.
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Re: Mojotone Bassman very low output
Hey there. I am certainly no expert but was the spark you saw, something like an arc between the plate ( pin 3) and and the heater ( pin 2 ) ? If so I don't think you really damaged the OT. Look for char marks on the tube socket, if you've got some, consider changing the socket out, it could be conductive.
There's a good uncle doug video that will help you examine the transformer in question. I bet it's fine.
And a quick simple thing to verify is that you have the speaker cable in the right jack. Recently I built a trianwteck and it had very low output. I was scratching my head and posted a pic. Someone comically pointed out that it was plugged I to the extention jack, thus the low output. Lol.... a humbling moment hahah
Hopefully it's something that simple.
Also , have a look at rob robinette troubleshooting web page.
With low output it can be a fried screen resistor or bad solder joint etc.
There's a good uncle doug video that will help you examine the transformer in question. I bet it's fine.
And a quick simple thing to verify is that you have the speaker cable in the right jack. Recently I built a trianwteck and it had very low output. I was scratching my head and posted a pic. Someone comically pointed out that it was plugged I to the extention jack, thus the low output. Lol.... a humbling moment hahah
Hopefully it's something that simple.
Also , have a look at rob robinette troubleshooting web page.
With low output it can be a fried screen resistor or bad solder joint etc.
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Stevem
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Re: Mojotone Bassman very low output
A accidentally shorted out test probe to any of the pins on a output tube will not kill any parts, but if you shorted a heater wire ( pins 2 or 7) to ground that might blow the main fuse.
A arc jumping to your test probe from pins 3 or 4 or the other side of that 470 resistor is not a concern.
Please take a bunch of close in shots of the amp guts and post them up for us to look at.
But first and most importantly confirm that with the output tubes not in place that on pin 5 of each output tube socket you at least have a - 48 volts dc, ( minus 48 ) if not do not put the output tubes in there sockets and report back here with that info and the photos.
A arc jumping to your test probe from pins 3 or 4 or the other side of that 470 resistor is not a concern.
Please take a bunch of close in shots of the amp guts and post them up for us to look at.
But first and most importantly confirm that with the output tubes not in place that on pin 5 of each output tube socket you at least have a - 48 volts dc, ( minus 48 ) if not do not put the output tubes in there sockets and report back here with that info and the photos.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Mojotone Bassman very low output
Thanks guys for answering! I totally gutted and resoldered the leads. I am getting the exact same results as the first. I checked all but the power tubes in my Super so I know they are good. As for the pin 5, I have -45 dc with the tubes in. Steve, I'm a newb. Do I take that reading with all tubes in or just leave in the rectifier? Sorry. Again been question: do I have the 4 8 ohm speakers wired right? Using all 4 jacks. They are rca. Positive is center and neg is shoulder.
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Re: Mojotone Bassman very low output
The -45 means you have bias voltage, that reading was with the rectifier in and power tubes out, so you don't fry the power tubes if the bias is wrong, but leaving the tubes in was fine.
I think you have the 4, 8 ohm speakers wired correctly, in parallel, this should give you 2 ohms total. I would double check the resistance of each individual speaker to make sure the male rca plugs are wired correctly and not shorted. Do this one at a time, unplugged from the amp. You should get somewhere around 6 ohms for each speaker.
Hard to see much detail from the picture, but some of the solder joints around the tube sockets look a bit rough, might want to touch them up, making sure to heat the wire and the socket pin at the same time.
Poking each solder connection with a non-conductive chopstick while audio is playing through the amp can help you find bad solder joints. Better to use something other than your guitar so you can safely use one hand for poking and keep the other in your pocket.
I think you have the 4, 8 ohm speakers wired correctly, in parallel, this should give you 2 ohms total. I would double check the resistance of each individual speaker to make sure the male rca plugs are wired correctly and not shorted. Do this one at a time, unplugged from the amp. You should get somewhere around 6 ohms for each speaker.
Hard to see much detail from the picture, but some of the solder joints around the tube sockets look a bit rough, might want to touch them up, making sure to heat the wire and the socket pin at the same time.
Poking each solder connection with a non-conductive chopstick while audio is playing through the amp can help you find bad solder joints. Better to use something other than your guitar so you can safely use one hand for poking and keep the other in your pocket.
Re: Mojotone Bassman very low output
6.4 on the speakers so they are good. How about this (diode?) Is it wired right direction? Grey stripe to left.
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Re: Mojotone Bassman very low output
What if I didn't have the filter caps in right? I followed the diagram. Would that cause a drop in volume across the board?
Re: Mojotone Bassman very low output
Negative bias voltage confirms the diode and the bias circuit are correct, filter caps not exploding confirms they are installed in the right orientation.
Re: Mojotone Bassman very low output
Mains power OFF Ohm checks:
1) Verify that the input jacks are wired correctly. Plug a guitar cord into one of the inputs. Measure Ohms between the sleeve and tip of the other end of the cable. You should measure either 1 Meg or 136K. Verify that there is a low Ohm reading from the tip to one of the 68K resistors on the eyelet board. Try all the input jacks.
2) Measure Ohms across the speakers at the jacks inside the chassis. Should be a very low reading, less than one Ohm. Now, unsolder one of the wires from the output transformer that goes to the speakers. It doesn't matter which one, choose the one that will be easiest to re-solder after the test. The resistance across the speakers should jump up to about 1.5 Ohms. As you unplug speakers one at a time, the reading should go up until you have no speakers connected.
3) Turn all the pots half way up. Measure the Ohms from the center terminal to ground. The Volume controls, Treble and Bass should have readings 100K or higher. The Mid and Presence controls should have lower readings about half the pot's rated resistance.
1) Verify that the input jacks are wired correctly. Plug a guitar cord into one of the inputs. Measure Ohms between the sleeve and tip of the other end of the cable. You should measure either 1 Meg or 136K. Verify that there is a low Ohm reading from the tip to one of the 68K resistors on the eyelet board. Try all the input jacks.
2) Measure Ohms across the speakers at the jacks inside the chassis. Should be a very low reading, less than one Ohm. Now, unsolder one of the wires from the output transformer that goes to the speakers. It doesn't matter which one, choose the one that will be easiest to re-solder after the test. The resistance across the speakers should jump up to about 1.5 Ohms. As you unplug speakers one at a time, the reading should go up until you have no speakers connected.
3) Turn all the pots half way up. Measure the Ohms from the center terminal to ground. The Volume controls, Treble and Bass should have readings 100K or higher. The Mid and Presence controls should have lower readings about half the pot's rated resistance.
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Stevem
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Re: Mojotone Bassman very low output
A fender Bassman schematic calls for -48 volts.
At -45 volts your output tubes will get getting very hot to the touch very fast, in fact in a darkened room you might see there plate structure glowing red and you do not want that taking place!
At -45 volts your output tubes will get getting very hot to the touch very fast, in fact in a darkened room you might see there plate structure glowing red and you do not want that taking place!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Mojotone Bassman very low output
A fender Bassman schematic calls for -48 volts.
At -45 volts your output tubes will get getting very hot to the touch very fast, in fact in a darkened room you might see there plate structure glowing red and you do not want that taking place!
At -45 volts your output tubes will get getting very hot to the touch very fast, in fact in a darkened room you might see there plate structure glowing red and you do not want that taking place!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Mojotone Bassman very low output
Are those Voltage readings you took in the layout picture of post #1 ? Check connections around the middle preamp tube. Is pin 1 connected to pin 7 ?
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Mojotone Bassman very low output
Do the pop test with your voltmeter.
Turn all the amp’s controls up to full and starting at pin 5 of each output tube touch the probe to pin 5.
You should hear a light pop out of the speaker.
If this checks out then the output section is working, if so then move back up stream to the preamp tube and touch pins 3 or 8 and you should get an even louder pop.
If that checks out then move to pin 2 on the next tube up stream and you should hear a good pop.
If that checks out then move back to the next preamp tube and touch pins 2 and 7 and both should make a really loud pop.
Where ever you don’t hear a pop then that’s the section with a problem.
If you get a pop at the output tubes then I would first review your wiring of the input jacks.
Another easy fast thing to try with the controls all up full is to tap the first preamp tube (V1) with your finger nail.
You should hear ting out of the speaker.
Turn all the amp’s controls up to full and starting at pin 5 of each output tube touch the probe to pin 5.
You should hear a light pop out of the speaker.
If this checks out then the output section is working, if so then move back up stream to the preamp tube and touch pins 3 or 8 and you should get an even louder pop.
If that checks out then move to pin 2 on the next tube up stream and you should hear a good pop.
If that checks out then move back to the next preamp tube and touch pins 2 and 7 and both should make a really loud pop.
Where ever you don’t hear a pop then that’s the section with a problem.
If you get a pop at the output tubes then I would first review your wiring of the input jacks.
Another easy fast thing to try with the controls all up full is to tap the first preamp tube (V1) with your finger nail.
You should hear ting out of the speaker.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!