Blowing Fuses in Mojotone 18w TMBish Build

StratMatt7376

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Hey everybody. So, I just finished wiring up a made from scratch version of the mojotone 18 watt tmb (except with a 5y3 instead of an ez81, with an included cathode follower that is the regular jcm 800, with the heater CTs grounded to power tube cathodes, and using a power supply that is inspired by the 5e3 power supply. So it's not exactly the same amp, but it follows the mojotone schematic very closely with those minor changes). I was starting it up, and it didn't do anything strange when I didn't have any tubes in it, but when I put the 5y3 in and powered it up, it blew the fuse. It was a 1 amp fuse, but it seems to me like there definitely shouldn't be 1 amp flowing when none of the other tubes are in and I'm not playing through it. I put in a 3 amp fuse just for kicks, and I noticed some smoke and then arcing in the 5y3, then the fuse blew again. The 130 ohm power section cathode resistor was hot. I repeated this a few times, pondering what it could mean.

I'm using a hammond 372dx power transformer (600V ct @ 144mA). The PT secondary is supposed to have a CT for the rectifier heater, and it's supposed to be yellow with a black stripe, but the only wire it could be is beige with a black stripe. Does anybody know if that is really supposed to be the rectifier heater CT? I'm just trying to figure out where the current came from that heated up my resistor and caused the arcing. It seems these things are indicative of a short. I'm at a loss as to where any voltage could be that could potentially lead to such high current, since the standby switch is off and only the CTs have access to ground. Any pointers?
 

King Fan

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Lotta issues here. Folks will chime in. Lemme just mark off the obvious.
• You totally need to build a light bulb limiter.
• Upping the fuse for kicks can make for expensive kicks.
• You want to follow a known safe stepwise startup procedure.
 
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AntonyB

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Quadruple check that all of your polarized capacitors are oriented correctly
I made 2 mistakes on my BYOC JTM45 clone and those were that, with the same symptoms as you describe
My newbie mistake honestly was to be anal about component orientation… and in a moment of less attention, 2 polarized caps got inverted, so I could align the writing…
Good intention, bad execution

Blowing fuses and drawing too much current are signs that something is going to ground when it shouldn’t
 

andrewRneumann

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Hey everybody. So, I just finished wiring up a made from scratch version of the mojotone 18 watt tmb (except with a 5y3 instead of an ez81, with an included cathode follower that is the regular jcm 800, with the heater CTs grounded to power tube cathodes, and using a power supply that is inspired by the 5e3 power supply. So it's not exactly the same amp, but it follows the mojotone schematic very closely with those minor changes). I was starting it up, and it didn't do anything strange when I didn't have any tubes in it, but when I put the 5y3 in and powered it up, it blew the fuse. It was a 1 amp fuse, but it seems to me like there definitely shouldn't be 1 amp flowing when none of the other tubes are in and I'm not playing through it. I put in a 3 amp fuse just for kicks, and I noticed some smoke and then arcing in the 5y3, then the fuse blew again. The 130 ohm power section cathode resistor was hot. I repeated this a few times, pondering what it could mean.

I'm using a hammond 372dx power transformer (600V ct @ 144mA). The PT secondary is supposed to have a CT for the rectifier heater, and it's supposed to be yellow with a black stripe, but the only wire it could be is beige with a black stripe. Does anybody know if that is really supposed to be the rectifier heater CT? I'm just trying to figure out where the current came from that heated up my resistor and caused the arcing. It seems these things are indicative of a short. I'm at a loss as to where any voltage could be that could potentially lead to such high current, since the standby switch is off and only the CTs have access to ground. Any pointers?

Your 5V rectifier filament does not require a center tap and should not use a center tap. Is that beige/black wire coming out of the PT next to the yellow wires? If so, I would say that's the 5V center tap and it should be capped and tucked away. It should NOT be connected to anything. Yes, that would cause a short if connected to the chassis (or very close to a short if connected to the power tube cathodes).
 
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StratMatt7376

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Your 5V rectifier filament does not require a center tap and should not use a center tap. Is that beige/black wire coming out of the PT next to the yellow wires? If so, I would say that's the 5V center tap and it should be capped and tucked away. It should NOT be connected to anything. Yes, that would cause a short if connected to the chassis (or very close to a short if connected to the power tube cathodes).
Andrew, your input is appreciated. I disconnected the rectifier heater CT from the power tube cathode, and it stopped blowing fuses. Following the startup instructions found in the manual for the TMB kit, I found that when I insert my power tubes, the preamp B+ goes down to 135V and the B+ at the rectifier goes down to 200V (down from 340V and 427V respectively), which I wasn't expecting. As stated earlier, the PT is rated at 600V CT @ 144mA. Referencing the datasheet for EL84s, it looks as though the current through the EL84s ought to be around 114mA, leaving me plenty of current for the PI and preamp tubes. Did I not buy a big enough PT? It's not blowing 1A fuses anymore, but could there still be a short? Any ideas? Also, here is a picture of my build.
 

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Snfoilhat

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Screenshot from 2022-05-24 17-47-06.png

I don't have a complete enough understanding of what's going on with the amp, but one thing jumps out about what might be a misunderstanding of the EL84 data sheets. This tool is a great resource. Check out the (approximate, since I don't know what you've built) currents for each EL84.

Others can give more granular advice, especially if you provide the info they'll need. Good luck!
 

andrewRneumann

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Andrew, your input is appreciated. I disconnected the rectifier heater CT from the power tube cathode, and it stopped blowing fuses. Following the startup instructions found in the manual for the TMB kit, I found that when I insert my power tubes, the preamp B+ goes down to 135V and the B+ at the rectifier goes down to 200V (down from 340V and 427V respectively), which I wasn't expecting. As stated earlier, the PT is rated at 600V CT @ 144mA. Referencing the datasheet for EL84s, it looks as though the current through the EL84s ought to be around 114mA, leaving me plenty of current for the PI and preamp tubes. Did I not buy a big enough PT? It's not blowing 1A fuses anymore, but could there still be a short? Any ideas? Also, here is a picture of my build.

I’m afraid you smoked your rectifier tube, your power tube cathode resistor, or both. Do you have another rectifier?
 

StratMatt7376

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Check the power tube cathode resistor for proper resistance. Also, the bypass capacitor that is in parallel to it was probably exposed to over-voltage, so that is also suspect.
It was both the resistor and the capacitor. I improvised a little something for while the other parts are on order.. heh

The amp works great now. Not as loud as I was expecting, but all the B+ are spot on now. Sounds great. You have to give them credit for designing a good amp. Thanking the Lord for you all.. my dad is a ham radio guy, but he prefers transistors, so he's not much help. You all are the best. Thanks guys. I'll build a light bulb limiter now..
 

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andrewRneumann

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It was both the resistor and the capacitor. I improvised a little something for while the other parts are on order.. heh

The amp works great now. Not as loud as I was expecting, but all the B+ are spot on now. Sounds great. You have to give them credit for designing a good amp. Thanking the Lord for you all.. my dad is a ham radio guy, but he prefers transistors, so he's not much help. You all are the best. Thanks guys. I'll build a light bulb limiter now..

That’s some creative engineering there. Post your power tube voltages on all pins to chassis, AC for the heaters. Do you have another 18W-ish amp you can compare it to?
 

StratMatt7376

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That’s some creative engineering there. Post your power tube voltages on all pins to chassis, AC for the heaters. Do you have another 18W-ish amp you can compare it to?
Sorry for the wait. Here are the voltages:

pin # tube 1 tube 2
1..........nc..........nc
2..........0V..........0V
3..........12V........12V
4&5 .....5.9V......5.9V
6...........nc..........nc
7..........366V......367V
8...........nc..........nc
9...........358V .....360V

I have a Bogner Alchemist that can be run at (a very loud) 20W, so that's what I was comparing it to. Does 12V at the cathode seem too high?
 

andrewRneumann

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Sorry for the wait. Here are the voltages:

pin # tube 1 tube 2
1..........nc..........nc
2..........0V..........0V
3..........12V........12V
4&5 .....5.9V......5.9V
6...........nc..........nc
7..........366V......367V
8...........nc..........nc
9...........358V .....360V

I have a Bogner Alchemist that can be run at (a very loud) 20W, so that's what I was comparing it to. Does 12V at the cathode seem too high?

Hmmm... let's investigate the heater voltage. When you measure Vac from pin 4 to pin 5 do you also get 5.9Vac? On the rest of the valves, when measured across the heater at the socket, do they show 5.9Vac? (eg. on 12A*7, measure from pin 4/5 to pin 9)

Low heat would negatively impact performance.
 

StratMatt7376

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Hmmm... let's investigate the heater voltage. When you measure Vac from pin 4 to pin 5 do you also get 5.9Vac? On the rest of the valves, when measured across the heater at the socket, do they show 5.9Vac? (eg. on 12A*7, measure from pin 4/5 to pin 9)

Low heat would negatively impact performance.
I appreciate your help. The 12ax7s are all measuring 5.8V.

I clipped some extra resistors in parallel with the power section cathode resistor/bypass capacitor, and it got noticeably louder.. probably at the expense of my tube life.. but man is this thing a joy to play through. I just need to put in a larger capacitor for the power supply, and this amp will probably be about finished.
 

StratMatt7376

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I thought -12V might be a little bit cold, but according to that calculator, it's not far from 100% dissipation... hmm
 
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StratMatt7376

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View attachment 987111
Seems like the cathode resistor may be smaller than recommended already. It may be there other are issues and one masks another
The cathode resistor that I rigged up was actually 163 ohms.

I was able to make a 138 ohm network of resistors. It gave me 11.4V across the cathode resistor, and according to the calculator, it puts me at 100.7% dissipation.
 
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