Blowing Fuses in Mojotone 18w TMBish Build

andrewRneumann

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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.

Maybe your wall voltage is a little low--I expected the 372DX to give you at least 6.3Vac if not a little higher. Never had a Hammond that didn't give me the rated voltage.

Going back to the original sin of connecting the 5Vac center-tap to the power tubes cathode... I'm now concerned that you may have damaged some preamp tubes. The mistaken connection basically elevated your heater supply by your HT voltage. (It probably never got to 370V because the poor 5Y3 was struggling to conduct that much current and the cathode resistor was in the process of melting down.) There is a maximum heater-cathode voltage for the 12AX7 and it is 180V. For the EL84s, it's only 100V, but you were putting HT on the cathodes and heaters of the EL84's, so they were probably safe from arcing. Could there have been some arcing and damage between the heaters and cathodes? This is just theoretical, but might be worth exploration.
 

andrewRneumann

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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.

Hmmm... there is a math error somewhere. I get 13.8W (about 38.8mA plate current per tube) with those numbers if the plates are at 366V (to ground, 355V to cathode). I thought the 168-ohm numbers were close to 12W. If I recall I was getting 12.5W with the earlier numbers.
 

StratMatt7376

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I switched out the 12ax7s, and it sounds the same.

I reran the numbers and now they agree with yours. I initially missed the box at the top of the page for Plate to Cathode voltage.
 

StratMatt7376

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Update: I discovered that I was missing a capacitor between the tone controls and the phase inverter. I placed one in the signal path, and now it's ridiculously loud. The new problem: squealing. I introduced some negative feedback, and it helped a little, but it still squeals with the master at 9 or 10 o'clock.
 

StratMatt7376

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I wasn't sure if I should start a new thread for this, but the amp has kind of a reverberating screech to it when the master volume is in a certain range. The link is to a sound file with it. Can you guys hear it in the recording? Does anybody know how to get rid of it?

 

andrewRneumann

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I wasn't sure if I should start a new thread for this, but the amp has kind of a reverberating screech to it when the master volume is in a certain range. The link is to a sound file with it. Can you guys hear it in the recording? Does anybody know how to get rid of it?



I can hear it. You have narrowed it down to a certain range on the MV? What range?

And speaking of MV, can anyone explain why this cap is necessary in this design? Seems like it just sits there doing nothing. I'd be tempted to remove it / jumper over it.

1653938378406.png
 

Lowerleftcoast

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And speaking of MV, can anyone explain why this cap is necessary in this design? Seems like it just sits there doing nothing. I'd be tempted to remove it / jumper over it.
I assume it is there to keep DC from the PI off the volume pot. All of the 5F6A (cathode follower) variants have this cap in between the pots and the PI.
 

StratMatt7376

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King Fan, yes I did, and it alleviated a lot of the squeal. The recording was after I switched the OT primaries.

It was the substitute cathode bypass cap that was causing that sound. I put a 16uF in parallel with that capacitor/resistor tower thing and that sound went away. Credits to Deepblueharp on youtube for helping me realize that it might have been that.

After implementing that, the amp started oscillating with the MV between 8 and 9 o'clock, so I put in bigger grid stoppers, and it stopped oscillating, but then the note decay got inconsistent and unstable sounding, so I unclipped them. The oscillations haven't come back, but the MV is still funky. It doesn't get louder after 9 or 10 o'clock and it sounds unstable and sloppy after that point. I'm about to investigate. It has been a big, long learning process today, working on this thing.
 
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andrewRneumann

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King Fan, yes I did, and it alleviated a lot of the squeal.

It was the substitute cathode bypass cap that was causing that sound. I put a 16uF in parallel with that capacitor/resistor tower thing and that sound went away. Credits to Deepblueharp on youtube for helping me realize that it might have been that.

After implementing that, the amp started oscillating with the MV between 8 and 9 o'clock, so I put in bigger grid stoppers, and it stopped oscillating, but then the note decay got inconsistent and unstable sounding, so I unclipped them. The oscillations haven't come back, but the MV is still funky. It doesn't get louder after 9 or 10 o'clock and it sounds unstable and sloppy after that point. I'm about to investigate. It has been a big, long learning process today, working on this thing.

I once wired all my pots in reverse. I swore I checked them with an ohmmeter to make sure I had the terminals right, but they still ended up reversed. Make sure it’s not something simple like that!
 

StratMatt7376

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Hey everybody. I'm still working on this amp. It sounds great when the master volume is between 7 and 9:30, but above that, the sound has a small crackle to it, and the note decay doesn't sound very good. Here is a link to the sound file of the decaying note.



Does anybody know what could be causing this?
 

StratMatt7376

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My dad helped me out with it, and he discovered some coupling happening between lugs of the master volume and also between one of the output transformer primary wires and a grid. The coupling was causing oscillations above audible frequencies, and that once those were mitigated, the amp got a lot louder and more stable.
 

RetiredUnit1

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Most of the layouts I've looked at (which is probably ALL of them) have a 2amp fuse. This was recently an issue with a new amp builder on the 18Watt forum. I'm getting ready to build one as well and I'll be using a 2A slow blow. Like to also point out the Fender 5e3, a 12 watt, also has a 2A fuse.

I wanted to create my own layout because I enjoy drawing layouts before I build an amp just to get to know it better, plus I make my own circuit boards by making a copy of the board and removing everything but the turret and screw holes. Then I print at 100% ratio out of visio. I use an elmers glue stick to attach the printout to the G10 board and start drilling and swagging turrets.
 

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StratMatt7376

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Most of the layouts I've looked at (which is probably ALL of them) have a 2amp fuse. This was recently an issue with a new amp builder on the 18Watt forum. I'm getting ready to build one as well and I'll be using a 2A slow blow. Like to also point out the Fender 5e3, a 12 watt, also has a 2A fuse.

I wanted to create my own layout because I enjoy drawing layouts before I build an amp just to get to know it better, plus I make my own circuit boards by making a copy of the board and removing everything but the turret and screw holes. Then I print at 100% ratio out of visio. I use an elmers glue stick to attach the printout to the G10 board and start drilling and swagging turrets.
This is a great amp. I highly recommend it. It was actually my fault that the fuse was blowing, though. I had one of the heaters hooked up wrong. On the other note, making your own circuit boards is awesome, though I'm not sure if I totally understand your process.
 

RetiredUnit1

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This is a great amp. I highly recommend it. It was actually my fault that the fuse was blowing, though. I had one of the heaters hooked up wrong. On the other note, making your own circuit boards is awesome, though I'm not sure if I totally understand your process.
Visio allows you to 'draw to scale'. When you start a document you set metric or inches, and general size of the drawing. It's used on everything from building layouts, to circuit boards to flow charts for programming (I used version one for that).

So as I add a resistor or capacitor, I measure it's exact size and that goes into the shape database. When I'm done moving stuff around and getting the layout made, I can make a new blank page and copy the circuit board layout to that page, giving it a name like "board drilling layout". Then I delete all the resistors and capacitors that I used to be sure I was putting everything in relatively correct positions, leaving only the turret and screw holes as shown in the picture below.

I saw the epoxy board, called a dozen different names like G10/FR4, on my diamond tile saw. This stuff stinks to high heaven when sawed so the water on the diamond saw stops the stench, and the diamond cuts nice smooth edges.

I print out the drilling template, remove the excess paper and glue that to the epoxy board I just cut. Now I have epoxy board with paper on it that shows me exactly where to drill the holes for the turrets and screws!
 

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