5E3 Build Smoke Comes out of Resistor

[apeontheweb]

I built a chinese Ebay 5E3. I plugged it in and it sounded great. I smelled smoke and shut it down. The smoke came from a 4.7K voltage dropping resistor in between the first and second filter capacitors. On some schematics, it's noted as a 5K resistor. To help orient you on the schematic, it is tied to a 22K resistor I was wondering if anyone could help me troubleshoot my problem. Edit: I realized when I the resistor started getting hot that I was playing with the amp in the orientation so that the open side was laying down. There was no ventilation. Could that maybe have caused the resistor to overheat?

[Banjan73]

My first thought is that the resistor has too low wattage. But since this is a kit... I don't know.
Have you measured current/voltage drop over the resistor (if it still measures 4.7k)?

[JPK]

This layout shows it as 2 watt. I checked a Weber and Robinette layout and they're all 2 watt. You can see them on my populated, un-soldered board on the bottom left. Did they supply a lower watt resistor? Got a BOM or pics of it?

[apeontheweb]

The little bag the resistor came in said "2 Watt 4.7K" and I measured it and it was 4.7K. I don't know maybe it wasn't a 2 Watt but how would I know? The resistor still reads 4.7K. What voltage drop might i expect to see over it? (Edit note: I originally wrote 47K here and it was a typo. I went back and corrected this to read 4.7K)

[SEL49]

The little bag the resistor came in said "2 Watt 47K" and I measured it and it was 47K. I don't know maybe it wasn't a 2 Watt but how would I know? The resistor still reads 47K. What voltage drop might i expect to see over it?

Should be 4.7K, not 47K.

[stratomaster]

Something at that node is drawing enough current to kill that resistor.  I'd be inclined to think it's the filter cap for that node, but it could be anything there.

[JPK]

Should say 4k7. Not 47k. Should read close to 4.7k. If they actually supplied a 47k that's not gonna work. The other one needs to be around 22k. Both 2 watt as my layout shows.

[apeontheweb]

Yes it is a 4.7K not a 47K. I also verified with my multi meter. I apologize for my lousy communication when I accidentally said a 47K resistor.

[JPK]

Was it 2 watt? It should look like it. Much bigger. If not look at the tube socket wiring. Make sure it's all correct.

[apeontheweb]

Yeah the resistor is a bit bulkier than the others.

[AlNewman]

Sounds like a good time to build a light bulb limiter.

[tubeswell]

Something at that node is drawing enough current to kill that resistor.  I'd be inclined to think it's the filter cap for that node, but it could be anything there.

^This^


The reason the resistor is smoking is its drawing too much current.

The two pathways that draw current from that resistor are the 6V6 screens and the preamp B+ supply.

Therefore, assuming the amp was built properly, there could be either a failure (short) across the screen node filter cap, or a short on one of the 6V6 screen grids that is dumping excess current to ground (causing all B+ voltage to be dropped across that resistor). Or it could be that you have a short to ground behind the board at that filter cap's power supply node.  If the screen filter cap voltage node measures '0VDC', then it's most likely one of this possibilities.

Please measure the PS rail voltages on all filter cap nodes and report back

[Banjan73]

As said by tubeswell: current is too high.
I've experienced this myself, and then there was a short to ground.
Another longshot thing is that resistors actually have voltage ratings. Since this is in the B+, they should have voltage ratings well above the actual voltage you have.
Since this is a chinese thing, I may question the quality of those resistors (rating wise). But hopefully, they have considered this.

[apeontheweb]

I'm pretty sure that 4.7K resistor is a 2 watt -- some folks were asking. I attached a photo. The red arrow on the photo points to the resistor that heated up. I am going to get a replacement resistor today and then begin to test the circuit. I'll report back. Thanks so much.

[tubeswell]

Further to my previous post - some possible fault pathways

[apeontheweb]

Here are some numbers I can share with you.
B+1 347 VDC
B+2 300 VDC
B+3 237 VDC
Mains amp draw: 0.55A
Voltage drop across 4.7K 2 Watt resistor with guitar plugged in and volume cranked 109VDC at its peak but hovered around 90VDC. (Edit: I calculated that at 90VDC that's right about 2 watts and 109VDC is 2.5 watts!)
I think these numbers are all normal?
I wonder if the resistor overheated because I was testing the amp with the open side faced down? No ventilation. I was playing with the amp cranked. Also I was using a 16 ohm speaker because Chatgpt said that wasn't a problem. But I've since read from Weber speakers that you must use an 8ohm or 4ohm load for an amp that wants to see 8 ohms. But that is an aside. What do you think is there any evidence pointing to why my resistor overheated? Could be an intermittent problem as well.

[apeontheweb]

Hey Tubeswell, Thanks for the help. I'm sorry I didn't get all the values you were asking me to get but here's a few. I had a variac and current limiter in the circuit when I did these tests. They were done at 110VAC.
5Y3 tube socket:
P2 : 275VDC
P4: 262VAC
P6 : 262VAC
P8 : 267VDC
Please also see my above comment where I listed some other specs. Any thoughts? I'm feeling like everything looks good? Maybe an intermittent problem? I dunno.