Need 5E3 Build Troubleshooting Help

[paul-e-mann]

The 4.7K 2W resistor started to smoke on first fire up of the amp and I need some troubleshooting help. I pulled the resistor its a little burned on the outside but it measures correctly at 4.7K so I dont think the resistor was the problem, I'll order a new one any way since its burned. So I'm not sure where to go from here, everything looks to be in the right place. One weird thing I wasnt expecting, I fired up the amp without tubes to check the voltages and was expecting to get 7.5V I was getting more like 2.8V so I wasnt sure about that. Then when I put tubes in it and took it off standby the 4.7K resistor started smoking. See pictures below, it looks messy cuz I didnt do any lead dress, I'll get to that later, I just wanted to make sure it worked first. Any help is appreciated thanks.

 

[Gene Ballzz]

There is a very good reason (and this is a perfect example) why every amp build website and/or advice giver tells builders to first build a $5-$10, incandescent light bulb limiter for starting up amps the first time. There is obviously "SOMETHING" shorted or mis wired in the power supply/string/connections or a reversed polarity capacitor, or…….? My first guess would be one of the under the board connections, like the one from the third 16uf/450v cap from the left in image #6 or first from the left in image #7!

Step back, read up on how to build the simple limiter, head to the home store for the parts and build it before you go any farther!

Just Sayin'
Gene

 

[playloud]

There is a very good reason (and this is a perfect example) why every amp build website and/or advice giver tells builders to first build a $5-$10, incandescent light bulb limiter for starting up amps the first time. There is obviously "SOMETHING" shorted or mis wired in the power supply/string/connections or a reversed polarity capacitor, or…….? My first guess would be one of the under the board connections, like the one from the third 16uf/450v cap from the left in image #6 or first from the left in image #7!

Step back, read up on how to build the simple limiter, head to the home store for the parts and build it before you go any farther!

Just Sayin'
Gene

Plus cheaper than a fire extinguisher!

 

[paul-e-mann]

There is a very good reason (and this is a perfect example) why every amp build website and/or advice giver tells builders to first build a $5-$10, incandescent light bulb limiter for starting up amps the first time. There is obviously "SOMETHING" shorted or mis wired in the power supply/string/connections or a reversed polarity capacitor, or…….? My first guess would be one of the under the board connections, like the one from the third 16uf/450v cap from the left in image #6 or first from the left in image #7!

Step back, read up on how to build the simple limiter, head to the home store for the parts and build it before you go any farther!

Just Sayin'
Gene

I started it up slowly with a variac, does that do the same thing as the light bulb limiter?

I'll check all those things closer you mentioned...

 

[Gene Ballzz]

Here's a link to multiple, yet similar designs and good dicussion on the subject:

The Amp Garage Light Bulb Current Limiter - The Amp Garage

And don't be scared off, if you get a "warning" from goggle or such! That's an artifact left over from when they got hacked, many years back!

And no, using a variac doesn't do the same thing. The light bulb "TELLS" you when something is wrong, whereas the variac simply allows you to gradually increase the voltage, until the fuse blows, or something else cooks! The light bulb limiter might likely have shown the issue, before putting the tubes in!

Check your wiring to the power tube screens, as well as all pins. I heard of "a guy" (who shall remain nameless) who actually wired up his power tubes, one pin off, all the way around! He was lucky to catch it before fire up! In his defense, not all brands of octal sockets are as clearly numbered as others! That's his story, and he's stickin' to it!

Best To Ya,
Gene

 

[playloud]

I started it up slowly with a variac, does that do the same thing as the light bulb limiter?

I'll check all those things closer you mentioned...

Does your variac have a current/power meter? If so, you can get an indication that something is wrong if you see excessive current draw at a low voltage.

However, this is still a manual method (requires you to spot the issue before increasing the voltage to a dangerous level) and subject to human error. The beauty of the light bulb limiter method is that it takes care of this problem automatically!

I have both (variac with power meter and light bulb limiter) and use both.

 

[South Park]

check your pin conections on the tube sockets

 

[Gene Ballzz]

And oh yeah, I've experienced that some octal sockets have the locator hole slight oversize and some tubes (particularly JJs) have slightly undersize locator barrels/tabs. This can allow a tube to be inserted in incorrect orientation, and still not break the locator! That was a scary thing to discover! Luckily no damage!
Again, Just Sayin'
Gene

 

[Spanngitter]

Unfortunately no detailed pics of the Power Valve Sockets as seeing that 4.7k going up in smoke normally indicates that somethin' ain't right with that 6V6 Screen Grid connection at Pin #4...Also, I do not see any PE Connection so you built a infernal machine very well.

 

[Ken Underwood]

No easy way to say this but If you cannot go as you say any further then you should not be trying to fix your amp, give to a qualified tech and get the job done properly

 

[Pattherocker]

The builds that work best and are easiest to troubleshoot are ones that are built with patience, attention to detail and proper soldering techniques.
I would recommend cleaning up your work and you will more than likely find your mistake

 

[Pattherocker]

Also check your grounding, I’m seeing some major issues there unless you have the turrets jumpered underneath

 

[neikeel]

I am far from a Fender type build expert and, of course, I am based in the UK
The point about having a dedicated ground wire from the mains with a star washer and ideally a nyloc to the chassis is non-negotiable (whatever else your wiring scheme).

The fault that causes that sort of dropping resistor in a series chain to blow will be a fault on that node (as has been pointed out pin 4 of your octals which are the screen supply) or the filter cap for that node.
I presume the toasting happened with tubes in?
The 22k does not show signs of roasting (so less likely that the V1 plates or the next cap in the chain is faulty) but you have to reason that there is something shorting to ground causing high current flow in that particular resistor.

I highly recommend the light bulb limiter. I confess that I am a relatively new convert to this (couple of years) but as I have been doing builds/projects/repairs off piste on some odd things out of my normal comfort zone (I can probably build a plexi in my sleep) then they are really worth while.

 

[paul-e-mann]

Lots of good advice here and thank you, I'll build the lightbulb limiter and start checking everything suggested. I'll try to get some better pictures if needed. This is my first build so I dont mind the learning curve.

 

[paul-e-mann]

This guy had the same problem with the same kit, he said he had a bad tube that caused it, go to 28:00 in the video below to hear what he says. All my tubes are lit, I dont have a way to test my tubes otherwise. I checked over every connection in the amp and everything is solid and not touching anything else, I havent looked under the board yet, will require uninstalling to some degree to get at it. Here's what it looked like last I saw it:

 

[paul-e-mann]

I built the bulb limiter and put the resistor back in since it reads correctly and tried out the limiter, the bulb lit dimly which I suppose means theres a short somewhere, and the resistor also started to smoke so I shut off the amp. Tomorrow I'll have to remove the board to inspect the back, everything on the front looks good except that burning resistor. I dont think power is getting to the board, none of the capacitors have any voltage measure they are just zero, I would think they would have something but no.

 

[Chris-in-LA]

I built the bulb limiter and put the resistor back in since it reads correctly and tried out the limiter, the bulb lit dimly which I suppose means theres a short somewhere, and the resistor also started to smoke so I shut off the amp. Tomorrow I'll have to remove the board to inspect the back, everything on the front looks good except that burning resistor. I dont think power is getting to the board, none of the capacitors have any voltage measure they are just zero, I would think they would have something but no.

If the bulb glows dimly, that means no short.

 

[playloud]

I built the bulb limiter and put the resistor back in since it reads correctly and tried out the limiter, the bulb lit dimly which I suppose means theres a short somewhere, and the resistor also started to smoke so I shut off the amp. Tomorrow I'll have to remove the board to inspect the back, everything on the front looks good except that burning resistor. I dont think power is getting to the board, none of the capacitors have any voltage measure they are just zero, I would think they would have something but no.

Putting it back in there is asking for trouble in my opinion.

Have you traced the circuit against a schematic? It is also worth checking every connection for continuity and every resistor value.

 

[paul-e-mann]

If the bulb glows dimly, that means no short.

I thought if no short the bulb doesnt glow at all? Well, at least I dont have to worry about a short and I can look elsewhere.

 

[paul-e-mann]

Putting it back in there is asking for trouble in my opinion.

Have you traced the circuit against a schematic? It is also worth checking every connection for continuity and every resistor value.

Yeah I know I just wanted to try the bulb limiter, its coming back out and a new one put in when I get it.

Yes I traced the circuit, how do I check continuity and value without removing everything? Maybe I have something wired backwards? Transformer wire? Power cord wire? I'll post a drawing later what wires are where since its hard to see in the photos.

The first red flag I saw was the voltages were low powered on without tubes, I got 2.8V everywhere when I was expecting 7.5V (based on the Mojo instructions), I don't know if thats ok or not.