5E3 fuse issues

[cmattdabrat]

Hey guys,

My 5E3 has been occasionally blowing fuses. The build is almost 2 years old. I've played around 50+ 4 hour shows with it. Been transported and bounced around.

I figured out what the initial problem is. The cover material on the back cover started coming off the back and touched the standby or power switch. I have aluminum tape for electrical insulation. So that was touching the poles on the power / standby switch I fixed that.

One of the issues I noticed was the power switch would spark from pole to pole if you blew it? How can I correct this? Hot glue? Electrical tape?

The other day during practice, the amp seemed to fade out, come back in, and then blew a fuse. Is this a sign of time to replace the tubes?

What else could cause them to blow? They blow gradually and not right when I turn the amp on.


Thanks for all the help. I need to get her reliable again before my next show on 10/27.

Matt

[Chuck H]

If the power switch is stock with a the amp or you otherwise know it's rating to be acceptible, don't sweat it. Those switches may spark or pop or seem fussy in some way. But they keep working. Fender actually used under rated switches on some amps that are now over four decades old. Using the same switch that has been arcing and popping for the last three and a half decades.

The power fading and then the fuse blowing is probably indicative of failing power tubes. If your 5e3 is cathode bias (as per stock) just plug in a new pair and check that the bias is acceptible. If you have a deviant 5e3 with fixed bias the problem could still be bad tubes. But it could also be a bad bias supply. So check the bias supply first and if it's fine get new tubes and set the bias.

[cmattdabrat]

If the power switch is stock with a the amp or you otherwise know it's rating to be acceptible, don't sweat it. Those switches may spark or pop or seem fussy in some way. But they keep working. Fender actually used under rated switches on some amps that are now over four decades old. Using the same switch that has been arcing and popping for the last three and a half decades.

The power fading and then the fuse blowing is probably indicative of failing power tubes. If your 5e3 is cathode bias (as per stock) just plug in a new pair and check that the bias is acceptible. If you have a deviant 5e3 with fixed bias the problem could still be bad tubes. But it could also be a bad bias supply. So check the bias supply first and if it's fine get new tubes and set the bias.

Thanks, Chuck! I'm unsure how to check the bias. I built it and really didn't do much but a smoke check. It's given me very little problems though. Just the fuse issue.

How do I check the bias? My 5E3 build is a Tube Depot build. Attached is the board with values. Here's shot of my populated PCB.










Thanks for the help.

Matt

[Chuck H]

It's cathode biased.

After plugging in new tubes, measure the DC voltage across that 250 ohm resistor on the left in your photos. Then divide that reading by the resistor value (250 ohms). This will be the current your power tubes are drawing. Now read the DC plate voltage at pin 3 of either power tube. Subtract the voltage you measured across the 250 ohm resistor from the plate voltage. This is your working voltage. Multiply your working voltage by the current. This is your wattage at idle. It should probably be about 20 to 24 watts (10 to 12 watts per tube).

[cmattdabrat]

It's cathode biased.

After plugging in new tubes, measure the DC voltage across that 250 ohm resistor on the left in your photos. Then divide that reading by the resistor value (250 ohms). This will be the current your power tubes are drawing. Now read the DC plate voltage at pin 3 of either power tube. Subtract the voltage you measured across the 250 ohm resistor from the plate voltage. This is your working voltage. Multiply your working voltage by the current. This is your wattage at idle. It should probably be about 10 to 12 watts.

Thank you! I'll give it a whirl.

[Chuck H]

I edited my post above. That's 10 to 12 watts per tube (20 to 24 watts is what to expect). The cathode resistor is shared so your current reading will be for both tubes.

[cmattdabrat]

I edited my post above. That's 10 to 12 watts per tube (20 to 24 watts is what to expect). The cathode resistor is shared so your current reading will be for both tubes.

Thanks again!

[stokes]

From the pictures it looks to be cathode biased,should be able to drop in a new pair of tubes.
"One of the issues I noticed was the power switch would spark from pole to pole if you blew it?" By this,do you mean the fuse blows when it arcs?If so,you should change the switch.In the meantime,I guess you could insulate each of the posts with some heat shrink tubing or tape,but I would change the switch.

[cmattdabrat]

What are common causes that would make my amp hiss and crackle? It used to be really quite. Now it's has a his that comes and goes with loudness, but is always there. Also, it's still fading in and out after about an hour of playing it.

I don't know what to do and I need it to act right for an upcoming show.

[cmattdabrat]

Also, it sounds like the ocean too.

[52 Bill]

Did you ever replace the tubes?

[cmattdabrat]

Did you ever replace the tubes?

Hi Bill,

Yes. I forgot to mention it, but I replaced all the tubes with new JJ tubes.

[Bruce / Mission Amps]

Hi Bill,

Yes. I forgot to mention it, but I replaced all the tubes with new JJ tubes.

Preamp tubes and including the rectifier?