Fender Twin blowing fuses

[Dub8er_e]

I have a fender twin amp that is blowing fuses. I noticed that it had a bad rectifier tube as well. Any help would be appreciated.

[e_c_mccollum]

Check the condition of the main filter capacitors located on underside of the chasis. Sometimes they will blow.

Are the pwr tubes black? Is the rectifier tube black? Does the fuse blow right away, or does it take time?

[acorkos]

are you sure you don't have a solid state rectifier?

[e_c_mccollum]

What year is this amp?

[Dub8er_e]

The amp is a 5E8A and it is a 2008. I dont think I have a solid state rectifier. It has 2 5U4GBEH tubes that I believe are for the rectifier and one is bad. They arent burned though. The fuse blows about 5 seconds after the amp is turned on.

[acorkos]

ok....not solid state rectifier (i thought you had a blackface twin reverb). you say that it has one bad rectifier tube....how do you know it's bad? if it is, the solution is replacement. you might want to pick up 2 and keep one as a backup.

[Diablo]

That amp will work fine on one rectifier tube. So, I would try each of your rectifier tubes by themselves to determine if the amp works properly, and which rectifier tube is bad. If you are handy with a soldering iron and feel comfortable working with high voltage equipment, you can modify your tube rectifier sockets by adding "protection diodes" 1N4007 to pins 6 and 4. One side of the diode connects to the socket pin and the other side connects to the transformer secondary AC. The striped end of the diode goes toward the socket. If your tube rectifier shorts again, the amp will keep on working, but at a higher voltage, since there won't be any voltage drop across the tube rectifier.

[Dub8er_e]

I can tell the tube is bad because there is a piece that is rattling around inside. Its the little round piece that is usually at the top of the tube on some designs. The main problem is that it blows fuses immediatly after turning it on. I tested across the fuse socket with my dmm while I turned it on but it didnt register a spike in voltage that would cause a fuse to blow. it just hovered right around 121 and 122. The fuse I was useing had a 3 amp 250v rating. When it blew it left a black mark in the fuse. Seems like the markings of a surge. I am thinking there is a short somewhere. Now just have to find it.

[Diablo]

I can tell the tube is bad because there is a piece that is rattling around inside. Its the little round piece that is usually at the top of the tube on some designs. The main problem is that it blows fuses immediatly after turning it on.

Did you try turning the amp on after removing the rattly rectifier tube?
Does it still blow the fuse with that tube removed?

[R.G.]

How many times has "XXX amp blows fuses" appeared here?

And how many time has "light bulb limiter" been a response?

[Prairie Dawg]

How many times has "XXX amp blows fuses" appeared here?

And how many time has "light bulb limiter" been a response?

Many times.

[Phostenix]

How many times has "XXX amp blows fuses" appeared here?

And how many time has "light bulb limiter" been a response?

One advantage of the light bulb limiter over my variac - if I don't watch the current meter on the variac as I turn it up, it still blows the fuse. Doh!

[R.G.]

One advantage of the light bulb limiter over my variac - if I don't watch the current meter on the variac as I turn it up, it still blows the fuse. Doh!

That's one reason I'm so much in favor. I actually have a deluxe variac - metered, with 2A and 10A ranges, from zero to 132Vac with 120V in. I still find I use the light bulb limiter more specifically because of what you note - if my attention drifts or I sneeze and the dial goes to the max, I don't fry the unit I'm working on.

Anyone who does amp repair very much ought to have a deluxe light bulb limiter, with an on/off switch, bypass switch (with fail-safe guard on the switch!) and a selection of maybe 10, 25, 50 and 100W light bulbs. Ham dandy.

[Dub8er_e]

Thanks guys. I am getting my light bulb limiter tomorrow.

[Albert Kreuzer]

Anyone who does amp repair very much ought to have a deluxe light bulb limiter, with an on/off switch, bypass switch (with fail-safe guard on the switch!) and a selection of maybe 10, 25, 50 and 100W light bulbs. Ham dandy.