# VibroVerb blowing fuses...



## StevieMac (Mar 4, 2006)

This is a super-general question with little or no information for me to add but here goes...


BF VibroVerb build worked perfectly for a long time and then just started blowing fuses. Powers up but fuse blows almost immediately. Power tubes are both fine however, in fact they're being used in another amp right now with no issues whatsoever. WTF?


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## dtsaudio (Apr 15, 2009)

> Powers up but fuse blows almost immediately


How long does it power up for? Do the tubes get warmed up or does the fuse blow almost immediately. Can you power it up in standby?

Could be bad rectifier tube, bad cap, or heaven forbid power transformer. If you can answer the above questions we can narrow it down.


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## StevieMac (Mar 4, 2006)

dtsaudio said:


> How long does it power up for? Do the tubes get warmed up or does the fuse blow almost immediately. Can you power it up in standby?
> 
> Could be bad rectifier tube, bad cap, or heaven forbid power transformer. If you can answer the above questions we can narrow it down.



Thanks Dan. Tubes appear to warm up normally but then the fuse blows as soon as Standby is switched off i.e once it's in "play mode". It has a SS rectifier (plug in type).


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## Church-Audio (Sep 27, 2014)

Rectifier diode in plug rectifier could be bad. When I get this type of issue I do this
1 pull power tube does fuse blow? 
If no replace power tubes. If yes check diodes or rectifier tube. 
If the amp blows fuses after rectifier is replaced pull all tubes does fuse blow? 
If fuse blows with transformers secondary leads disconnected heater and high voltage, suspect bad power transformer
if you have a neon power indicator cut one lead I have seen these short out and cause a blown fuse condition particularly in Ampeg amps, from the 1970s. Obviously fender does not have this issue. Some amps have false center tap resistors on the heater check these as well usually near the lightbulb.


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## StevieMac (Mar 4, 2006)

Thanks Chris. I'll try that sequence and report back.


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## StevieMac (Mar 4, 2006)

UPDATE: Here's what I found inside, which probably explains blown fuses...


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## Church-Audio (Sep 27, 2014)

1.5k grid stoppers. Looks like you had a problem with a tube. I personally don't like 5 watt resistors in fender Amos 2 watts is all you need and they fail when you have a bad tube leaving your Output transformer safe from damage. Want to go a step further on some touring musicians amps I install 100ma fuse to ground from pin 8 I put a diode in parralelle with the fuse with a 4.7k resistor, the led will glow if the fuse blows. Allowing you to not have blown screen resistors and to simple replace the bad tube on tour and the fuse and party on. Since this is not an original vibroverb IT would be a worthwhile mod. Food for thought. All amp companies should do this! Also now that I look at your picture again bad solder joint or short from ground to the screen resistor. I would replace the pair of sockets in this case and both screen and grid resistors. Looks like the heater shorted out onto the plate !!! Yikes.... I don't like these tube sockets because the solder tabs move around so much. Another good reason to use small two watt resistors... And cinch the resistor tight so this can't happen. I always check for this stuff when I am doing new builds or socket replacement.


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## StevieMac (Mar 4, 2006)

Good advice. I'll be sure to pass that along to my tech. Visually at least, it looked to me like the tab for the OT lead contacted the ground tab beside it at some point, though I have no idea if that's just incidental. The OT lead let go at the tab as soon as I touched it though. Anyway, going to replace both with ceramic sockets.


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## Church-Audio (Sep 27, 2014)

No ceramic unless they are the ones with the two pins on either side of the hole. They are made by belt on and they will outlast your amp. They are a brown plastic type.


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## jb welder (Sep 14, 2010)

Looks like the high voltage at the plate (pin3) arced across to the heater (pin2).
The socket and the 1K5 grid stopper will need to be replaced. The 470R screen resistor should be checked and may need replacing also.
I agree with Chris that 2W (rather than 5W) screen grid resistors offer better fusing for bad tubes and may help protect the amp better.

One other thing, you need to find out how the heater winding is center-tapped. If it has one (usually green/yellow wire) connected to ground, no problem. But if it has virtual center-tap for the heaters, you need to check for burnt 100R grounding resistors, or a burnt hum-balance pot. The 100R virtual center-tap resistors are usually at the pilot lamp or a tube socket, one 100R resistor from each heater wire to ground.

Edit: Also check your cathode resistors, the 1R's going from pin 8 to ground.


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