Fuse blows when Standby switched off ?

[kolandscapes]

Fender DeVille Amp ..

Warms up no problem. As soon as I switch off Standby the fuse blows. Using 3 Amp fuse. All tubes are clear with no sign of burnout.

Thoughts ?
Thank you...

 

[Milspec]

Failed diodes is my guess. Had that same issue with a SF Twin years ago. Cheap parts, easy repair.

 

[2L man]

Fender DeVille Amp ..

Warms up no problem. As soon as I switch off Standby the fuse blows. Using 3 Amp fuse. All tubes are clear with no sign of burnout.

Thoughts ?
Thank you...

Shorted HV filter capacitor is my first quess. Depending where the HV switch is the rectifier short is second quess. Power tube or its socket is third quess.

 

[Michael Smith]

Others will suggest diagnostic procedures, but I would add now is the time to build a light bulb current limiter.

 

[corliss1]

Hey new member - welcome!

What troubleshooting tools do you have? Do you have a current limiter so you're not just blowing through fuses? Any spare tubes? It's almost certainly a power tube, but you'd need some tubes and a fuse to confirm this.

 

[King Fan]

https://robrobinette.com/5e3_Modifications.htm#Light_Bulb_Current_Limiter

 

[kolandscapes]

Shorted HV filter capacitor is my first quess. Depending where the HV switch is the rectifier short is second quess. Power tube or its socket is third quess.

So I'm guessing I need a tech to look into this ? Or is there a way I can check beforehand ?

These things are somewhat foreign to me in terms of knowing what I'm looking at. The amp was recently given to me and appears barely used. My first experience with a tube amp so I'm not exactly familiar with the nuances you could say.

 

[kolandscapes]

Hey new member - welcome!

What troubleshooting tools do you have? Do you have a current limiter so you're not just blowing through fuses? Any spare tubes? It's almost certainly a power tube, but you'd need some tubes and a fuse to confirm this.

Don't have trouble shooting tools really but a neighbor might. No spare tubes but they look brand new practically. Do have 8 fuses left.

 

[kolandscapes]

Failed diodes is my guess. Had that same issue with a SF Twin years ago. Cheap parts, easy repair.

How do I know where the Diods are ?

 

[Powdog]

How do I know where the Diods are ?

This question alone leads me to emphasize that you should take the amp to a tech.

 

[Michael Smith]

Don't have trouble shooting tools really but a neighbor might. No spare tubes but they look brand new practically. Do have 8 fuses left.

Even if a tube looks brand new, if the amp was jostled around in the trunk of a car or bed of a pickup truck, it could have an internal short. Blowing the fuse is due to something in the circuit drawing too much current or a dead short. You could try pulling the power tubes, then power it up and take it out of standby. If the fuse doesn't blow, that might point to a bad power tube. I guess it could be something else in the output section of the circuit, but you would need a schematic and the ability to measure voltages to track down the problem. Probably time to take it to a tech in that case. A tech might have a tube tester that can detect an internal short in the power tubes.

 

[Michael Smith]

This question alone leads me to emphasize that you should take the amp to a tech.

I would agree, even when unplugged a tube amp can have 4 or 5 hundred volts inside the chassis.

 

[kolandscapes]

Even if a tube looks brand new, if the amp was jostled around in the trunk of a car or bed of a pickup truck, it could have an internal short. Blowing the fuse is due to something in the circuit drawing too much current or a dead short. You could try pulling the power tubes, then power it up and take it out of standby. If the fuse doesn't blow, that might point to a bad power tube. I guess it could be something else in the output section of the circuit, but you would need a schematic and the ability to measure voltages to track down the problem. Probably time to take it to a tech in that case. A tech might have a tube tester that can detect an internal short in the power tubes.

Are the power tubes the larger ones ? There's 2 large ones on the left an 3 small ones to the right. Should I pull ALL the tubes when doing that ?

 

[Michael Smith]

Are the power tubes the larger ones ? There's 2 large ones on the left an 3 small ones to the right. Should I pull ALL the tubes when doing that ?

The power tubes are the larger ones. I would start by just pulling those, then plug the amp back into the wall, power it on and let it warm up for a minute and then take it out of standby. Note that the power tubes have an indexing key that has to fit in a slot in the tube socket when putting them back in.

 

[schmee]

Diodes or filter cap is a good idea. But first, how about replacing the switch? They can arc inside causing spikes.

 

[Michael Smith]

Diodes or filter cap is a good idea. But first, how about replacing the switch? They can arc inside causing spikes.

I assume that amp is diode rectified, so the standby switch should be used. But if you had a Variac you could temporarily alligator clip a jumper across the switch and slowly bring up the voltage.

 

[King Fan]

@kolandscapes , beware. This is a friendly crowd of folks who *really* want to help, and we're a bunch of guys who work around 600V amps and love to suggest (lots of different, so far) fixes and fancy tools. Do you know how to use a multimeter? Build a cap snuffer and safely drain filter caps? Solder? If not, I'm not saying you couldn't learn, but you'd need to learn all that before you ever open up your amp.

Here's what's safe. You could try replacing the output tubes (yeah, the big ones) and see if the fuse still blows. If it does, tech time.

 

[kolandscapes]

The power tubes are the larger ones. I would start by just pulling those, then plug the amp back into the wall, power it on and let it warm up for a minute and then take it out of standby. Note that the power tubes have an indexing key that has to fit in a slot in the tube socket when putting them back in.

So I pulled the power tubes as directed ... and the fuse didn't blow Most appreciated

GT isn't around any more. What do you recommend I get ?

 

[Michael Smith]

So I pulled the power tubes as directed ... and the fuse didn't blow Most appreciated

GT isn't around any more. What do you recommend I get ?

Hmm, that's a Sovtek 5881, also known as a 6L6GB. Let me check the schematic and let you know. If you gently shake the tubes near your ear, do you hear anything rattling around inside?

 

[Michael Smith]

Schematic for the Fender Hot Rod Deville calls for a pair of 6L6 GC's. Does that agree with the tube chart inside the back of your cabinet? I would probably go for JJ brand. By the way Groove Tube didn't manufacture the tubes, they just put them on their tube tester to sell them as a matched pair, or matched quad, etc.