63' fender super keeps blowing fuses

[goldstache]

I have this issue with my super.  It's been blowing fuses as soon as standby/HT is engaged.  I replaced fuse.  The amp says 3A, the schematic says 2A.  I went with schematic.  If I yank the rectifier and power on I get proper voltages on all PT secondaries.  I put a rectifier in (tried several) it stays on for a play.  Then the next time I power up (at a gig for instance), fuse goes!

Any pointers?  It needs recapped, but there isn't any leakage.  All tubes test good and have been swapped just to be sure.

I miss this amp so much and am thankful for any thoughts!
Thanks!

[eleventeen]

Too many unknowns. It's not just the amp rating on the fuse, it's the "slo-blo" characteristic as well. A slo-blo 2A might work fine, a regular one probably would blow. It depends upon whether you have a GZ34 or 5U4 rectifier, the 5U4 uses 5 more watts with its 3 amp filament versus the GZ34 a 2 amp filament. Maybe your home voltage is lowish, maybe the line volts at your gig tend to be higher. A few of these conditions nudged this way versus that way could make the difference between the fuse blowing or not.


If there's no leakage on the electrolytics (virtually unheard of on a 50 year old amp, assuming they are stock) then why do they need replacing?

[John]

If it were me, I'd try a 3A slow blow. If *that* blows, I'd think it's time to replace those electrolytics. Do you have a light bulb limiter? That might help you to figure out where the current drain is happening.


If OTOH that 3A sloblo doesn't blow, problem solved. 

[goldstache]

Thanks for replying! 
I am using a slo-blow 2A fuse.
The rectifier is a 5U4
My home voltage is 119VAC, also brought it up slowly for some measurements with a variac.
I've replace some but not all of the caps since I've been the owner. 
Initially just changing blistered leaky ones.

All voltages seem within spec during a stable power up.
Any hints on where to start my trouble shoot?

[goldstache]

Does anybody have the patience to take me to school on fuse rating basics?!
 

[eleventeen]

Not that much to know, really. On the input of a tube (or transistor, for that matter) amp we have a transformer, which if you measure the DC resistance of the winding, can be very low. Almost a short circuit. Consequently, there is biggish inrush current. A slo-blo fuse is intended to absorb this without blowing. There are also "fast blow" fuses designed to protect much more delicate equipment. Tube amps are not really considered "delicate". They can handle and are often called on to handle inrush initial turn-on currents bigger than what they use during normal operation, and indeed, they might use that kind of current if they are turned up and played full blast.
 
The purpose, obviously, is to prevent some fault from creating such a high input current that stuff burns up. That's the purpose of *any* fuse.


For the most part, you just stick the right rating fuse in the fuseholder and that's really all one should have to know about it. The issue nowadays is that fuses are expensive! It's no fun blowing 3 of them at almost a buck each.

[goldstache]

Ok thanks for the help!
Interesting what was mentioned about filament current for the rectifier!  Maybe I will try a 3A fuse in lieu of the 2A in the schematic.  I'm thinking if it were a rectifier issue it would blow immediately upon power being switched on.  It only seems to blow when HT is engaged via standby.  So I'm thinking further down the circuit!
The quest goes on!

[goldstache]

Okay.  I was all wrong in my details above.  I have a 6g4-A super the rectifier is a 5ar4/gz34 not a 5u4 as stated before.  The schematic says 3A slo blo and the back of the amp says 2A slo blo.  I installed a 3A fuse and it is powering on and off without failure!  Well at least here at my house!
Do ya think the schematic gives the most accurate fuse suggestion over the screen printed chassis backplate?

[DummyLoad]

transformer is rated at @ 190VA. should draw around 1.6A fully loaded. fuse for 1.5-2 x max. current load. in this case round down and use a 3A. use a slo-blo for inductive load.


http://www.hammondmfg.com/pdf/EDB290DX.pdf


you could just experiment: you need at least a 2A SB, but it doesn't hold, what about 2.5A SB? 3A SB holds, so 3A SB it is...


--pete

[PRR]

> use a slo-blo for inductive load.

For capacitive, hot heater, or (what you were thinking) MOTOR load.

Motor has inductance but this limits turn-on current.

Motor is normally operated at speed which makes back-EMF which holds running current down to match the work on the motor. However from a dead start the speed is zero, the back-EMF is zero, the motor current is maximum (essentially locked-rotor). Typically 5X-10X higher than running current.

Assuming the motor gets up to speed in "a few seconds", this high current will not melt the motor or its wires. But we do need delay-blow fuses.

[DummyLoad]

For capacitive, hot heater, or (what you were thinking) MOTOR load.

i was thinking the mains see an inductive load. a transformer is an inductor? or did i miss the boat again somehow?

--Pete

[thermion]

A transformer winding and a choke are both long coils of wire, they have inductance.

[goldstache]

It is indeed a 6G4-A.  And yes the schematic calls for 3A slo blo.  My confusion stems from the 2A slo blo silk screened on the chassis backplate.  Someone on the forum told me to go with what was printed on the amp.  It seems clear now that 3A is the way to go for cold start powering up! 
Pics to come when I'm home!
Thanks guys!

[DummyLoad]

It is indeed a 6G4-A.  And yes the schematic calls for 3A slo blo.  My confusion stems from the 2A slo blo silk screened on the chassis backplate. Someone on the forum told me to go with what was printed on the amp. It seems clear now that 3A is the way to go for cold start powering up! 
Pics to come when I'm home!
Thanks guys!

that was me. apparently fender put the wrong rear face-plate on that amp or it was replaced.


http://blog.guitarcircle.de/2012/04/09/fender-super-amp-brown-6g4-a-1962/fender-super-amp-62-back-left-corner1/


--pete

[goldstache]

No worries Dummyload!   It got me thinking and in process I learned stuff!
Thanks!

[goldstache]

Here are the pics

[goldstache]

and another

[Willabe]

Hey she's in wonderful shape!


                 Brad   

[eleventeen]

Definitely handsome looking amp/amps. I like reverb in the amp but I wouldn't turn down such an amp under any conditions. I used to have a brown vibroverb (which I sold to Mr. Groove Tubes) which of course looked "about" the same but I didn't especially like the way it sounded....believe it or not. Don't ask me if I regret selling it.