Blowing fuses

[gui_tarzan]

So this is a stumper. Fender SF Twin Reverb that's blowing fuses when the standby is switched off (running, not idling).

I replaced the filter and bias caps and diodes, some of the caps were original (and leaking) and some mis-matched and the diodes looked very crusty. When the amp is in standby it's fine but within 10-15 seconds of switching it off it blows the mains fuse.

It does not blow the fuse when the tubes are out.

I tried a new set of 6L6s, same thing. Checked all screen and grid resistors and replaced a couple that were out of spec. Removed the secondary wires and it doesn't blow a fuse so it's only when the PT is in-circuit. But I can't see anything to indicate a problem, other than the fuse popping.

[Deric]

Are you using the correct Slo-blo type fuse?

[gui_tarzan]

Yep. There was a fast-blo in it when I got it, I was hoping that was the problem.

[Deric]

So much for the easy fix.

Is this a later silver face that has the standby before the first filter cap? If so, moving the standby to after the first filter might be a good idea and be easier on the fuse. Just a thought....

Edit: Scratch that.... Just checked and it looks like all the Twins had the standby after the first filter caps...

[Colossal]

Replace your rectifier diodes.

[gui_tarzan]

Yep, after the first pair.

[rp]

It works in stby, it works w/o tubes, it still blows fuse with different power tubes. If power section really is fine, could you have a bad preamp tube? Like broken bad?

[Leo_Gnardo]

How's your bias department? Any sort of failure in bias would send your amp into the kind of failure you're experiencing.

A bad output tube could do it too but I wouldn't go sacrificing new ones until 100% sure about the bias supply and its distro network.

Least likely but possible, a bad signal coupling cap feeding output tube control grids. These hold up well in old Fenders, but every now & then I find a baddie.

[Stevem]

This sounds like a voltage potencial issue to me!
The heater voltage potencial does not change in standby or our out, this leaves the B+ winding and the bias tap that will jump up in voltage when then current load is removed in the standby position .

Like Leo posted it could be the bias section, replace that diode and also note that dependant on the year amp the bias filters where run much too close to higher then there voltage rating and with the amp in standby that voltage goes over the top!

Replace those filters, that diode and see if it flys !
You can test just the PT itself by disconnecting all of the secondary wires and on the primary side after the fuse and before the power cable install a 1 ohm 5 to 10 watt resistor.
Next hook up a AC voltmeter across that resistor and turn the amp on and if the meter reads over .4 volts then the transformer is toast!

Since you state that it takes a few seconds to blow the fuse my bet is on the bias filters and you will likely find that they are warm to the touch after the fuse blows.

[martin manning]

So this is a stumper. Fender SF Twin Reverb that's blowing fuses when the standby is switched off (running, not idling).

The fuse blows when the amp is taken off standby, so we can assume that the heater and bias circuits are not directly causing the problem.

I replaced the filter and bias caps and diodes, some of the caps were original (and leaking) and some mis-matched and the diodes looked very crusty.

All filter caps and diodes have been replaced, so let's assume they are now "good."

When the amp is in standby it's fine but within 10-15 seconds of switching it off it blows the mains fuse.

It does not blow the fuse when the tubes are out.

Adding the idle current draw to the HT is what takes the fuse out. If the idle current draw is reasonable then (and only then) would I suspect the PT is at fault. I assume when you say "tubes" you mean power tubes. If the preamp tubes are installed does the fuse hold?

With the power tubes removed:

What is the plate voltage?

What is the bias voltage at the power tube sockets (pin 5)? A reasonable level would be around -40V.

Have you checked the power tube idle current? It may be that the idle bias is just too hot. SF Twins have a bias balance pot but no level pot, so you'll have to change a resistor or add a pot to adjust it.

[gui_tarzan]

Replace your rectifier diodes.

Just to reiterate, I replaced all the filter and bias caps and power supply diodes. The dropping resistors all checked out good, I have 335-335 on the secondaries, -36v at pins 5, 465v at the plates.

[martin manning]

-35V sounds hot to me. What is the idle current and plate dissipation on the 6L6's?

[gui_tarzan]

I don't know, it won't stay running long enough once I throw the standby switch.

[gui_tarzan]

With all pre-amp tubes out I stuck the power tubes back in and let them warm up, then took it off standby and they redplate immediately. No time to test voltage without endangering the tubes.

[gui_tarzan]

Second test, took it off standby and within 5 seconds it blew a fuse.