Fender Twin Reverb 65 RI keeps blowing 8 amp filament fuse

[Alf]

http://www.ametherm.com/thermistor/d...tc-thermistors

Why not try these in the primary of your OT , worked for me with a toroidal OT which draws a large inrush current.

Just my 2 cents,

Alf

[tim]

Try being scientific. Pull the fuse and clip in your current meter. What is the draw? Pull all the tubes, now how much current is there? Your two 50 ohm resistors are 100 ohms across the voltage, so 63ma there. Or is there more than that? Oopss, wait, the pilot light draws a little, or pull the bulb.

Hi Enzo...I've done all that. There is a current spike as to be expected when amp is turned on cold but not when it is turned on warm. Current draw after it's on a few seconds is about 5 amps. With all tubes, pilot lamp and balancing resistors removed, no current at all as to be expected as well. If I install pilot lamp and balancing resistors only, just a few hundred ma surge on turn on cold, and expected current after it's been on a few seconds.

[Jazz P Bass]

At this point, with the setup you have, I would install one tube at a time.

Start at the preamp tubes & leave them in as you go along.
When you get to the power tubes, carefully monitor the draw.
I think you may have a duff power tube. (new or not).

[tim]

At this point, with the setup you have, I would install one tube at a time.

Start at the preamp tubes & leave them in as you go along.
When you get to the power tubes, carefully monitor the draw.
I think you may have a duff power tube. (new or not).

I've tried three different quad sets of power tubes. All exhibit the same symptoms. I measured current draw on each and every tube, pre and power, and all are well within spec. I think there is a reason why Fender changed their filament fuse ratings from 8a to 10a. It's not like the filaments are drawing excessive current when they are warm. It's only for a second or two when the amp is first turned on. The fuse can take only so many of those "turn on when cold" cycles before the metal in the filament stresses and breaks.

[g1]

I would suggest contacting Fender about this one. Especially if still having the problem with the T10A.

As well as the possibility that the fuse is still "too fast", also consider the thermistor may also be "too fast". It may be worth trying a replacement.

[Pdavis68]

This is a real long shot, but I read somewhere about a 65 reissue arcing with one of the PCB screws. Try removing all of the screws holding the PCB in that are anywhere near electronic components or traces. Long shot, but who knows?

[tim]

I would suggest contacting Fender about this one. Especially if still having the problem with the T10A.

As well as the possibility that the fuse is still "too fast", also consider the thermistor may also be "too fast". It may be worth trying a replacement.

I have an email into Fender right now. If I don't hear anything by late afternoon, I'm going to call them. I do this stuff in the evening after my 9-5 job. Usually most places are closed by the time I get into my shop unless they are west coast.

I did consider the thermistor but in all the tests I did on it, it seemed to work normally...measured resistance heat vs cold, hung a scope on both sides of the thermistor and turned amp on and watched the voltage drop across it over 5-10 seconds...seemed normal. No instantaneous heavy current draw like I see on the filament secondary. It should be interesting to see what Fender says.

[tim]

This is a real long shot, but I read somewhere about a 65 reissue arcing with one of the PCB screws. Try removing all of the screws holding the PCB in that are anywhere near electronic components or traces. Long shot, but who knows?

Thanks...I know about that...I ran into it a few years back. That is caused by one of the traces that carries plate voltage getting too near one of the PCB hold down screws. The easy way to fix it is before it happens (the arc), using a nylon or teflon washer/spacer under that screw which gets the head far enough away from the trace so it doesn't arc over. If it has already arced, I usually cut the damaged trace away, clean the area well with fluxoff or alcohol and jumper it with Teflon covered wire

[demundo]

I would like to help, what schematic are you using?

[Jazz P Bass]

I recently had an Eric Clapton Twinolux come through.

It had a 12 Amp fuse on the heaters.
(there is even a factory label 12A T)

And a bad tube took it out.

[g1]

demundo: up in post #3 is the schematic. But the fuse being discussed is not shown on the drawing.
Welcome to the forum.

[Enzo]

It is on later revisions, I'll try to post it soon.

[nickb]

Just throwing out a wild idea. I do find it more than a co-incidence that this only started happening right after a rectifier blew. Now, since you can reproduce the problem, does it still happen if the standby is kept off (i.e no HT)?

[demundo]

yeah I see that. I will take mine apart and look. fuse xf4

demundo: up in post #3 is the schematic. But the fuse being discussed is not shown on the drawing.
Welcome to the forum.

[kurtsa]

I have an email into Fender right now. If I don't hear anything by late afternoon, I'm going to call them.

Did you receive more information from Fender?

I'm currently having a reissue Twin Reverb on my desk with similar symptoms. It came with a blown 10A heater fuse. 10A is also written to the PCB. All the tubes, pilot light and hum balance trimmer are ok. Wiring and tube socket contacts seems to be ok. Digital multimeter shows about 12A peaks during the cold start for a second or so. Probably the meter (Fluke 189) is too slow (and measurement range limited) even in peak measurement mode to show the real peak values. Anyway peaks should be substantially higher to blow a 10A slow blow fuse. I checked specifications of two fuse manufacturers (Littelfuse and Schurter) and they both state that slow blow fuse should stand 15A for minimum of 30minutes. At 27.5A a slow blow fuse should withstand 0.6seconds minimum, 10seconds maximum. For 40A the specification is 0.15s minimum, 3s maximum.

One explanation could be raised fuse temperature due to a loose contact between the fuse and the fuse holder or maybe faulty solder joint of the fuse holder. Or maybe even the too narrow PCB traces.

Update:
Today I measured the current with a shunt resistor (0.01 Ohm) and an oscilloscope. The highest cold start peaks were repeatedly around 20.5A (RMS). Current dropped below 10A after 1.2seconds. I tested about 15 times waiting for 0.5-1 hour between the tests. Every time pretty much the same result.
_
Kurtsa