'71 Fender Twin Reverb blowing mains fuse

[Danglin' Fury]

I just re-capped a 1971 Fender Twin Reverb and now when I turn it on, it plays in either channel for approximately seconds to 1 minute and blows the fuse... I made 100% sure everything was correct polarity and like I said, it sounds great for about a minute then it blows the fuse... all the tone controls work, she gets loud AF.... just, this is weird. Any ideas?

[Randall]

Obviously pulling too much current, so you must find out why. Do you have a light bulb limiter? Does it blow the fuse with the power tubes out? With different tubes? I keep a Kill-A-Watt meter plugged in for these occasions, it tells you how much current it is pulling, better than just "blows the fuse or doesn't".

[Mick Bailey]

Did it blow the fuse before you recapped it?

Pull the output tubes and see if the problem persists. Ideally you need a light bulb limiter or variac and ammeter, otherwise every time this happens the PT (and other parts) are getting stressed. Is the bias cap connected positive terminal to ground? Re-check the polarity of all of the caps with reference to the printed layout - especially the two series connected caps and their 220k balancing resistors.


EDIT: Randall beat me to it!

[g1]

And a reminder (sorry if you already know this) that bias filter caps must have positive end connected to ground.

[Enzo]

And if it helps, I have to admit I have soldered a bias filter in backwards before.

[Danglin' Fury]

Lol! I checked and I do indeed have the positive on ground for the bias filter. I'm going to try it with the power tubes pulled this evening and report back. I will say, in the 45 seconds to a minute that it stays on before the fuse blows, it sounds REEEEAAALLY good. No buzz or hum. And then them the fuse just says "nope, I'm done". Lol! Man you guys had me worried about that bias cap! Lol! And I installed all the caps and resistors in the dog house correctly as well...I will report back later. Thanks guys!

[g1]

Why not try with power tubes removed first. Check pin5 DC voltage at power tube sockets.. Make sure it holds up for several minutes.

[Danglin' Fury]

Yyyyyeah. Power tubes out and standby off, I'm reading -5.5 volts at pin 5. But the fuse isnt blowing. Lol! Wow..

[Danglin' Fury]

While on,I let it sit for a while... now pin 5 reads -4.5... after approximately 5 minutes.

[The Dude]

You should have approximately 10 times that for proper bias voltage (see schematic). There must be something wrong with your bias supply. There isn't much there- a rectifier, cap, pot, and resistor(s). Check the circuit. You were red plating tubes, which caused the fuse to blow. You're not blowing fuses with tubes out because there's no output tube conduction.

[Danglin' Fury]

That's what I was thinking. That bias voltage is waaaaaay too low.

[Greg_L]

That's what I was thinking. That bias voltage is waaaaaay too low.

It is. You're murdering those output tubes. This totally explains why it sounds pretty good for a bit and then goes into meltdown.

[Danglin' Fury]

Oh I was murdering them indeed! Luckily, they are a test quad I've had around for a bit and they aren't the ones meant for this amp.

[Danglin' Fury]

Well....I had the dumb... everyone was asking "Hey is the polarity of the bias capacitor right? Plus on ground" and yes... the one was....the other, however, was not... I looked at the schematic and realized 2 of them need to be positive to ground, and the one mounted to the rectifier board was not. Apparently whomever was in the amp before me positioned it like that as I simply reinstalled the new caps 1 by 1...and placed their orientation How the old ones were. My bad. But I do indeed now have a substantial negative bias voltage and now I'm fixing little things like leaky navy blue caps and a bright switch that doesnt do anything. Thanks again guys for helping me shed "the dumb"...