Blowing fuses

[Rob Livesey]

Hi Chaps,

I have an original 1965 blackface Deluxe Reverb. About 10 years ago it was recapped, serviced and retubed and has been one of my favourite and most reliable amps of all time.

Last week, the bias supply cap failed, the tubes drew loads of current (loud hum) and the mains fuse blew. The cap has now been replaced.

At the weekend, I played a festival over two days, the first day, the amp was fine, the second day it would not power up and it appeared that the fuse blew as I turned the amp off the previous day. I replaced the fuse and powered up and down a few times without any problems.

Last night, on a fresh set of power tubes, I used the amp again for a recording session and as I switched the amp off at the end of the session, it blew the fuse again.

This seems to be intermittent, what could be the possible cause of this fault?

Cheers,
Rob.

[lastwinj]

did you check the screen resistors? i dont remember if that circuit has any or not, though.....


good luck!!!

germ

[Rob Livesey]

Hmm, ok. My amp does have them and I think they were replaced in the service too, but I could check them all the same. In fact, they could have been affected when the bias cap went......what do you think?

This happens when powered off from standby, so all that's running are the heaters and perhaps the negative grid bias?

Rob.

[mlp-mx6]

Are the switches original? Especially on the standby switch, there could be internal arcing that is *just for a moment* shorting the B+ to ground. That switch is not officially rated for the +DC that it sees.

Safest bet would be to replace both switches.

[BobW]

Pull the 6V6s, measure the screen resistor values.

With the 6V6s still out of the amp, turn it on and check the bias voltage, adjust it up and down to see if the bias pot still works. I've had a pot go bad in the past, but if it does work OK, then set the bias voltage to the most negative value.
switch the amp from Standby to ON a few times.
If the fuse isn't blown by now, the problem isn't in the power section.
If the fuse does blow, you probably have a weak filter cap in the first stage.

The first filter stage voltage rises when it's unloaded and in a standby mode. I don't remember, but on some models the first filter was either before or after the standby switch. If your layout is before the standby switch, your first filter cap voltage is rising when switching from ON to Standby. This cap may be at the threshold of breakdown when in Standby mode. I would change out the first cap if the fuse blows when switching from ON to Standby with the 6V6s pulled. hth

[drz400]

Get rid of all the electrolytic caps Liftetime is no more than 20 years

[Funkalicousgroove]

One of your tube sockets could have a carbon trace across it where it arced(possibly) when the C- supply failed, it's hard to tell on the black fender sockets, I'd clean them up as best as you can and examine them with a magnifying glass.

When you have a carbon trace on the socket the electricity will flow across it intermittently, especially during power-up, but not usually during power down.

Have you tried powering down with the stand-by switch in the ON position?

[Johnhenry]

If on power up it blow's, yes, all the above could be wrong, but on going to standby a fuse blow's, that tells me that a standby switch is going to ground, to check, like Funk said, power off with the AC switch, if you blow no fuse's useing this method of powering down, then you would know you got a bad standby switch !
Johnhenry

[Rob Livesey]

Thanks to everyone who replied. I've managed to get through a couple of gigs now without popping fuses. I still have the new power tubes installed, I might go back to the RCA blackplates I had in there before to try my luck shortly.
It appears that my new fuses are more robust than my old ones, a new box has managed to do the trick. Whether or not I had quick blow instead of slow blow is a mystery, they were labelled T1A, but the new ones are definately Slow Blow and all seems well.

Thanks again,

Rob.