Valveking 112 blowing the flyback dodes

[GainFreak]

HI,

This amp came to me blowing the mains fuse. Problem was one of the flyback diodes was shorted. I replaced both with RGP02-20E which is a 2kV/500mA diode. The amp was working properly for a month but then it blew the fuse again although at that moment it was only ON without being used. The problem is the same - one of the flyback diodes is blown again. What I did so far:
1/ Checked and resoldered all connections on the speaker board
2/ Removed and connected the speaker wires to the connectors several times
3/ Tried to clean the speaker jack contacts
4/ Installed 1 Ohm resistors to the power tubes' cathodes for easy biasing and monitoring
5/ Checked the OT for resistance, inductance and shorts
Output power before clipping is ~42W. The sine wave is normal (in full power mode). Power tubes are RUBY 6L6GCMSTR. One of them has the paint more "baked" and glows more than the other however they bias within limits.
This time I intend to put 3x1N4007 in series on both sides of the OT but obviously there was a problem (and maybe it's still there) so I would appreciate your comments about what it could be.

[Jazz P Bass]

My first thought is that you have a dud 6L6.

[Enzo]

Maybe, but I can't think of a way for a tube to harm the parallel diode. If the tube opens, then it is like an ampty socket. If the tube shorts, then current shunts through teh tu rather than the diode.

In my eye I only see images of a break in the secondary circuit: intermittent winding, connection jack, speaker wires, speaker.

What does the diode face? One end is grounded, the other end might somehow short to something. Shorting to B+ is more or less normal for it. If somehow the negative bias were to short to it, it would be a forward shunt across that supply, however the B+ would be ther countering the bias supply, so I see no way for that to happen.

Was it the same position diode or on the other side this time?

[nickb]

Could it be that the owner is using it with an external cab that has a flaky connection? Also the ext jack switch interrupts the current to the internal speaker so that is a prime candidate for trouble. It might be best to simply replace it.

[GainFreak]

Thanks for your comments.

My first thought is that you have a dud 6L6.

That thought crossed my mind as well especially if you have like ~7V on the heaters. I wrote about that in another thread but I was told not to worry about it.

In my eye I only see images of a break in the secondary circuit: intermittent winding, connection jack, speaker wires, speaker.

Yes, too many variables. I tried to eliminate some of them but dealing with something intermittent is the problem.

Could it be that the owner is using it with an external cab that has a flaky connection? Also the ext jack switch interrupts the current to the internal speaker so that is a prime candidate for trouble. It might be best to simply replace it.

He's not using an external speaker. The jacks however look oxidized although the amp is relatively new. I tried to clean the contacts of that one but maybe replacing it would be a better idea.

[Enzo]

7v on the heaters would not blow those diodes regardless.

By the way, those tube heaters are all wired in series. You should find about 6v ACROSS each heater, but measurements to ground will not mean much.

[GainFreak]

I measured those voltages across each heater. Here's my previous thread:

http://music-electronics-forum.com/t44211/

[Chuck H]

If the jacks look oxidized (for whatever reason - perhaps the amp is kept in a poor environment occasionally?) there may also be oxide on the tube pin sockets or even the tubes themselves causing intermittent contact.

[GainFreak]

I'll look into that and maybe resolder the sockets as well.