Bugera 1960 NON infinium

[Danglin' Fury]

I have a Bugera 1960 on me bench. (Obviously lol) I turn it on, all tubes glow....I turn the Standby on (or....off?) And it makes a buzzing sound and then blows the mains fuse... the power tubes are good as i just took them out of my amp to test this one. I measured the OT from each leg to the center tap. It reads acceptable... Any ideas?

[Enzo]

Measure each power tube plate resistance to GROUND. Transformer could be shorted, but more likely would be one of the flyback diodes has shorted. Check OT for winding short to frame and primary short to secondary.

[Danglin' Fury]

Hey Enzo. Thanks for the reply. With the power tubes out, plate resistance to ground is pretty high. The primary of the OT is reading pretty good. Balanced. 26.4 ohms from center tap to one side, 28.5 on the other. Primary is not shorted to secondary....buuuuut I just measured every leg of the secondary to frame....apparently they are ALL shorted to the frame....shit... How in the hell did that happen? Also, there are no fly back diodes in this amp. None that I can see anyway.

[Enzo]

The secondary is a low number of turns of heavy wire, so of course it measures a very low resistance. Do you have some other amplifier? Measure the secondary resistance on that. I bet it seems "shorted" as well.

[Danglin' Fury]

True. I see your point. Non of the primary wires read a short to the secondary so that is good... when I turn this thing off standby, I get like maybe 2 or 3 seconds of static like sound and then the mains fuse blows. No other fuses in it go, just the mains... aaaaand unfortunately, my "buddy" I let borrow my lightbulb limiter made off with it... bastard. So I cant take any voltage measurements with the tubes in. Could the choke be a turd?

[Helmholtz]

The secondary is typically grounded at the output jack. As all secondary windings have a DCR below 1 Ohm, all secondary wires will look shorted to ground.
Does the amp blow the mains fuse with power tubes out?

Schematic?

[Danglin' Fury]

Indeed....thank you sir....I had "the dumb" for a minute. This is actually the same Bugera 1960 I had on the bench in 2018. I found the problem to be a bad OT and replaced it. All was well... now, even with known good power tubes, I turn it.on and within seconds it torches the mains fuse. Very frustrating. The OT still seems to be in good shape though... as it doesnt have any fly back diodes in this thing, what are the opinions on Ken Fisher's 3 diode mod off pin 3 of the power tubes? We can leave that for another time, just wondering if it would protect the OT or not. Anyway, I cant keep this thing on long enough to measure voltages. I guess I can try to put the standby in the "play" position without power tubes in it and see if it even stays on.

[The Dude]

......Schematic?

Yes, that.

[Danglin' Fury]

The schematic is.posted on this site. That's where I got it. I dont know how to post the link.... sorry. If you do a search for Bugera 1960 schematic it will pop up.

[g1]

Here it is

[The Dude]

It looks like the standby switch is before the HT rectifier circuit, so you could have something low resistance in the rectifier circuit. If it were a dead short, I suspect F4 would open, but maybe mains fuse goes first. Or are we calling F4 the mains fuse? Loss of bias voltage would give you lots of current, so check that bias voltage is present. Also check the rectifier diodes D10-D13. Does the amp draw excessive current with power tubes removed?

[Danglin' Fury]

All of the diodes are good. F1-F4 are all good. The mains fuse is the one on the back of the amp attached to the PSU6 A/C input board that has the MOV s on it... 3 amp I believe...

[Danglin' Fury]

Ok looking at the back of the amp with the chassis out, the 2 right hand EL-34 sockets on pin 5 have - 35.12 VDC... the 2 left hand sockets on pin 5 read -1.43... definitely no Bueno

[Helmholtz]

Ok looking at the back of the amp with the chassis out, the 2 right hand EL-34 sockets on pin 5 have - 35.12 VDC... the 2 left hand sockets on pin 5 read -1.43... definitely no Bueno

I guess this is with power tubes out?

Lift one end of coupling caps C26 and C29 each and measure pin 5 voltages again.

[Danglin' Fury]

Yes it is with the power tubes out. The mains fuse blows within seconds of the amp being fully on, as in the standby in the play position. Also, I measured the 220k ohm resistors (R15 and R23) one of them read 220-ish k ohms, the other one reads ~ 680-ish k ohms.