50W Build - First Startup, Bias Issue

[Littlewyan]

Things did improve last time the OT leads were swapped but the speaker jack wasn't grounded. Another quick test would be to just disconnect the NFB and see if the squealing stops.

[Cathode Ray]

Things did improve last time the OT leads were swapped but the speaker jack wasn't grounded. Another quick test would be to just disconnect the NFB and see if the squealing stops.

Will do.

Want to be clear.. "disconnect NFB" would mean the connection points marked in this image, right ?

[Littlewyan]

Yeah you can disconnect it from any point. If it's easier just disconnect it from the presence pot, or lift one side of the NFB resistor.

[bepone]

yes and if the problem is in the output stage, squealing will stop. if is in preamp then will remain. any amp without NFB connected must be totally stable

[Cathode Ray]

Ok, I put the wires from the output transformer back the way they were originally.. This was the only change I made.

Connected AC power and connected the speaker output to a 4x12 cab.

As soon as I flipped the power switch on, pop.. smoke.

Instantly switched power off and drained filter caps.

Smoke came from this area :



Cannot feel any components are hot. Not sure exactly what just got burned up.

[bepone]

bad sign..we are going backwards.
this is bridge rectifier.. or first input capacitor there..somwthing is connected wrongly or very bad bridge rectifier.. need to be removed, checked, replaced,4700uf/25v capacitor also!

[Cathode Ray]

bad sign..we are going backwards.
this is bridge rectifier.. or first input capacitor there..somwthing is connected wrongly or very bad bridge rectifier.. need to be removed, checked, replaced

Yeah, this circuit provide DC heaters for pre-amp tubes and PI.

It's a 14VAC tap off the PT that goes thru bridge rectifier, some filtering caps, and a voltage regulator.

In the end, it's a 12V DC power supply for the 12AX7 heaters.

[Cathode Ray]

bad sign..we are going backwards.
this is bridge rectifier.. or first input capacitor there..somwthing is connected wrongly or very bad bridge rectifier.. need to be removed, checked, replaced,4700uf/25v capacitor also!

The bridge rectifier is this part here :

RC206


It's clearly marked on the package top where the AC leads go, and I have it correct.

It's a 68 cent part from Mouser, but (obviously) my real concern is why would this component go.. at this point ?

[bepone]

blown because you have low impedance (short) in some heater wiring ? remove it and power up without this rectifier.. also you can measure 14VAC voltage if still alive

[Cathode Ray]

blown because you have low impedance (short) in some heater wiring ? remove it and power up without this rectifier.. also you can measure 14VAC voltage if still alive

Doesn't seem to make sense, because that would have failed long before now, considering my testing thus far..

Not saying that's not what has happened - just baffled as to why that would happen after swapping OT primaries back to their original positions.

[bepone]

who knows what you did. doesnt matter now, you have burned amp. you will find out what happen later, at the end all will be clear. just now you need to double your power to repair this, before was "easy" now is one level up. so dig again, start from 0, start without small bridge rectifier and see the 14 VAC present

[Littlewyan]

Have you got a light bulb limiter? If so, use that for now to avoid anymore destruction.
Can you see any burn marks on anything in that area? Was it definitely the rectifier? Have you tested the rectifier with your meter to see if it's shorted?

[Cathode Ray]

who knows what you did. doesnt matter now, you have bourned amp. you will find out what happen later, at the end all will be clear. just now you need to double your power to repair this, before was "easy" now is one level up. so dig again, start from 0, start without small bridge rectifier and see the 14 VAC present

Yeah.. before I do that I'm going to remove the turret board so I can get a better idea of what just happened.

[Cathode Ray]

Have you got a light bulb limiter? If so, use that for now to avoid anymore destruction.
Can you see any burn marks on anything in that area? Was it definitely the rectifier? Have you tested the rectifier with your meter to see if it's shorted?

Yeah, that's just it.. I performed the initial start-up using a variac and LBCL.

I'm not certain it was the bridge rectifier that just burned up.. I was looking directly into the amp when I flipped the power switch on, and that is where I saw a puff of smoke.

I have clipped the 14VAC taps off the turret board, but before I do any more testing with power I wanna see under the board. Get a better idea of what happened.

[pdf64]

Use a low power bulb for initial etc testing, ie when checking for such shorts.