Fried Amp - Transformer advice

[Middleman]

My 1981 Vibrolux Reverb fried yesterday. Here is the order of events and some questions I have of any kind soul.

I was in the process of replacing the opto-switch to get the vibrato working. The amp had prior worked, no problem but no vibrato.

I replaced the opto and turned the amp back on. The #2 6L6 next to the rectifier started having an internal light show i.e. sparks, super glow then smoke came out of the amp on the underside (tube side was up on the bench. I shut down the amp.

I turned over the amp over and saw no damage to anything. All of the resistors and caps were fine but there was a smell of smoke. I assumed the tube had just blown and that the smoke had come out the underside of the socket. I examined the resistors and caps on that socket and they were fine. The ohm resistance was within range as well. No swelling or cracks on anything touching the socket of the blown tube.

I looked down at the speakers and noticed the connecting wire between the two 10 inch speakers had become disconnected. I assumed this had dropped the ohms down to a point that made the tube fail, this being a 28 year old amp. The tubes were less than 6 months old. A matched set of Groove Tubes. I connected the wire back to the second speakers.

I had some extra 6L6s and replaced the blown tube. Turned on the amp and this time smoke came out again, but it was at the base of the other 6L6 in position 1. On the underside of the socket the 470 ohm 1W burned and cracked. Turned off the amp.

I thought, this is not so bad. Just replace that resistor and go. I decided at this point however, to go over all connections with a fine tooth comb. Upon closer examination, I found two melted/cracked 100 ohm resistors that connect to the pilot light had actually been the source of the first smoking event. The heater wires connect at the pilot light as well to both ends of these resistors.

So total damage so far: 470 Ohm 1W and two 100 Ohm 1/2 resistors.

Before I just replace all these and potentially sacrifice another set of 6L6s. I decided to check the Power and Output transformers. I removed all the tubes, drained the caps. Here are the measurements I took.

Power Transformer:
B/W Primary Coil - 0 Ohms
G/G Heater lines - 0 Ohms
R/R Rectifier feed - 50 Ohms
Y/Y Rectifier feed - 0 Ohms
RB/YR coil = 2 Ohms

Then I crossed checked the above for shorts.
RedBlue to Red 1 - 27 Ohms
RedBlue to Red 2 - 24 Ohms
YellowRed to Red1 - 26 Ohms
YellowRed to Red 2 - 24 ohms
All other cross checks yielded 0 Ohms

Next I measured the Output Transformer

Brown to Blue 150 Ohms
Black to (Blue/Black -these two connect to the same point) 154 Ohms
Blue to Black 233 Ohms
Brown to Black 225 Ohms

I am not a transformer expert and so want to know if these measurements indicate a blown transformer which I would want to replace before powering up the beast. If they are in within range then I can pick up some resistors and get the amp back up today.

Any thoughts or suggestions would be appreciated.

 

[z8894]

If the primary, B/W is 0 Ohms (dead short) the fuse should have blown. The G/G is your filament voltage and the Y/Y is your rectifier filament voltage, none of those should read 0 Ohms. I can't see how any of that could be caused by replacing the opto coupler. If those are all reading 0 Ohms, your power transformer is toast. Do not replace it till you determine what happened and why the fuse didn't blow. Something is definitely not right and if you replace the power transformer or anything else before you figure it out, you'll loose another transformer. If in doubt, take it to somebody. Good Luck.

 

[jefrs]

Difficult to measure windings on a transformer, you will likely get zero ohms, best way is to measure volts disconnected from load. Same with output transformer, but with that put primary across mains and measure secondary volts to get turns ratio etc - be very very careful. It is practically impossible to blow an output transformer and not quite impossible to blow power supply transformer if it is fused properly.

But something is wrong if valve seat it smoking, too much current going on there and it has to be fixed first, suspect a resistor has cooked. Look for barbecued object.

 

[Middleman]

The opto had nothing to do with any of this. It was the loose speaker wire which created the load problem. Thanks for the responses although they seem to contradict each other.

The fuse did not blow because, after inspection, it was the wrong fuse. The rating was 2A @250V and should have been 2A@125V according to the specs. That might have caused the backup. This fuse has been in there for over 25 years, not sure why it was the wrong value.

I talked to an amp tech today and he looked at the ohm measurements above. He said the transformers were fine based on those numbers. The key thing was that my meter was measuring something but it was so low it may not have registered. The meter went from 0L to 000.000 which means the lines were good. He said the heater line resistors were probably failing and having the wrong speaker load took out both these and the first 6L6. After plugging in another 6L6 it just passed the problem to the next tube in line. He said if I replace the bad resistors, it should work OK.

Off to order parts. If anyone else has any input, would love to hear that.

 

[Bruce Bennett]

Brown to Blue 150 Ohms
Black to (Blue/Black -these two connect to the same point) 154 Ohms
Blue to Black 233 Ohms
Brown to Black 225 Ohms

If I read this right. then your problem is stareing you in the face.

MOST Fender center taps on the primary of the output are RED not BLACK... BUT if yours IS black then you should never have DOUBLE the omhs on either side as you would across the Blue/Brown which is the outside ends of the primary coil... to clarify whatever blue to brown reads then blue to center tap will be half of that, never double. but I just checked again. Your output should have a RED center tap.. I think you may have read your choke lead. which will add the power resistors into that reading.

also I' s bit suspect of your power trans readings as well you should never see zero across the primary input... that would equate to a direct short at thewall socket.

It's my belief that whatever you did replacing the opto-coupler. either some solder went where it shouldn't have ( maybe even UNDER the board where you can't see it) or you put the opto-coupler in upside down. (Silver paint dot should be closest to the pots.) I would go back and test carefully while looking over the schematic. anything out of the norm should be pulled and checked again.

also, don't forget that Fender Tremolo circuits WILL NOT WORK unless the "vibrato pedal" RCA jack on the rear panel is shorted to ground. I keep a shorted "dummy plug" on the bench to test Fender tremolos with.

and 250 volts on the fuse IS correct. thats a slo-blo fuse rateing. your worry is NEVER with the voltage but with the AMPS. if it says 2 amps then thats what you want to worry about.. the volts is about the speed of blow.

 

[Middleman]

If I read this right. then your problem is stareing you in the face.

MOST Fender center taps on the primary of the output are RED not BLACK... BUT if yours IS black then you should never have DOUBLE the omhs on either side as you would across the Blue/Brown which is the outside ends of the primary coil...

also I' s bit suspect of your power trans readings as well you should never see zero across the primary input... that would equate to a direct short at thewall socket.

It's my belief that whatever you did replacing the opto-coupler. either some solder went where it shouldn't have or you put the opto-coupler in upside down. (Silver paint dot should be closest to the pots.)

also, don't forget that Fender Tremolo circuits WILL NOT WORK unless the "vibrato pedal" RCA jack on the rear panel is shorted to ground. I keep a shorted "dummy plug" on the bench to test Fender tremolos with.

and 250 volts on the fuse IS correct. thats a slo-blo fuse rateing. your worry is NEVER with the voltage but with the AMPS. if it says 2 amps then thats what you want to worry about.. the volts is about the speed of blow.

Sorry, I typed that wrong it should have read as follows on the output transformer.

Brown to Blue 150 Ohms
Black to (Red/Black -these two connect to the same point) 154 Ohms
Blue to Black 233 Ohms
Brown to Black 225 Ohms

It's my belief that whatever you did replacing the opto-coupler. either some solder went where it shouldn't have or you put the opto-coupler in upside down. (Silver paint dot should be closest to the pots.)

Now that's interesting. The original had the light bulb side toward the tube sockets and away from the pots (mind you not working). So I put the replacement in the same way. The silver dot is currently on the tube socket side of things. I was concerned that the silver dot was mis-marked. I need to check to see if that might have blown the opto. Good read.

 

[Bruce Bennett]

Sorry, I typed that wrong it should have read as follows on the output transformer.

Brown to Blue 150 Ohms
Black to (Red/Black -these two connect to the same point) 154 Ohms
Blue to Black 233 Ohms
Brown to Black 225 Ohms

the Black to red/black measurement is your choke reading NOT the output.

pull your power tubes , turn off your standby switch and pull the red wire off the board. then measure.
BL to BR = "X"
Rd to BL = Half of "X"
RD to BR = Half of "X"

also if you really want the help call me

423-643-4999

also make sure your speaker is unplugged from the amp.

 

[z8894]

On second thought. Jefrs is right you couldn't get an accurate measurement unless you disconnected the leads. Although you should see something higher than 0 Ohms 2 to 4 maybe. If it was a dead short the fuse would have blown, 125VAC or 250VAC. Since it didn't blow, please disregard my previous post. I promise to think first and type later in the future. Let us know how it turns out.

 

[Middleman]

Bruce Bennett, you rock. Thanks for the help on the phone. I will keep you posted on the outcome.

 

[Middleman]

Just wanted to circle back on this thread to announce the Vibrolux IS ALIVE!!!

After hours of research it turns out that the problem was indeed the loose speaker wire. This dropped the speaker load below 4 Ohms which took down the heater wire ground resistors and then one of the power tubes, followed by the 470 ohms resistors on both power tubes. I replaced all power resistors with new fire resistant types. This should not effect the tone.

Also replaced all of the .002 and .01 caps with orange drops. Then added some new 6L6s and the amp she is back up and running. In fact it sounds awesome.

Thanks again Bruce, you were right. The transformers were all fine. The opto was in the right position by the way which was light bulb to the tube side in this amp. The vibrato is back up and running after almost 15 years of non-operation. Feels like a new amp.

There are still some small cap values that will need to be replaced in the future but for now I am just going to enjoy the fact that this 1981 Vibrolux is operational.