Ampeg Smokey Burning and Blue Flashes!!!

[Gibricfenderpeg]

Hello I have a problem with a 73 Ampeg SVT Head, I changed power caps on the preamp section and the power section changed all tubes but the two 12bh7 driver tubes(they tested good) here lately it has been blowing the 15 amp fuse directly connected to the ac line and the ac accessory at first i thought it might have been just a power spike but it has happend three times in less than two weeks of use with a power conditioner connected to it and today after repairing a broken joint on the pcb board i get this buring electronic componet going bad smell and the sound after the b+ is put to the tubes sounds like a broken speaker and all crackle and grainy and a few people say they have seen some blue flashes of light coming from it, i plan to get the 12bh7 tubes replaced they help control the bias from what i gather but my question is really does anyone think this could possibly be the output transformer, p.s. checked the plate resistors they are still good. can anyone help?????????
T.J.

[txstrat]

Did the amp play well BEFORE you changed the caps?
If so, I'd check all the solder joints again and if you're not sure resolder them. This might be the crackle you noticed.
Check if you accidentally damaged any other part on the PCB.
Where were the flashes? In the tubes? Inside the chassis?
Looks like a cold solder joint to me since flashes like this are usually accompanied by crackling sounds.
I assume you biased the power tubes properly.

[Gibricfenderpeg]

Did the amp play well BEFORE you changed the caps?
If so, I'd check all the solder joints again and if you're not sure resolder them. This might be the crackle you noticed.
Check if you accidentally damaged any other part on the PCB.
Where were the flashes? In the tubes? Inside the chassis?
Looks like a cold solder joint to me since flashes like this are usually accompanied by crackling sounds.
I assume you biased the power tubes properly.



the amp Sounded like an amp that needed caps and after they were replaced it cleaned up and i just bridged the pcb joint with a solid wire cause the pcb's little metal film that is used to connect was pulling off the board so if it did eventually break the connection would still be there there were no other connections that were near the two and the one looks unburnt and tests ok the flashes i never saw someone else said they saw it when i was playing through the amp, the burning smell smelled like ozone and it is in the vicinity of the output tranny, the crackle is when its really pushed it get distorty and muddy, I biased from point k1 to ground .072volts and between k1 and k2 .01volts so im not sure going to check the chassis out tonight and look for burnt paper on the output tranny just to be sure

[izzybell2810]

Hi,
are the blue flashes coming from all the tubes, or just 1? If it's just 1 tube, try swapping them around and see if the flashing follows the tube.If so you have a bad tube.

[Amp Kat]

Flashes usually indicate bad power tubes or grounding issues. Are you sure all 6 of your tubes are good ? Whenever you change caps in those amps it is posible that all of the voltages rise to higher levels after all those cap were pretty old and the carbon comp resistors have increased in their values drifting over the years. Like the idea you changed them as it is the best thing. Make sure all of your polarities on you caps are correct. Be very careful as you know what type of lethal voltages are in that monster. (700V) R-49 is an 8.2k 15 watt dropping resistor that sets the preamp voltage thru connector P1 #5 which is node B and if my memory is right is an orange wire. Check your voltages on the preamp power rails and see if they are around 300. Right at the 8.2k R49 is 700 volts and on the other side is 485 and then to the connector and to R47 another 8.2k but 2watt down to the 300 volts at the 6C4 (v5) . If the voltage is to high after that dropping resistor it could cause those problems

[Gibricfenderpeg]

ok thanks for all the posts man you guys are giving me a real idea on what to look for (I usually work on fenders and they are childs play compared to this monster and well its a little scary messin around with all that voltage floating around so check r49 and put my power tubes to the short test and go from there one thing i smelt around the output tranny that burning smell and thats what im worried about that if its that its a $265 fix and just my luck that would be it. but either way i think we are on to somethin

[Gibricfenderpeg]

ok checked the power tubes and one was bad, but can that make an ozone smell???? checked the caps and resistors all seem good and checked all 14 of the tubes and like i said one of the power tubes are bad i guess it could possibly be that i have a vaccum leak in one of the tubes i saw this once on a vt 40 i was working on It had the original 7027a's in it and one of em made a horrible smell but i wasnt sure i was sure worried it was an output transformer either way no offense i hope this is the last post on this matter thank you so very kindly to each of you again for trying to help me out if you have any more input please let me know again thanks
T.J.

[Amp Kat]

All it takes is one bad tube to spoil it for the rest. What happens is when a tube fails it's just like a direct short and usually the cathode to grid are the ones that short and it should blow a fuse but more times than not it doesn't and will start frying things in it's way but hopefully you have it under control. Just keep an eye out and if you hear treble loss or volume loss it's probably time for a tube change and SVT's are known for eating tubes.

[Gibricfenderpeg]

ok changed tubes ,fixed screen grids that cracked, biased and powered up slowly so as to make sure caps would be ok from sitting for these few weeks even check each solder joint, but i had a horrible smoke burning smell ????? what could it be, I didnt let it stay on for very long after i smelt it could it be my windings burning in output or even a choke or a power transformer???? I am about to just buy a bassman 100 silverface and never lug out the heavy artillery anymore

[Amp Kat]

I would go down the schematic and check all of the voltages very carefully. Burning smell means something shorted or something open causesing something else to get hot or a backwards cap. Did you replace the can caps with Axials or replacement can caps. Check your grounds on the caps and make sure they are connected. The can itself on those are tied to chassis ground.

[Gibricfenderpeg]

I replaced the caps with jj 50x50 500v and a 100x100 500v i looked on the power section and there are no signs of any burning but how would i get it to work to check the voltages cause of the two part preamp and power section i do know that it only happens when the b+ is turned to the tubes and after a few minutes of playing you get a burning smell that leads me to believe thats its a transformers winding going bad?? any thoughts?

[Amp Kat]

Those trannies are very stout and would be the last thing I would suspect going bad. If they are burning something is causing it to burn. If I suspected transformer damage I'd disconnect the primaries and test them. If they have the correct voltages I'd say it is good because aside from the heaters there just isn't lots of current draw in the plate windings. You are getting all the correct voltages on preamp tube plates at around 300 v?

[Gibricfenderpeg]

ok been away from the bench a while i had a 100x100 500v cap leaking and replaced it and when i got it all together this power/preamp in jack with a resistor blew and i replaced it and the resistor blew again?? Any thoughts something on the board is causing an over load any ideas???