WTF just happened

[bigmik]

finished up a el 34 hrm build last night and it fired right up or so I thought
voltage seemed fine throught biased it kind of cold and plugged in,something wasn't right had sound but the input volume was all that worked the clean and the gain volume had no affect on anything. cool miswired somthing no big deal ,pulled the board this afternoon and went back through through the amp didn't find anything so I suspected a cold solder joint, reinstalled the board and reflowed all the solder joints checked all of the tube pins to ground to make shure nothing was touching and fired her up again . sill not right only the OD channel was working kind of really weird distortion happening ,SHIT !!! .strum a few more cords and FIRE starts shooting from one of the power tube sockets !!!!! fuse blows and I am shakeing like a leaf WTF .didn't know what happened ,drained the caps and started checking and found that the heaters has contunitity to ground ,disconected the heater wires fron the tube socket and they are shorted to ground.what would cause this to happen after the amp was on for quite a while????


thanks
neil

[talbany]

Neil
If your using the 100 ohm resistors on the heaters for a false CT if you put your meter on continuity check it will beep at you since the 100 ohm resistors won't provide enough resistance to keep your meter from beeping at you.. By chance did the 100 ohm resistor fry on you.. pull the tubes check around the sockets especially pin 3.. If there is a problem with the opt section and your tubes can't dissapate the power through the OPT transformer it looks for some place to go pow right to the chassis and sometimes flames follow.. It could also be a bad set of 34's I would suspect..Double check your screen resistors while your at it..I would address that issue first.. Good Luck!!

Tony

[Deric]

If the heaters have a CT the usually measure as a short to ground... I think. I'll double check but I'm pretty sure....

[LPSGME]

Could be arcing. Debris on the socket can result in a carbon arc that burns a crack or channel, into and across the socket material, that will continue as a path for the arc. You can get smoke and sparks etc. when that happens, and it can take out a power tube or output transformer.

[Cliff Schecht]

I would try firing up the amp with no tubes. If it still blows a fuse then you obviously have power shorted to ground somewhere. If it doesn't blow a fuse then you're probably alright. For tests like this, I put in an underrated fuse, usually 500mA or less, so that it blows quickly if there is a short somewhere. Another trick I'll use is to hook up the hot and neutral connections of the power plug to a function generator and see that I'm getting some voltage on all of the nodes. It won't be correct, but it shows you if a certain part of the power supply isn't working without risking actually hurting any of your power supply or components. Also, like others said, closely inspect the sockets for any shorts. Without tubes installed, it's easy to Ohm most parts out with a DMM because they are only connected on one side. Check all of the components surrounding the tube sockets.

[bigmik]

thanks for the suggestions I'll do some more checking this afternoon gave up pretty quick after the firework session .

the transformer is a magnetic comp 100 watt marshall PT the heaters are center tapped so no 100 r resistors.

just to make sure it wasn't opperator error here how I connected the tranny

black -heaters
2 green -ground
white-bias tap
grey-HT
blue -HT
yellow -not used, full wave bridge rectifier

thanks again
neil

[bigmik]

ok had a few minutes before work to do a bit of checking an here is what I found the heaters contunity to ground was caused by the center tap pulled the tubes and checked that no tube pins had contunity to ground again also checked the gris resistors and they measure fine,put in a light fuse as suggested and turned her on . everything seems fine with the PT ,heater voltage was correct and B+ before the standby swicth was good .

now what ? blown OT bad tube?

[talbany]

Neil
The windings on the heaters are still very low resistance... with the CT grounded you will still get continuity to ground is normal...I would try another set of tubes fire it back up..I have had the same problems w/34's right out of the box.. POW!!.. Cheap power pentodes!!!

Tony

[Structo]

Yes could be the power tubes.
But, I like to always start a new build with a light bulb current limiter.
It can save expensive components if there is a short or wiring fault.

I start first with no tubes and a 25 watt light bulb, this limits the current to the amp to less than 250ma.

Once that initial test goes ok I install a 60 watt light bulb and repeat the test.
Then if that is ok I install the tubes, plug in a speaker load and try again, if no problems then I plug straight into the wall.

[bigmik]

thanks for the tips i probally do need to build a light bulb current limiter .

looking the amp over at lunch and realised I have the NFB wire running next to the signal access wires at the back of the amp could this cause osscilation ? which would explain the weird distortion. and if the osscilation is bad enough couldn't it take out a power tube or OT?

could be wrong but just doing some brain storming,as much as my brain will allow at this moment

[talbany]

thanks for the tips i probally do need to build a light bulb current limiter .

looking the amp over at lunch and realised I have the NFB wire running next to the signal access wires at the back of the amp could this cause osscilation ? which would explain the weird distortion. and if the osscilation is bad enough couldn't it take out a power tube or OT?

could be wrong but just doing some brain storming,as much as my brain will allow at this moment

NFB wires are generally out of phase with the signal wires so I doubt that is an issue..Internal oscillation (That you cannot hear) generally causes red plating then a blown fuse not fireworks.. Flames and small explosions are generally mechanical failures such as heater cathode shorts or screen failures un grounded caps.. Now if you didn't have the speaker plugged in and the NFB backwards that will give you a nice fireworks show!!

Tony

[bigmik]

back home from work, installed two of the four tubes in the amp and fired her up.pretty shure it was a bad el34 as tony suggested . nothing blew up and i have signal going through the amp still a distorted ,splatty sounding signal and the input vol is all that is working , but a least it is good to know that the trannys are not fried

thanks to all who chimed in with great suggestions it is appreciated

now back to trouble shooting

later
neil

[talbany]

Neil
Glad it wasn't anything serious.. Just a suggestion.. Don't push the output section too hard until you figure out the other problem.. Cool..!!...Run a signal into the return and make sure the OPT section is solid.. I suspect it is.. FWIW.. EL34's don't like being in the mail.."Hey Juan toss me that box labled FRAGILE"

Tony

[Marvel]

"Hey Juan toss me that box labled FRAGILE"

http://www.youtube.com/watch?v=uRFcbWdn360

Marcin

[Structo]

I would double check all resistor values.
It's easy to mistake a 470R with a 470K or similar.
Over at the Metro forum there is one resistor that frequently gets the wrong value installed, the guys there know the symptoms very well and when questioned, the builder quickly realizes his mistake.
Not saying that is why your OD is screwed up but it can certainly cause symptoms like that.

I can read color codes pretty well but whenever I start a new build I like to meter all my resistors then tape them to a piece of heavy stock paper with the value notated.
Not unheard of to have an out of spec value.