Peavey Classic 50 Sparking Tube Pin

[speedcity89]

Hi all,

I just acquired a Peavey Classic 50 212 combo at the cool price of $250 yesterday. When I tried it out (for about 30 minutes) it worked just fine; however, once I got it home after a bit more playing the amp made a few scratchy noises and then I saw a large blue spark and heard a much louder noise.
Now when I turn the amp on there is a blue spark which occurs about every 1-2 seconds coming from one of the pins of one of the power tube sockets. This amp uses a printed circuit board design and the spark actually occurs on the pin of the side opposite of the tube. Additionally, I can see a bit of charred circuit board where the spark is coming from. My question is, what is causing this?? I am not brave enough to leave the amp on for any extended period of time.

I would think that a tube is bad, but I dont want to simply retube the amp if it is something else and I will just ruin another tube.

Any help??

Thanks.

[Amp Kat]

I can almost promise you it's a bad tube and you need to quit turning it on with it in there or you will do more damage to it if you already haven't. When a tube shorts and you are lucky it will blow either the mains or the HT fuse but unfortunately that rarely happens and what does happen is you blow a screen resistor and then you have to go in and replace it. You can check the screen resistors from the outside with the power down but you want to make sure the filter caps are discharged to do an ohm test are you could fry your meter. If you put the new tubes in and it doesn't work than you likely fried a screen resistor or hopefully a fuse.

[speedcity89]

I am new to tube amps, although not to electronics (I am a EE) but where are the screen resistors. I took a look inside and nothing looked fried. Also is there a way to test tubes to see if they are bad--I really don't want to replace all of the tubes if I can't confirm this is an issue.

Thanks for the reply.

Just to be clear, the amp has probably been on for a total of 10 seconds (tops) since the blue spark occured. Also just to make sure, there is nothing I could have done to blow the tube other than have no speakers plugged in to the amp right (I did, of course, have the cab plugged in the the 8 ohm output)?

[Amp Kat]

You could take it to your local Amp tech and if they are pretty good they will likely have a tube tester and could check for grid to cathode shorts or shorts period but they won't tell you when they about to go. That's the bad thing about tubes is sometimes they don't give you a warning but I wouldn't play around with it and would go get a matched pair and throw them in there. Those are plug and play amps so you should be fine with the bias. Some signs of tubes going out are loss of volume,swelling up and down and treble loss. You did nothing wrong the guy just put his old tubes in on you when he sold it. doh !

[speedcity89]

Would you retube the entire amp or just the 4 power tubes?? A lot of other replies I've recieved on this is simply "take it to an amp tech", but given my background I am feeling a bit more brave but I certainly don't want to make this $250 debacle any worse than it already is.

[speedcity89]

I took the "power board" (the board which houses the 4 EL84s) out of the amp and found the problem--albeit interesting. If you look at the pictures you can see that one of the traces between one of the pins of the tube was all but broken. This was the exact location of the spark and since it is right between two tube pins I thought it was the pin. I am going to solder a jumper wire and see what happens. As you can see this isn't the first time that this has happened as there are other locations on the board which someone has already done this at least once. By the way, fuse is obviously blown .





Additionally, I don't know if this guy kept the amp in a high humidity room or what but I've never seen so much oxidation on a circuit board .

[speedcity89]

Scratch that fuse not blown--apparently I can't recognize a porcelain fuse .

[Jazz P Bass]

O/k.
In the second photo, the previous repair, the output tube pins are numbered from the top right. 1-9
The arcing ocurred from pin 2 (grid) to pin 3 (cathode)
Pin 3 is at ground.
In the same photo, right next to pin 7 & pin 8, is a diode.
The diode connects pin 7 (plate) to ground. Reverse biased.
Make sure that this & the other one (CR5 & CR 6) are o/k.
All of that arcing carbon stuff has to be scraped away or it will jump again.
These Peavey EL84 amps kinda blow when it comes to troubleshooting.
The tubes are in series, so you cannot "test" one output tube at a time.
I guess you could put in dummy load resistors in the heater strings.
You are going to have to do some careful checking of everything on that output board.
Make sure the bias circuit held up, seeing that it was arcing to ground.

[speedcity89]

I soldered a jumper wire and cleaned the board up a bit--although I am afraid I may take one more look at it just to be safe. Like I said I am an Electrical Engineer;however, I have not researched the pin outputs of these tubes so thanks for your explaination. It is interesting that the same thing has already happened once (as this is the same place on the blown trace). Maybe a design flaw which makes me curious if Peavey has an updated version which they may be willing to send.

Thanks for that--I'm not off to fix the non-functional equalizing circuit. Although there is an obvious issue with a cap that is missing (leg broken) so hopefully this will fix it.

[Jazz P Bass]

Here is the first page of the 6BQ5 (EL84) datasheet.
If interested, Franks Electron Tube Datasheets
Link: Frank's Electron tube Pages

[Jazz P Bass]

[QUOTE=Jazz P Bass;191686]O/k.
In the second photo, the previous repair, the output tube pins are numbered from the top right. 1-9
The arcing ocurred from pin 2 (grid) to pin 3 (cathode)
Pin 3 is at ground.
QUOTE]

The copper trace that runs through Pin 2 & pin 3, the one that opened, is your B+ high voltage buss.
This gets attached to pin 7 (plate) of all the output tubes.
So the arcing looks like it was from the B+ buss to ground.
And maybe the grid also (pin #2)
that explains the blue arc.

[speedcity89]

Thanks for the input--That really was the problem. As you can see from the pictures it looks like this amp was stored in some interesting conditions (judging by the amount of oxidation/dust/etc I found when I opened the amp up. I am going to give the board one more going over when I change the aforementioned cap.