Strange output

[stokes]

I am experimenting with running a 6V6 and EL84 parallel SE output.It is basically a Champ circuit.I have the 2 output tubes running parallel except for the cathode resistors,I have a different resistor for each tube to keep the bias at about 70% for each tube,will try each into class A later.So I put a signal thru it,hooked it up to the scope and everything looked good,was getting about 8 watts clean out of it, about 13watts full bore.I disconected it to put it in the cab and test it out,but decided to hook it up again to check the pre amp section on the scope.When I fired it up this time I had neglected to connect the dummy load momentarily.Connect it and now I am getting a very distorted wave on the scope about 3 volts max ( about 1 watt).Did the chopstick test,checked the volts everything seemed to be okay except the output wave on the scope.Put a meter across the load and confirmed I was only getting 1 to 3 volts output.So I assumed the OT died when I ran it without a load.Disconnect the tranny and check the dc resistance on both sides and that looked fine.Ran a .5 volt ac signal into the secondary to confirm I still had the same primary impedance and that was okay.Reconnect everything and it is okay again,thought it might be a heat induced failure so I ran the signal thru it for about an hour,now I cant get it to repeat the malfunction.FWIW this OT is not a Champ OT,it is a heftier 15 watt 5400ohm primary.Anybody got any thoughts on this oddity?While it was malfunctioning neither tube was redplating,the preamp was operating normal and all the volts were what I would expect to see on the power tubes,it seemed to be "clamping in the OT".Maybe the plate impedance of the 2 tubes is too far off?In the meantime I am going to try to get it to repeat the malfunction.Just wanted to see if any of you guys have any thoughts on this.Thanks.

[stokes]

Okay,I got it to malfunction again.I changed the cathode resistors to get it into class A and it just started doing it again.Everything in the amp seems to be fine,all the voltages are good,no leaky caps no bad connections,signal thru the preamp is good right up to the input cap on the power tubes,voltage on the power tubes plate and cathode is good,no redplating only thing out of the ordinary is the low output at the speaker connections on the OT.Its gotta be the OT but I cant for the life of me see what could cause this to happen intermittently.

[Steve Conner]

I guess you could have some bad insulation in your O.T. that causes some turns to short out intermittently. It's quite common to have these things look fine when tested with a low voltage, but crap out once they start getting subjected to heat and hundreds of volts of signal.

Probably the easiest troubleshooting move is to try another O.T. and see if that fixes it. Or do a ringdown test on the O.T. when it's known to be malfunctioning. (Explaining what a ringdown test is is beyond the scope of this thread though.)

[stokes]

Its definately not heat induced,as I have had it running for two hours with a signal pushed to the max and it was fine.Let it cool down a few hours,went back to it and 2 minutes later it starts again.It seems to be caused by the EL84's screen.I checked it while it was malfunctioning and it was drawing 7+ma.Put a 1k grid resistor and got it down to 1ma,we'll see how it works now but I gotta figure that is the problem.Let you know after I run for a while.

[davoux]

Probably too simple but I like to eliminate them first. Could it be a bad speaker cable? It sounds like you are going for the tougher more fun stuff first. Hope I didn't insult you. You obviously know a hell of a lot more than me.

[stokes]

Davoux,no speaker involved yet,it is connected to a dummy load and 'scope.No insult at all,quite often the simple things are overlooked.Thats what is great about these boards,you get a lot of opinions from different people who look at a particular problem from a different point of view.Thanks,but I think the screen on the EL84 was the culprit,have had it running on and off a couple of days and it seems to be okay now.But dont ever think an idea you have is too simple,if you have an idea throw it out there,that is why we post here.

[Amp Kat]

Internittent problems are a pain and I have about a half dozen as well right niow. I think the only way really tell is what Steve said and swapping it out however I can't really say I've ever seen an intermittent OT SE are DE. They are either good or bad Do you think it's possible the Coupling cap is breaking down after a few minutes ? If you scope it during breakdown at the front and back and it's passing signal and you don't have any voltage present on the back side it has to be after that right ?

[stokes]

KB-Everything seemed to be fine right up to the coupling cap into the power supply,while it was malfunctioning.The only thing that I discovered was the excess current in the EL84's screen.I installed a 1k screen grig resistor and it seems to be okay.I assume the excess current was causing the OT to saturate?I assume the same about the SE OT either it is bad or not.I would assume that if,as Steve suggested,there was a short it would rear its head as soon as the high voltage was applied.I can understand it not showing when the low voltage signal was applied,but once the high volts are there it would appear.When it was working okay with out the 1k on the screen it was only drawing about 3ma.When the malfunction was occuring it was drawing 7+ma.Now with the 1k in place the current stays down to 1-2ma's.As far as swapping it out I did that,I have 2 of these OT's and when it first happened I put the other one in,assuming I had fried it when I applied the signal with no load.It seems to be okay but I will keep trying it for a few more days.Like I said earlier it didnt seem to be heat induced since I had run it for up to 2 hrs full tilt and nothing,let it cool down and after a few minutes it would reappear.I gotta figure it was the screen current.I now have each tube idling in close to class A and it hasnt malfunctioned since I put the 1k in there.Thanks for the suggestions,guys,if it comes back I'll post again and start over.If anybody has reason to think my theory on the screen current is wrong,please let me know.

[stokes]

That first line should read ...up to the coupling cap into the power tube,while it was malfunctioning.Gotta talk to my editor.