arcking from 6L6 between pin 3 to pin 4

[danotron]

I am getting arcking between pins 3 and 4 when I power up. Any ideas as to why this wouols happen. With the tubes out I measure 444volts at both nodes....there verry little to no drope across the choke I am using.

[Bob-I]

I've had that happen with low quality tube sockets and with a small strand of teh wire hanging out from the side of the connector. Another cause may be the incorrect screen resistor. You said both nodes read the same, what about at the socket after the screen resistor?

Also, typically once you get an arc the tube socket can be damaged, burned so that the arc will return.

[heisthl]

New build or has it worked in the past? New tube or has it worked in the past? New socket or has it worked in the past? Usually a short between 3 and 4 would only happen at loud volumes when a tube failed, not on power up. So my guess would be a bad socket or an already failed tube.
I suppose it would be possible on a new build that the socket was wired wrong or the tube was plugged in wrong (on some sockets you can plug them in wrong even though you have a guide pin), although I'm having trouble visualizing why it would fail that way in either of those 2 cases.

[danotron]

I have changed the socket and the arking is gone but now there is a lound 60Hz hum......the speaker is putting out 120Hz when measured. When I measure the supply with a scope there is no sign of it...only seems to be on the out put transphormer...any sudgestions on isolating it?

[heisthl]

Is this a new build? Output tube sockets wired backward?

[danotron]

its is a new build..but I placed my first build (works great) open beside it and all is exactly the same.....

But one thing....I am using an old hammond tranformer on the output....the original scimatis shows a 180ohm 15 watt resistor in parallel with the 8 ohm speaker....I left this out resistor out....could it be the problem...should I add it back in?....also a 100k feed back resistor is shown....

[Structo]

So I take it the sockets were new too?

About the only time I have seen arcing is in old amps where the sockets would carbonize and become slightly conductive.

I suppose you could add the feedback circuit, and or the 180R 15W just for giggles.

Center tap from OT to power supply bridge rectifier?

What size grid resistors are you using on the power tubes?

Kind of weird not to have screen resistors right at the tube, no?

[heisthl]

The 180 ohm resistor is probably just there for protection when no speakers are plugged in.

[novosibir]

I am getting arcking between pins 3 and 4 when I power up. Any ideas as to why this wouols happen.

Are you shure, that the arcing not occurs between pin 2 & pin 3?

I've never seen yet arcing from 3->4, but uncounted times from 3->2
This happens with a speaker mismatch 1-2 steps upwards or w/o a speaker connected (if the spk. jack isn't shuntig to ground w/o plugged sth. in)

Larry

[danotron]

The sockets are new....and the screen resistors are 470ohm and the control resistors are 4K7......I tried the tubes in another amp and they are ok....the OT was originaly for EL34's and I am using 6L6's, could that be a problem?

[stine]

it should not matter if you are using 6l6's as long as you bias the tubes.I have found it helpful to get a magnifing glass and take a good look at the socket. if it is stranded wire you could have a strand laying across connections. does this happen with one or both tubes? try diffrent set of tubes(old tubes hopefully)

[danotron]

its seems that there is somthing funky with the phase inverter. If I disconnect the input to the inverter the arcking goes away. I double checked the wiring and it looks good.....I'm frustrated!

[heisthl]

Check and recheck actual against schematic and if you don't find anything wrong, get a dental mirror and look under the board, or alternativly use an ohm meter to check for unclipped component leads or solder throughs. This would seem like it's a build error of some sort.

[danotron]

Well after farting around for almost a week, I found the problem. It was my feed back loop....when ever I connect it, the power amp starts to oscilate out of control. When disconnected, the amp works great.

Now, how do I solve this....do I just increase the feed back resistor from4k7 to say 100k?....100k is the value that was used in the donar amp....

[tubetek]

You're getting positive feedback instead of negative...
correct this by reversing the 2 OT primary leads; the ones that go to pin 3 of the output tubes. Then re-connect the feedback loop and all should be well.
Tom