Arcing in power tubes

[atmars]

I got a hold of a filmosound with two 6v6 power tubes, 5y3, two 12ax7s, 5879 and another 6v6 as an oscillator. Powered up and got low distorted output and what appeared to be flashes ("did I just see that or imagine it?") in the tubes. Removed oscillator and excess components - same result. At this point i figured, why mess with this crazy circuit and try and figure out what's wrong with it, I'll gut it and and try and build a deluxe. I wired it up (all new components except for tubes and transformers). The new build has almost nonexistant output and major arcing in power tubes. Plucking of bass string will makes arcs in time with notes (only tried this for a couple of seconds). Removed the tubes and put them in another 6v6 amp and they seem to work fine with no arcing so it does not seem to be a tube problem. After reading the posts about "arcing" I thought it might be the OT. I did this test

OT's are easy to test. Use your meter and make sure the caps are discharged. If you put the meter on DC volts and go from chassis(black lead) and (red lead) to pin 3 and verify low voltage there. Switch the meter to resistance and measure across the two wires going to the OT primaries that are hooked to pin 3 of each tube side. It should be around 70 to 140 ohms. Now locate the B+ wire on the primary of the OT and measure from it to each wire on pin 3 you just measured and it should be half of what you just measured before. If it is your tranny is almost certainly good. I haven't found one yet that passes that test and is bad. If it is bad it will read high resistance or OL on the meter. Also make sure the 250 ohm Cathode resistor is ok and the capacitor. If the cap has black around the positive side it could be bad and cause the amp not to sound.

resistance is some thing like 230 ohms. Does this mean it is toast? if not, any ideas what might be the culprit?

Thanks for you input.

[MWJB]

"resistance is some thing like 230 ohms. Does this mean it is toast? if not, any ideas what might be the culprit?" Which resistance is 230ohms? End to end (plate to plate) or each plate to centre tap? Either way if each plate to CT resistance is about the same (within 15% or so) you are more than likely OK - though I have seen shot OTs that pass this test.

You don't mention what voltages you have at 6V6 plate (pin 3), screen (pin 4) & cathode (pin 8)?

Any pics of the build?

[Rick1114]

I just had a similar arcing issue (buzzing with low note) on my 57' GA-5. It turned out to be the 12AX7 tube socket. Are you using the old sockets? I cleaned them and retentioned them but they still give me minor problems because they were cheap to begin with and are now 50 years old. I'll replace them when I have time but it won't be fun since they are bolted to the chassis.

Rick

[atmars]

Which resistance is 230ohms? End to end (plate to plate) or each plate to centre tap? Either way if each plate to CT resistance is about the same (within 15% or so) you are more than likely OK - though I have seen shot OTs that pass this test.

231 ohms between plates. Each plate to center tap: 109 ohms, 122 ohms.

You don't mention what voltages you have at 6V6 plate (pin 3), screen (pin 4) & cathode (pin 8)?

Voltages: Pin 3 - 407v, Pin 4 - 357v, Pin 8 - 16v.

I am running this amp through a reversed isolation transformer so the wall voltages are closer to the 117v called for on the original schem. Plugging straight into the wall would bring them up even higher. The original filmosound schem. says there should be 321v going to the plates. Also, there is a 500 ohm ten watt resistor between the rectifier tube (5Y3GT) and the first big cap on the original schematic.

Any pics of the build?

i don't think i'll post a picture because it's a rat's nest in there. The chassis is to small for a turret board, and since I knew there was a possibility that one of the transformers might be damaged, I just threw it together for a test. I figured if it worked then I would try some value substitutions and when I had it nailed down I would do a real layout.

Are you using the old sockets?

Original 50 year old sockets.

Thanks for the help

[MWJB]

16v at the cathodes is dodgy at the plate voltages you mention, should be 25v+ with a 250-270 ohm cathode resistor & a pair of 6V6s, I'd use 330ohms for a pair of 6V6 with those voltages.

DC resistances accross primaries look OK.

[atmars]

16v at the cathodes is dodgy at the plate voltages you mention, should be 25v+ with a 250-270 ohm cathode resistor & a pair of 6V6s, I'd use 330ohms for a pair of 6V6 with those voltages.

I'll give it a shot - do think this issue would cause the crazy arcing?

[Rick1114]

Try the chopstick test inside the chasis. That how I found my bad tube socket. If nothing makes a noise when hit with a chopstick, then I'd be thinking about transformers, but I don't think so based on the experience I had. I was told "note triggered" noises are not usually the transformer, but may reside in the preamp. But I'm just a newbie....

[6267]

In my experience transformer problems are obvious...generally you will find an open on the primary or secondary of an OPT if it blew... In one case I had a blown reverb transformer that was acting as a flyback on the B+ or something, was getting 1-2kv spikes! yikes! I suppose something like that might cause arcing, but my guess without seeing anything is that maybe the speaker jack is loose (or bad cable) and the bass notes are intermittently disconnecting the speaker. That would probably cause arcing, since the tubes would be working into an infinite impedance. A crappy speaker could do that too, if it was going open on large signals. I would check solders on the speaker terminals, and try a different speaker and/or speaker cables (if you use an external test speaker or something). Arcing probably means the anode voltages are getting really, really high for some reason, probably beyond the safety design of the amp or its caps, so be very careful.

It could also be a power supply problem of some kind, so if the tubes are ok, and the OPT is ok (it doesn't seem like it's blown, relatively unlikely it's partially blown somehow) and the mechanical connections between all these parts are ok, then I would probably be looking at the power supply.

[MWJB]

"I'll give it a shot - do think this issue would cause the crazy arcing?" With 16v at the cathodes you don't have sufficient resistance to bias up the 6V6s to a reasonable plate dissipation, even with 330ohms the 6V6s might be dissipating 13-15W, your amp is undoubtedly dissipating MUCH more than that. This needs fixing first, before any other symptoms are dealt with.

[hasserl]

Arcing at the power tubes is often a result of improper speaker load. Are you certain you've got the correct load? Is it possible it's too high? i.e. a 16 ohm load on a secondary expecting to see 4.

Besides correcting the bias issues, verify you've got the correct load.

[atmars]

Arcing at the power tubes is often a result of improper speaker load. Are you certain you've got the correct load? Is it possible it's too high? i.e. a 16 ohm load on a secondary expecting to see 4.

original schematic show a 16 and 8 ohm tap that are color coded. i soldered them myself with new jacks so I thing they are ok. Is there a way to check secondaries to see if they are really what they are labeled as?

With 16v at the cathodes you don't have sufficient resistance to bias up the 6V6s to a reasonable plate dissipation, even with 330ohms the 6V6s might be dissipating 13-15W, your amp is undoubtedly dissipating MUCH more than that. This needs fixing first, before any other symptoms are dealt with.

I am in the process of doing this. The reason I thought of OT is what was mentioned above. I thought the incorrect bias resulted in non functioning tube or a red plateing tube - not arcing - but i am on the beginning part of the learning curve.

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Maybe this is a topic for another thread - but what is the best say to bring down overall voltages? I have read a tweed deluxe transformer is delivering some where in the vicinity of 320 to 350 to the power tube plates. The original schematic for the filmosound says there should be 335v with 117v at the wall. I am close to 117v running through a reversed isolation transformer, and I have 407v at the plates - I think it was 420 when it was plugged strait into the wall.

Thanks for the help

[atmars]

Idiocy on steriods

Sorry for all the trouble.

Arcing at the power tubes is often a result of improper speaker load. Are you certain you've got the correct load? Is it possible it's too high? i.e. a 16 ohm load on a secondary expecting to see 4.

Improper? Nonexistent! Speaker cable had been "disabled" by cat chewing the cable (he will be killed). i didn't realize I had no speaker hooked up even though it seemed I did. It's no wonder this issue was so hard to diagnose. Although, I did have some (much more minor) arcing when I first fired this amp up and I know i used a different cable because I had to hard wire it because of a atypical speaker jack (coincidence I guess, the kind that sends one off in the wrong direction).

So, anyway, thanks for all the help. Sorry i wasted everybody's time. But, I guess I learned a whole bunch of stuff in the bargain.