Power Tube Arcing

[Sleet]

First time post.

I have a Traynor Studio Mate (aka Guitar Mate) that is giving me trouble. I have worked on this amp in the past replacing filter caps and tubes with success.

Lately however it has taken to blowing the circuit breaker after flipping the stand-by switch. There was a slight burning odor afterwards. I pulled the chassis and observed some evidence of arcing at both power tube sockets. It seems that pin 7 (wired to the OT) on both is arcing to ground.

Everything else seemed okay (visually). I was hoping to get a few suggestions as to where to begin. After searching the forum for similar problems I have a few ideas:

1. Pull the power tubes and see if it still blows.

2. Check the stand-by switch (bad?).

3. Check and or replace the power tube sockets (arcing, cracked).

4. Test the OT (not sure exactly how to measure for that).

I do have a nice Fluke DMM and soldering station to help me along.

Here is a link to the basic schematic for this amp if that helps.

http://www.lynx.bc.ca/~jc/73_JoePiazza_YGM34.gif

I am working out how to host pictures with my ISP so I should be able to attach pictures or the actual schematic that came attached inside the combo cabinet.

Thanks.

[hasserl]

Arcing at the power tube sockets points to high flyback voltages from the OT, often the result of mismatched load with the load substantially higher than spec'd. Are you plugging into the correct speaker impedance? An open circuit between the OT and the speaker could also cause this, make sure your speaker connections are tight, speaker cable/plug/jack connections are tight and you have continuity between the OT and the speaker.

[Sleet]

Arcing at the power tube sockets points to high flyback voltages from the OT, often the result of mismatched load with the load substantially higher than spec'd. Are you plugging into the correct speaker impedance? An open circuit between the OT and the speaker could also cause this, make sure your speaker connections are tight, speaker cable/plug/jack connections are tight and you have continuity between the OT and the speaker.

Thanks for the reply. Traynor has a somewhat funky speaker cable running from the chassis to the cabinet speakers, its rather weak and uses a RCA style connection. I could test it by connecting to another external speaker and to see if it still blows the breaker.

How would I check for continuity between the OT and speaker?

[Bruce / Mission Amps]

Thanks for the reply. Traynor has a somewhat funky speaker cable running from the chassis to the cabinet speakers, its rather weak and uses a RCA style connection. I could test it by connecting to another external speaker and to see if it still blows the breaker.

How would I check for continuity between the OT and speaker?

For what its worth, I have found over the years that once a tube socket arcs over, it is VERY VERY hard to fix the socket so it won't do that again.
I just replace them at once, saving the the brain damage and time/energy to do it again and again.

[Sleet]

For what its worth, I have found over the years that once a tube socket arcs over, it is VERY VERY hard to fix the socket so it won't do that again.
I just replace them at once, saving the the brain damage and time/energy to do it again and again.

I am considering replacing them. Should I look for any particular kind of replacements? Some people are big on the older no longer in production stuff.

Come to think of it, I have a few of the white ceramic sockets with "gold" contacts that should work.

[Sleet]

I am considering replacing them. Should I look for any particular kind of replacements? Some people are big on the older no longer in production stuff.

Come to think of it, I have a few of the white ceramic sockets with "gold" contacts that should work.

Okay, I have replaced the tube sockets and it is still arcing off of pin 7 to the chassis. Should I look at replacing the OT now? If so how would I go about identifying a replacement unit?

Thanks.

[Bruce / Mission Amps]

The only thing I can think of that would cause the plate lug to arc over to the chassis is if there is no speaker load, the speaker connection is intermittent or the speaker impedance is WAY too high.
I can't imagine the secondary being internally shorted, creating a radical impedance ratio error, but maybe.

Check to see if the wires from the OT are connected properly to the socket and or the speaker jacks.
With respect to the secondary (speaker side), open is the dangerous mode and shorted is just broken but not dangerous.

[Sleet]

The only thing I can think of that would cause the plate lug to arc over to the chassis is if there is no speaker load, the speaker connection is intermittent or the speaker impedance is WAY too high.
I can't imagine the secondary being internally shorted, creating a radical impedance ratio error, but maybe.

Check to see if the wires from the OT are connected properly to the socket and or the speaker jacks.
With respect to the secondary (speaker side), open is the dangerous mode and shorted is just broken but not dangerous.

Bruce,
Thanks for the response. Possible cracked or cold solder joints? I will double check my connections again. Is there a method to figure out what kind of replacement OT I would need? If it comes to this.

[mykey]

First time post.

I have a Traynor Studio Mate (aka Guitar Mate) that is giving me trouble. I have worked on this amp in the past replacing filter caps and tubes with success.

Lately however it has taken to blowing the circuit breaker after flipping the stand-by switch. There was a slight burning odor afterwards. I pulled the chassis and observed some evidence of arcing at both power tube sockets. It seems that pin 7 (wired to the OT) on both is arcing to ground.

Everything else seemed okay (visually). I was hoping to get a few suggestions as to where to begin. After searching the forum for similar problems I have a few ideas:

1. Pull the power tubes and see if it still blows.

NO

2. Check the stand-by switch (bad?).

NO

3. Check and or replace the power tube sockets (arcing, cracked).

YES

4. Test the OT (not sure exactly how to measure for that).

YES

I do have a nice Fluke DMM and soldering station to help me along.

Here is a link to the basic schematic for this amp if that helps.

http://www.lynx.bc.ca/~jc/73_JoePiazza_YGM34.gif

I am working out how to host pictures with my ISP so I should be able to attach pictures or the actual schematic that came attached inside the combo cabinet.

Thanks.

You MUST replace the burned sockets, there is a carbon track that cannot be cleaned off.
This is usually caused by the wrong speaker impedance or a bad output transformer. It can also be caused by a bad speaker. very often this happens when using a shielded guitar cable instead of a two conductor speaker cable- to connect the speakers.

[Sleet]

You MUST replace the burned sockets, there is a carbon track that cannot be cleaned off.
This is usually caused by the wrong speaker impedance or a bad output transformer. It can also be caused by a bad speaker. very often this happens when using a shielded guitar cable instead of a two conductor speaker cable- to connect the speakers.

Here is an update to my problem:

I have received my parts and installed the two new power tube sockets. I fired it up and found the good news:

1. Circuit breaker held and no arcing from pin 7 to the chassis.

The bad news is that there is no output to the speakers when plugged in with my guitar. Also, when switching from on back to standby there is a loud "pop" heard through the speakers.

I did purchase a new set of tubes, but thought I would try plugging with the old tubes first. I am concerned that if the OT is bad, I might damage my new tubes somehow.

Is it safe to assume that my OT is blown? I didn't test the resistance on it prior to my repairs.

[mykey]

Here is an update to my problem:

I have received my parts and installed the two new power tube sockets. I fired it up and found the good news:

1. Circuit breaker held and no arcing from pin 7 to the chassis.

The bad news is that there is no output to the speakers when plugged in with my guitar. Also, when switching from on back to standby there is a loud "pop" heard through the speakers.

I did purchase a new set of tubes, but thought I would try plugging with the old tubes first. I am concerned that if the OT is bad, I might damage my new tubes somehow.

Is it safe to assume that my OT is blown? I didn't test the resistance on it prior to my repairs.

there are a number of things that could be wrong, including that the
tube sockets may be rewired incorrectly.
the B+ circuit could be malfunctioning
the output tranny could be bad
but at this point it's better to get it to a shop and have it checked.
rather than guess and randomly replace parts.

[Amp Kat]

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.

[Sleet]

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.

Sorry to be dense, but I don't need to unsolder the leads of the OT to check them?

Thanks.

[Enzo]

Actually you don't. The wires from the primary go to the tube sockets. If you removed the tube, then the wire is connected to nothing. SO it is just as good as disconnected. But even with the tube, the plate of the tube within the tube itself is not connected to anything else. it is just a metal plate that normally waits for electrons to hit it. SO with powr off, the tube is a non-connection as well.

[kg]

Use your meter and make sure the caps are discharged.

^^^^^^^ unless you like big sparks, blown meter fuses, and soiled underwear, this is a VERY IMPORTANT STEP. :lol:

[Sleet]

I am back with a little more probing of the OT:

I read the resistances of the primary sides to the center tap:

170 ohm and 150 ohms respectively.

I checked the secondary and found it to read 0.8 ohms (8 ohm speaker impedence). Does this rule out the OT as a problem? If so where should I look to next? I would guess I should fire it up and read some voltages. One other thing I am a concerned about. I purchased a new set of EL-84's with the power tube sockets I replaced, but have not yet installed them for fear of possibly damaging them.

Thanks to everyone for your input so far, I am learning a lot here.

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.

[Jag]

If I understand this correctly, after replacing the sockets you have no more problems with arcing but you get no output when you send signal in? Also, when you standy by you hear a pop?

Are you sure you got the connections right when you replaced the sockets? Triple check that. Even if you wired everything correctly, it's not clear that you've verified the integrity of the connections from the OT secondary through to the actual speaker. There should be continuity (ie: reads dead short on a DMM) from the ground side of the secondary to the negative terminal on the speaker and from the applicable tap to the positive terminal on the speaker. If you initially get a good reading, try wiggling the associated wires, all the way from the OT to the speaker, and see if the reading changes while you're wiggling the wires. It shouldn't change. I'd pay particular attention to the jack you described as weak earlier.

EVERYTHING I suggested in that last paragraph should be done with the amp unplugged and the filter caps drained.

Also, you may have fixed the original problem but inadvertently buggered something else by accident. Check general areas you were working in very carefully for anything out of place, disconnected, shorted or otherwise damaged.

If/when you do fire it up to check for voltages, do so with the tubes out. That will save damaging the amp if you've miswired it or there is some other problem, unless you got it really wrong. The fact that it isn't blowing fuses anymore is probably good.