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Arcing in Tube Sockets - SOLVED!

11/7/2004 9:28 AM
Arcing in Tube Sockets - SOLVED!
I just wanted to report back on my rebuild / homebrew problem that had arcing in the power tube sockets. In my particular case it turned out to be a problem with the output transformer. I am using both the transformers from the original unit. I thought I had the OT figured out, but in testing it I found that a couple of wires coming out of the secondary side were nearly "pass through" connections from the primary.  
I don't know what these were used for for in the orginal unit and don't really care. Bottom line, I taped these two wires off. Doing the testing also demonstrated that I have taps appropriate for an 8 ohm and a 16 ohm load with my 6AQ5 power tubes. I've temporarily connected the 8 ohm tap to my speaker jack and will eventually set up a switch for the 8/16 ohm loads.  
Also I installed ceramic tube sockets as a result of advice I got on this board. I'd like to thank Loudthud for that and the other advice. I'd also like to recognize Hasserl, who's been my long distance tutor/mentor for this project and has been EXTREMELY patient and helpful with this electronics noob.  
The solution to my problem here demonstrates the importance of understanding your OT, especially on rebuild/refurbish projects where you may have insufficient data on your components. I used the transformer testing procedures outlined at:  
I forget if I found that information here or through a google search. If it's not on the reading list, it should be - LOL. The trickiest part was finding a low voltage AC source. I don't have a variac but I found a little 12VAC wall wart at an electronics store that worked great for this purpose.  
Just wanted to report back to share the experience and learnings.
And now, a word from our sponsors:

1/23/2005 11:17 AM
Paul Rosk

Sounds like you had a real nice 'FLYBACK' working there! If you ever see 'arcs' on the tube sockets, the first place to look is the OT and (if any) the speaker selector switch. An open circuit (even temporary) in the high voltage primary connection of your OT, or the secondary can 'saturate' that OT! A nice, clean 460 Volts on the primary can 'flyback' with a 10,000 to 20,000 Volt POW! Makes a nice arc! This was a real problem with Laney and Marshall when they used those cheap speaker impedance selector switches. The connection would work it's way free (due to the amp vibrations caused by someone putting the head on top of the speaker cab), open the speaker connection and WHAM an arc would fly across the power tube socket!  
Hope this clears up your ARC questions, next time you'll know where to start looking! Wish I found this post earlier, could have helped sooner!  
Paul Rosk
1/23/2005 6:18 PM

Thanks for the response. That little amp has been doing great since I figger'd out the OT. This being a rebuild, I had no data on the OT so I was guessing a little on hooking it up.  
I've put those testing procedures in the reading list for this site because they were SO helpful in my defining that OT properly. That is a very useful technique if you have an unknown OT.  
Since then I've finished a cabinet for the amp as well as a matching speaker cab. I posted about that in a local (St. Louis) music forum. You can check it out here:  
There is a link in that message to another thread where I first wrote about the amp. That includes a link to some sound clips.
1/23/2005 10:54 PM
Paul Rosk

I was a bit confused at first when you mentioned a 'feed thru' primary on the OT, but I remember seeing that on several OTs. It's a primary "tap off" so to speak, a multiple 'tap' on the primary for selecting several input impedance settings, like a 'mains' transformer setup for 120/240 volt primary connections. I can see now if you left one of those 'taps' disconnected, it may cause a big problem! Just to be on the safe side, if I were you, I'd double check that OT one more time. Pull the power tubes, and with GOOD ohm meter (something that can go down to 0.01 ohms)measure the resistance from the OT's primary center tap to each outside lead, you should come within a few ohms of each other ( a good OT will be dead on!). If you have a big imbalance (like 50 ohms or more) you might check those primary taps again, just a suggestion. I suggest this because I've seen OTs with primaries 20 or 30 ohms out of balance (Fender is notorious for this!) being compensated by 'cross biasing' the power tubes to keep the output in balance. Check some old Fender schematics, you'll see a "bias balance" pot in the chassis to 'swing' the bias on the power tubes in both directions.
1/24/2005 7:28 AM

It confused me too!! There were actually 2 different secondaries doing this. One passed probably 95% of the primary voltage. The other one passed (I'm guessing from memory) probably 66% (2/3's) when measured against the secondary common ground. I've been using it for 2 months now with those isolated / unconnected and I'm having no problems with the amp. Remember, this was a rebuild of a chassis that was not a guitar amp originally.  
Going through my testing of the OT I tested both leads against the center tap (connected my 12VAC adaptor) and got identical readings on the secondaries from either configuration. I may check that resistance next time I have it out of the cabinet, but it really does seem to be doing fine.  
Another oddity (and there were several) that I had encountered with this chassis was a light that glowed brighter with more signal through the amp. I thought of connecting a VU meter to this just for a laugh. But when I did my stripping of the chassis I discovered that this was being powered by a wire simply stuck back in the paper wrap covering of the OT - not connected to anything. It was running on pure inductance! I abandoned the VU meter idea.
2/3/2005 9:58 AM
Paul Rosk

Check out this YaHoo group, there's a ton of info in the Files section about power output switching and the problem you were having ...
2/3/2005 11:11 PM

The standard color codes for OTs on the primary side is Red for the center tap, going to B+. Then the two plate leads are blue and brown. There are on some transformers extra taps called ultralinear taps. They are also brown and blue but with a stipe around each for ident. Are your extra wires a striped blue and a striped brown?  
The ultralinear configuration is where you connect the screens of the power tubes to these taps instead of to the power supply. Look up ultralinear for more. It gets discussed here from time to time. kg is a fan of them.  
There is absolutely no problem running with the UL taps not connected. Just tape them for safety.  
Watch out measuring resistance of the windings and expecting them to be even. SOme are, some are not. TRansformers work on turns ratios, not resistance, so uneven resistance readings can confuse. If you wind a transformer simply, when you get half the number of turns on the form, yuo connect the center tap and wind the rest of the turns. But as you wind, the diameter keeps getting larger, so the second half windings are longer in the wire. For that reason the resistance will be higher on the one side than the other. There is nothing wrong with this, but it can scare the unwitting.  
Many transformers are not wound simply and so do indeed come out even. But until you know how it is wound, don't trust the resistance reading for good/bad or you will find that certain amp models ALL have "bad" transformers.  
I suspect your extra "feedthrough" wires were UL taps.  
The other possibilty is if the amp was a Bogen or some other PA system amp that had CV outputs - 70.7v or 25v or both. These might be in series with the speaker outs. Take them off too and forget them.

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