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Could my Output Transformer be Shorting Against It's Frame?

Vez

Vez

Member
Has any one heard of such?
Hello everyone new member here. I have a 1949 Gibson GA30 with the OT mounted on the speaker basket. The amp is working. I just replaced some plate resistors and some coupling and tone capacitors.
When I touched the speaker I became acutely aware that there is something painfully wrong. The speaker frame and the output transformer housing have 316 volts on them. It's the plate voltage for the 6V6 power tubes. As I said the output transformer is mounted to the speaker frame. It could be the B+ wire insulation is worn and shorting as it is clamped to the speaker basket 68 years ago.
My question is could it be the primary is shorting on its housing inside the transformer?
Has anyone heard or seen such? Could a transformer short is such a way?
Today I will pull the speakers and inspect the wires to the OT. Btw the secondary and feedback wires shows no such voltage.
Thank you, Rick
 
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Welcome to AK Rick! .. http://archive.gibson.com/Files/schematics/ga-30b.gif

Actually I'm not a tube guy but I like to welcome new comers. I would not doubt your initial findings but hopefully somebody will chime in. I would go ahead put up another post in the tube forum. you won't go to ak jail.

piqued my interest because I haven't seen a transformer coupled speaker since an RCA upright console am,mw and short wave w/phono from the same period and similar tube design. I was about 7 years old and got the chassis out etc.. my mom made my dad get rid of it because I shocked myself a few times. 15in driver I think :D
 
Thanks Binkman. It sounds like you had an interest at an early age. We had a similar console. I'd like to rebuild one one day. You used to be able to pick them up cheap. I reckon you can still find them cheap in more rural areas.
Ill take your advice and post on the Tube forum.
Thanks again.
 
I think you're right to suspect the wiring first. Disconnect the wiring, and check the resistance between the speaker frame and the transformer primary with a volt ohm meter. If it's shorted, cut or bend open the metal strap that holds the wires to the speaker frame, hold them well separate, and measure again. If it's now open, put some heat shrink on the old wires and put it all back together.

If it is indeed the transformer, seek out a replacement. I would probably just abandon the old transformer in place, and mount a replacement on the speaker board off to the side. Chances are the new one won't fit the speaker mounting holes exactly anyway, and even if it did I would worry about damaging the speaker while grinding the riviets off. May be time to upgrade to a new production Jensen speaker.. those sound quite nice!
 
Maxfi !!!! seems to recog the driver/w/ transformer mounted? I just remember the dog and the speaker logo? rca logo dog with the curious look on the back of the speaker. damned that sounded good.45's to 78's lps.. Parents had a few parties dancing eves and that single driver was so clean.

whole amp uses pretty much standard tubes as i remember.. should be no issue there. speaker condition? but I recog a lot. minus the tuner and phono section at my young age. I hope that speaker transformer is still good. sh**T I'd love to hear decent les paul off that. Even so maxfi idea of a sep transformer is good idea too. swear eric clapton will buy it.

shoot some pics!! history restored.
bink
 
Lots of old radios, record players, televisions, etc. had the output transformer mounted on the speaker, it was more or less common practice at the time that amp was made. The practice was carried through to today, for public address speakers, which are still available with a transformer mounted on the speaker.

It is functionally and electrically no different from having the transformer mounted on the amplifier, or anywhere else for that matter.
 
Thanks max.. hopefully that pup will fly again. Did gloogle up some pics of the pup. nothing like all the front end pots and all you see today or from 60's with all the trim and effects pots.

Funny that console I had as a kid? I somehow switching to SW grounded a walkee talkee antenna to ground (somewhere) and ran about 20 feet of wire out the bedroom window, I could talk and receive through the speaker, too. but had to try to talk to somebody? I was kindly told by both parties talking a bit how old I was? that seemed impressive on their meters? I shouldn't be doing that. :D

Most of the marshall's tubers were and prob still are modeled off of wwII transmitters.
 
Here's some pix guys.I measured resistance from the power tube plates to the output transformer and came up with about 600 ohms. Which probably means that the transformer is shorting.
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recapped with vintage Ornage Drops.
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Jensen P12S 12" and 8".
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Output transformer
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Plate disssapation
 
Vez - Yes, it can certainly be shorted to the frame from the primary side (only). The secondary (and feedback line) could still be isolated. With 6V6's, the load resistance (primary) should be about 8000 Ohms, so your 600 would indicate a short near one end of the pri. coil. An 8K Ohm, 15w. transformer should fix the problem.
 
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