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#1
Posted to rec.audio.tubes
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Music Man HD-130 sequel.
I replaced the output transformer on this amplifier (see:
http://tinyurl.com/mf4u5) and as I was taking a look at the schematic, I found something that looked quite odd to me. Here is the schematic: http://www.ernieball.com/mmonline/te...s/2100-130.pdf I noticed that the two speaker output jacks are wired in series. But what would the consequences be if somebody inadvertently pulled out the speaker plug? In that situation, both speaker jacks then become wired in series effectively shorting out the output transformer. Can that be correct? Thanks for your reply. -- David Farber David Farber's Service Center L.A., CA |
#2
Posted to rec.audio.tubes
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Music Man HD-130 sequel.
"François Yves Le Gal" wrote in message ... On Fri, 7 Apr 2006 13:30:17 -0700, "David Farber" wrote: Can that be correct? Judging from the schematics, the output jacks are short to ground types in series. And yes, the wiring leads to a secondary short if no jack is inserted. And this secondary short won't cause any damage with the signal going through? Thanks for your reply -- David Farber David Farber's Service Center L.A., CA |
#3
Posted to rec.audio.tubes
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Music Man HD-130 sequel.
"David Farber" "François Yves Le Gal" Judging from the schematics, the output jacks are short to ground types in series. And yes, the wiring leads to a secondary short if no jack is inserted. And this secondary short won't cause any damage with the signal going through? ** You don't get to see too many tube guitar amps, do you David ? Nearly all Fender tube amps all have the same precaution - ie no speaker plugged in = shorted output. The fact is that it is more hazardous for a tube amp to be operated into an open circuit than a shorted one. Tubes have inherent current limiting, unlike transistors, so do not immediately self destruct when presented with such a load condition. However, operation into a open circuit can result is larger than normal voltage appearing on the tube plates and consequent possible internal arcing of the electrodes, socket bases or even the output transformer. You may note that "Music Man" amps take an additional precaution of having high voltage diode stacks wired from output tube plates to ground. This prevents the plate voltage from exceeding twice the DC supply voltage under overdrive conditions - which it otherwise will when presented with a typical inductive speaker load. ........ Phil |
#4
Posted to rec.audio.tubes
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Music Man HD-130 sequel.
"Phil Allison" wrote in message ... "David Farber" "François Yves Le Gal" Judging from the schematics, the output jacks are short to ground types in series. And yes, the wiring leads to a secondary short if no jack is inserted. And this secondary short won't cause any damage with the signal going through? ** You don't get to see too many tube guitar amps, do you David ? Nearly all Fender tube amps all have the same precaution - ie no speaker plugged in = shorted output. The fact is that it is more hazardous for a tube amp to be operated into an open circuit than a shorted one. Tubes have inherent current limiting, unlike transistors, so do not immediately self destruct when presented with such a load condition. However, operation into a open circuit can result is larger than normal voltage appearing on the tube plates and consequent possible internal arcing of the electrodes, socket bases or even the output transformer. You may note that "Music Man" amps take an additional precaution of having high voltage diode stacks wired from output tube plates to ground. This prevents the plate voltage from exceeding twice the DC supply voltage under overdrive conditions - which it otherwise will when presented with a typical inductive speaker load. ....... Phil Do I see many tube amps? How'd you know? lol. I have been doing business lately with the proprietor of a music store who sells guitars, amplifiers, keyboards, etc. He likes vintage tube equipment so I'm getting an education. Your explanation though solves another riddle I had a while back. I was repairing another amp (this one was solid state) for the same music store. He said all he did was add a speaker jack and then it started blowing fuses.So I replaced the shorted output and driver transistors and some small signal diodes. I turned it up slowly on my variac through my series 200 watt lamp and all seemed well. Then I turned up the input signal and the light started to glow. I hadn't turned up that much signal to draw a lot of current. In fact, there wasn't even a load on the amp. To make a long story short, I discovered the problem was the new speaker jack. If there were no plug in it, it shorted the signal to ground. Now I know to watch out for two types of 1/4" jacks. The ones that short, and the ones that don't. I told him the jack was defective but I guess that only applies to a transistor amplifier. Thanks very much for your reply. -- David Farber David Farber's Service Center L.A., CA |
#5
Posted to rec.audio.tubes
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Music Man HD-130 sequel.
"David Farber" "Phil Allison" And this secondary short won't cause any damage with the signal going through? ** You don't get to see too many tube guitar amps, do you David ? Nearly all Fender tube amps all have the same precaution - ie no speaker plugged in = shorted output. The fact is that it is more hazardous for a tube amp to be operated into an open circuit than a shorted one. Tubes have inherent current limiting, unlike transistors, so do not immediately self destruct when presented with such a load condition. However, operation into a open circuit can result is larger than normal voltage appearing on the tube plates and consequent possible internal arcing of the electrodes, socket bases or even the output transformer. You may note that "Music Man" amps take an additional precaution of having high voltage diode stacks wired from output tube plates to ground. This prevents the plate voltage from exceeding twice the DC supply voltage under overdrive conditions - which it otherwise will when presented with a typical inductive speaker load. Do I see many tube amps? How'd you know? lol. I have been doing business lately with the proprietor of a music store who sells guitars, amplifiers, keyboards, etc. He likes vintage tube equipment so I'm getting an education. ** My "education" with commercial guitar amps began back in 1971, while at uni doing engineering. Became my main job a few years later and I'm still at it - along with all manner of stage and PA gear. Your explanation though solves another riddle I had a while back. I was repairing another amp (this one was solid state) for the same music store. He said all he did was add a speaker jack and then it started blowing fuses. ** Customer carried out mods often prove fatal - but only to the amp unfortunately ! SS combo amps that have no ext. speaker jack are invariably NOT short circuit safe. If an owner wants such a jack fitted, then use a plastic one and wire it so it connects the ext. speaker in *series* with the existing one. This way it is safe even if some crappy jack to jack lead shorts out. ........ Phil |
#6
Posted to rec.audio.tubes
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Music Man HD-130 sequel.
"François Yves Le Gal" These are transformer coupled tube amps with diodes clamping the final stage: they should withstand shorts or nasty loads. ** Wow - I wonder who pointed those facts out earlier ............ BTW, a short is actually less dangerous than an open circuit when it comes to tube amps, that's why short to ground is quite standard. ** Wow - I wonder who pointed that fact out earlier ............ Do frog ****wits suffer from long range echos..... Errrrrruppppppp..... ......... Phil |
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