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David Farber
 
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Default 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


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David Farber
 
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Default 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


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Phil Allison
 
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Default 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







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David Farber
 
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Default 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


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Phil Allison
 
Posts: n/a
Default 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










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Phil Allison
 
Posts: n/a
Default 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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