Spot the Design Error 2

By unplugging the loudspeaker it appears that the output transformmer is short-circuited, and the feedback path is grounded!

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  When I tried casting out nines I made a hash of it.

Shorting the output by (apparently) unplugging the speaker whilst playing a deep, loud, note would not be good...

John :-#(#

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Phil Allis> >

** No shorted or open connections involved.

This is something more insidious.

..... Phil

Phil Allison wrote in news: snipped-for-privacy@googlegroups.com:

Plug in a quarter inch jack with nothing connected to it.

Open the "standby" switch? Inductive kickback from TR2?

piglet

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** Unplugging the speaker at the amplifier results in shorted output - a safety feature designed by Leo Fender. The output tubes see a low impedance and may get hot if driven for a while but no other risk.

Unplugging at the speaker box end results in an open input and some risk of high voltage events.

.... Phil

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** Piglet is getting warm.

Another step or two.

..... Phil

Phil Allison wrote in news: snipped-for-privacy@googlegroups.com:

Well, the standby switch creates a by design open or short (because that's what a switch does). And if that could cause a problem, it would qualify as insidious.

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** Well said.

What could be a bad time to operate that switch ?

..... Phil

Well I guess depending on the current flowing at the instant the switch was opened the kickback could be high enough to arc over inside the tubes or breakdown output transformer insulation?

piglet

And the worst time to open the switch would be when playing loud?

piglet

".

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eory.

I've heard this for years. I had a dummy load box with a rotary switch that went from 0, 2, 4, 8, 16 ohms. Over my 50 years on both tube and solid state amps, including Fender I've never seen a one misbehave. I fi gured the negative fb kept things sorted. It's rather interesting to switch thr ough the load range and see how the output voltage changes. Gives you a feel for t he DUT's output impedance. The few Crown amps I've worked on were very impressive. They didn't hardly blink. I'm well aware of inductive kickbac k. I've felt it's tingle when removing my ohm meter from a transformer's lea ds while checking for continuity.

Seriously, why doesn't the pole pig outside my house explode when I open the main breaker so it has no load at all on it's secondary winding. Am I mis sing something?

I've seen ignition coils fail when a spark lead developed such high resis tance it allowed the output voltage to rise high enough to cause internal breakdow n. How many Fender outputs have been replaced over the years?

Once, I had the boss's son call me from another shop telling me that he h ad replaced a Fender's output xformer and... before he finished, I told him "It oscillates". He then says HTF did you know. I said I read a lot and don't sit around s moking weed all day. It was worse than that. His dad offered me a grand (in 1970 ) to straighten him out. I said I thought it was too late. About 6 months afte r I'd left a friend told me that the son and several of his friends were busted in a n attempted armed robbery of a drug store.

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** Once the s/b switch is opened, the DC current in TR2 changes direction and goes up.

.... Phil

Oh, sorry I thought TR2 was a choke and tried to keep current flow in the same direction.

piglet

Apart from owner interaction - failure of the negative bias circuit for the output tubes will cause severe overheating of the end stage. I've seen melted 6L6's. Fortunately they were cheap then.

Arie

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** TR2 is a 2H choke with about 100 ohms R.

Telling you any more is like posting the answer.

..... Phil

OK, I was confused by the 'ground' switch with a big cap to ground... what's that about?

George H.

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** Potential user safety issue, when the AC outlet has no ground.

Those are know among guitarists as "death caps".

.... Phil

According to the schematic, unplugging the load shorts the output.

RL