Spot the Design Error 2

Let me hazard a guess:

- Start with the amp off, but with standby switch closed. - Turn on mains and immediately switch it into standby.

(with one hand movement, actuating both switches in opposite directions almost simultaneously - or maybe even in the same direction, depending on how the switches are oriented on the front panel wrt. each other)

If you leave very little time between "mains on" and "stdby open", it may be possible to catch the 20uF 525V capacitor in the middle of charging, with TR2 conducting the maximum peak current. Interrupting TR2 supply at this point will "discharge" TR2 into TR3 and the 6L6s in opposite polarity. Either TR3 or one 6L6 is likely to arc over.

Dimitrij

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** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.

..... Phil

Hmm, the owner decided to turn the amp on? 'cause it won't blow up if it remains powered off...

(ducking)

The idea is based on "Unsafe At Any Speed" - remember that book?

John ;-#)#

OK, another scenario:

- Mess up something with acoustic feedback so that the amp squeals.

- Panic and flip the standby switch to kill the squealing.

This would likely catch TR3 at maximum instantaneous current, which is way higher than TR2 maximum operating current could ever get to.

Alternatively:

- Get the amp to squeal and yank out the cable on the speaker side.

A user might deem that an acceptable "emergency stop" measure.

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** Reminds me of the false argument sometimes used by speaker repairers to deny warranty. Goes like this:

Speaker tech:

Your speaker has a burnt voice coil, so the amplifier has damaged it.

Customer:

So connecting the speaker to an amplifier with less power than recommended by the maker voids the warranty ?

...... Phil

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** One more step.

Think about instantaneous damage.

..... Phil

You've got me stumped here, not many things remaining that a normal user could likely consider natural. It has to be a simple action...

- Turn the power switch on with the standby switch open.

- Close the standby switch while the cathodes are still cold.

TR2 and the 20uF 525V cap together form a series LC circuit. Tubes with cold filaments pull no current and provide no damping.

The step response of a series LC circuit is an oscillatory transient with a peak amplitude equal to twice the step amplitude.

This could in theory produce a ring-wave of 890 V peak on that cap and therefore on the second grids of the 6L6s. In reality, the ringwave is damped because there is one more 20uF 525V capacitor connected through a 1k resistor. This resistor will provide damping and decrease the Q.

Not sure what the actual peak will be, this would need TR2 inductance and the ESR values for TR2 and the caps to model accurately.

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** You have the scenario correct.

Its what happens in the next few mS.

..... Phil

Please, Phil, seconds are lower-case s. S is Siemens, the unit of conductance, so it's ms, not mS.

--
 Thanks, 
    - Win

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** You get some sick pleasure out of being pedantic - do you Win ?

Compensates for getting so much else wrong?

** Go tell Rigol and other scope makers they have wrong front panel labelling.

S, mS and uS are very commonly used in electronics.

Two of my four scopes use capital S for the time base.

Long as there is no confusion, there is no error.

.... Phil

Good sniping. Does this cause a strike through to G3/anode and run full supply current through the tube to destroy it ?

--
mikko

There's no such thing as instantaneous damage in a tube amp, or a solid state one, either.

RL

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** You feeing all right ?

Haven't had a stroke or a blow on the head lately - have you ?

A visit to your doctorf might be advisable.

..... Phil