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Peter Wieck Peter Wieck is offline
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Default B Field collapse

Please note the interpolations:

On Monday, June 26, 2017 at 5:04:33 AM UTC-4, Xtrchessreal wrote:
While researching ignitors on gas arc tube lamps I read about how the magnetic field collapses when the circuit is basically opened as the AC input signal falls to zero and it causes a spike in the secondary as high as 2500-3000 Volt range for 150w to 400w lamps. I know the spike in a ballast is designed for the lamp to start. I should say the lamp is designed to take advantage of the spike.

SO:

A spike must happen to a tube push pull amp as well though perhaps not that high in voltage, I don't know.


This is the fallacy of 'leaping to conclusions' - reasoning from the specific to the general. My cat is grey, therefore all cats are grey. The ballast in a gas-discharge lamp is wound and designed to deliver that spike at the collapse of the field. Note that a lamp ballast is essentially a massive choke with no secondary winding(s).

A power-transformer with multiple secondary windings, the 'hottest' of which is perhaps 4:1 (but relatively low current) is not wound to deliver such a spike. What *does* happen is that the current through the filaments is a bit high until the filament heats and resistance increases. But this is hardly what would be defined as a damaging spike.

This does not include the potential of an external spike. But, nothing generated within the 'system' will do as you suggest. Once that is understood, your first question is moot.

I got to thinkin'. The Power Transformer, if opened while standby is closed, could do some real damage to the amp, right? If a spike like that is released across the rectifier and filter caps, the inductor, and the power tubes...I don't have testing experience in this so here is the question.

Q1 What happens to the amp as this spike works its way through a basic amp; e.g. http://drtube.com/schematics/marshall/2204u.gif

Part1: Assume a proper speaker is connected.
Part2: Assume without a speaker connected.

Slightly different note:
I've noticed that when I open the standby preparing the amp to be shut down after playing for a while that I can hear my guitar chord at lower volume and fades within a few moments through the speakers. This is a different discharging scenario since the standby is open no spike from the PT can pass.

Q2 What is contributing to the discharging of the amp, providing enough energy to the amp for the signal to still be heard for a few seconds? The heaters are still hot so is the amp dissipating the coils and caps stored energy only? Or is there stored energy in the hot tubes as well? Provided there is an input signal.


The capacitors within the amp discharging through the residual conduction in of the tubes. When the filament shuts off, there is still some minimal conduction from the residual heat. A few seconds - perhaps a bit more depending on the design of the amp itself and the amount of capacitance in use.

Peter Wieck
Melrose Park, PA