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Messages - Retrovert

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They drift in storage too...absorb humidity.

A once-retired engineer (working again to combat boredom) I crossed paths with, said it was standard to 'bake out' stored CC resistors to dry them before installing in (new) circuit boards.

CC resistors are a mixture of carbon black (conductor) and clay (insulator), with some phenolic binder to hold it all together.  The ratio determines the resistance.  The early phenolic coatings were often poorly applied or leaky, and moisture migrated in, swelling the grains and altering the characteristics.

The problem is that even when moisture is removed via baking the characteristics may not be fully restored to their original values.

CC is a horrible technology that was only used because it was inexpensive and nothing better existed.  We have access to wonderful technology, like 1% metal film resistors, which are rock stable with voltage and age.

As long as one is not building circuits in the hundreds of MHz or GHz, metal film is the way to go.

I sold all my CC's on eBay, as advised...

Wise decision.

You thereby simultaneously reduced their wallet thickness while enhancing your own wallet thickness in a totally honest fashion.

The problem with CC mojo is that the varistor effects only show up with large voltage swings.  So no magic could possibly occur for coupling stages, bias stages, etc.  The only place it could possibly matter is at the plate, with the large voltage swings.

Otherwise the resistor is just out of spec at the rated voltage.  (If a resistor's value declines with voltage, but the voltage is constant, than no mojo can occur.  The resistor is just out of spec at that voltage.)

Most claims about CC mojo are a combination of wishful thinking by those who either want to believe they've made some magickal change to their amplifier, thereby establishing a special relationship with it and conferring bragging rights, or by vendors who make outright fabrications to increase sales.

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Tubes and Hybrids / Re: Switching tube amp between two cabinets
« on: March 28, 2017, 06:26:32 PM »
It is correct, as noted above, that an output transformer may be easily destroyed without a load.

In somewhat more detail, the reason is that the core flux couples the primary to the secondary, with the number of turns determining the ratio between input and output.  The AC current at the primary induces an AC current at the secondary, with the relationship of voltage and current being determined by the winding ratio.  Current through the secondary is then dissipated as heat by the load (speaker).

The problem arises when no load is present at the secondary.  As the flux tries to collapse there is no place for it to go; it cannot be converted into current because the secondary lacks a complete circuit for current to flow.  So the voltage begins to climb until current may flow.  At some point that flow occurs via insulation breakdown, aka an arc.  This arcing problem happens with transformers, it happens with solenoids, motors, etc.  The voltage can be considerable: an output transformer with an input voltage of a few hundred volts may arc at several thousand volts.  That arc may go through the transformers insulation, destroying it, or it may go through tubes or adjacent terminals at the tube base, destroying them.

The standard technique is a reasonably high value resistor across the secondary terminals, in parallel with the normal load (speaker) to always provide a load of some sort.  As per Ohm's Law, E = I x R, if R is a few hundred Ω (say 470 Ω) it passes relatively little current at a terminal voltage of, say, 40 V (about 9 mA) compared to the speaker, say 8 Ω, which passes 5 A at the same 40 V.  This parallel resistor does not much affect the overall load impedance (470||8 = 7.9 Ω).

But there is an even better solution to be added in addition to the parallel resistor: the flyback diode.  These go by other names depending on whether they are used with transformers, motors, or solenoids.  In alphabetical order: catch diode, clamp diode, commutating diode, freewheeling diode, snubber diode, and suppression diode or suppressor diode.  If you've ever seen a relay circuit with a diode across the coil, that's the purpose: arc suppression.

When the inductive kick produces the reverse voltage the diode will conduct, and harmlessly dissipate the arc by completing the circuit.  It's like a switch that only triggers on an arc.

One thing that must be remembered is that the arc voltage is many times higher than the continuous voltage, so the PIV rating for the diode must be very high.  A few hundred V of B+ can result in a diode seeing 4 kV.  High voltage diodes are available and are not very expensive (few dollars) but one must be aware that the voltage ratings of common diodes at 1 kV (or lower) are inadequate for this purpose.

Placing a MOV (Metal Oxide Varistor) across the terminals is less idea.  These devices have a finite lifespan as each over-voltage condition which causes breakdown damages the device.  So a single cycle of operation without a load may be sufficient to destroy the device and then destroy the amplifier.  This is an inherent problem in the MOV construction, which in socket strips is known to fail as a short and then burst into flame.  One is technically supposed to regularly measure MOVs to detect a 10% difference in characteristics and then replace faulty devices.  (Yeah, right.  I don't make the specifications, I just report them.)

Anyway, flyback diodes and parallel resistors are inexpensive and highly effective.  No good reason exists to not protect hundreds of dollars worth of output transformer and tubes using a few dollars worth of components.

Oh, and since someone will ask, the reason manufacturers do not include protection devices is that it increases the cost which ripples into the retail channel at many multiples.  If consumers as an aggregate do not understand the difference and ask for better circuits, they will not receive them.

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