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Old 5th August 2004, 10:01 AM   #11
dhaen is offline dhaen  Europe
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Default A word of caution

The protection spark-gap is a good idea, but could lead to trouble if semiconductors are used anywhere in the amp. When a discharge occurs, a large amount of energy gets dumped. This can cause currents to be induced into other circiuts, causing damage. Small signal semiconductors are especially susceptible.
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Old 5th August 2004, 01:03 PM   #12
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I have seen a fast or ultra-fast diode with a voltage rating of at least *double* the HT voltage wired reverse polarity from anode to cathode of each power tube. The idea is that if one anode tries to go greater than 2x HT then it's mate will have it's anode try to go below earth. The diode clamps the voltage to no lower than earth potential.

Instead of a spark gap, how about a couple of 1500W transils/transorbs in series from anode to anode? These things have *picosecond* response time. I don't think MOV's would be very suitable here.

One thing I have never seen on a tube amp is a series RC network of say 10R + 100nF across the output like in many IC chip amps. Keeps a load on the transformer secondary at high frequencies when the speaker impedance begins to rise, or is in fact open cicuit.
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Old 5th August 2004, 01:41 PM   #13
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"One thing I have never seen on a tube amp is a series RC network of say 10R + 100nF across the output like in many IC chip amps. Keeps a load on the transformer secondary at high frequencies when the speaker impedance begins to rise, or is in fact open cicuit."

The HH Scott amplifiers (299C) have series RC networks on the transformer secondaries.
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Old 5th August 2004, 02:42 PM   #14
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Quote:
Originally posted by Circlotron
I have seen a fast or ultra-fast diode with a voltage rating of at least *double* the HT voltage wired reverse polarity from anode to cathode of each power tube. The idea is that if one anode tries to go greater than 2x HT then it's mate will have it's anode try to go below earth. The diode clamps the voltage to no lower than earth potential.
Precisely. Something that cannot happen in linear operation so stays out of circuit in normal circumstance. Turner Audio uses this, as well as zener diodes stacked to equal twice the power supply voltage.

Tim
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Old 5th August 2004, 05:45 PM   #15
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I hope they make Transorbs with silver leads, otherwise I'm not playing
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Old 5th August 2004, 05:45 PM   #16
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Default Re: A word of caution

Quote:
Originally posted by dhaen
The protection spark-gap is a good idea, but could lead to trouble if semiconductors are used anywhere in the amp. When a discharge occurs, a large amount of energy gets dumped. This can cause currents to be induced into other circiuts, causing damage. Small signal semiconductors are especially susceptible.

John,

You raise a valid point indeed. The only tube amp I used this technique on was a long time ago (in my teens) and it had no sand in it whatsoever. Even the B+ rectifiers were vacuum tubes (two 5u4's). This was a high power amp with a 120 watt Hammond O/P xfmer and four KT77's P-P-P.

CP Claire (the relay manufacturer) manufactures little precision spark gaps that have a small amount of radiactive material inside to promote ionization. They can be obtained in many different voltages and act very fast. I suspect this is what I would use today if I wanted to employ this idea. Simply adding a small series R would nullify the dangerous EMP issue of which you speak. Come to think of it, a series R would also make the open spark gap I suggested work better too!
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Old 5th August 2004, 06:05 PM   #17
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Hi RCAVictim,

The resistor idea with precision spark-gaps sounds good

TV manufacturers used an earthing scheme where all earths join at the earthly end of the spark gaps. This, with careful wiring paths and lead dressing increased the ability to survive flashovers from just a few to almost infinity.
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Old 29th July 2013, 11:42 PM   #18
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Can anyone explain more about what happens to an amp without load? Is there oscillation going on? Bias going unstable? Or?
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Old 30th July 2013, 12:52 AM   #19
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Quote:
Originally Posted by Doug Kim View Post
Can anyone explain more about what happens to an amp without load? Is there oscillation going on? Bias going unstable? Or?
The general consensus seems to point to an increase (buildup) of voltage on the primary side of the OPT, which will eventually cause the windings to break down, resulting in catastrophic failure. There may be "other" things going on at the same time, like arching inside the output tubes (or at the socket) due to the excessive voltage present.

Please realize I'm not an EE, hence the rather basic explanation.

jeff
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Old 30th July 2013, 10:03 AM   #20
DF96 is offline DF96  England
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No load means high gain in the output stage, so it easily clips. NFB can make this worse. Sudden clipping can dump a lot of energy into the OPT HF resonance and it has nowhere to go so tries to continue conduction via a spark. This damages the insulation and the likely outcome is either a shorted turn or an open circuit winding.
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