KT-88s: To the Pain
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eljefe
diyAudio Member

Join Date: Dec 2018
Quote:
 Originally Posted by Gnobuddy But I'm wondering what happens if the MOV starts conducting at the same time the corresponding output valve is also conducting hard. B+ is now clamped to ground directly via the MOV and valve in series.
Let's assume B+ is 500V and the MOV breakdown (for a single or for a string) is 1500V, just to make the numbers easy.

I think there are two cases.

The first is that the tube is in full conduction, plate voltage is very low, high current flows from B+ through half of the OPT to the plate and then through the cathode to ground. Now the signal changes, and because we're in heavy clipping, the tube current cuts off like a switch. Because current in the transformer windings cannot instantaneously go to zero, voltage piles up on the plate. When it reaches +2000V, the MOV "closes" and acts like a shunt from the plate to B+. Plate voltage is clamped to +2000V until enough current flows through the MOV to B+ to collapse the magnetic field in that half of the primary winding.

The second case is that the tube is in cutoff, at or very close to B+ and flowing little or no current. Since we're in Class AB instead of Class B, we'll say that a little current is flowing. Even through there is little current flowing through the corresponding primary half winding, the substantial current in the other half of the winding (that has just been cut off) has created a magnetic field that induces a voltage in the half of the primary winding that has "just been switched on". This induced voltage is negative with respect to B+ and can take the plate below 0V. Since "all tubes are diodes", plate current doesn't flow until the tranformer magnetic field collapses enough for the induced voltage on the plate to reach zero...or higher, if the tube actually obeys the diode line.

In this second case, the MOV doesn't shunt unless the plate drops below -1000V. And the tube dosn't really start conducting until the field collapses enough for the induced voltage in its half of the primary rises up near 0V, by which time the MOV should have stopped conducting and we've established a proper 500V drop from B+ to plate in that half of the primary winding.

So, unless my understanding of how all this works is faulty, I don't think the MOV causes any problem. It always maintains some rated voltage across its terminals, but once you exceed that voltage, it flows current "as needed" until either the transient passes or the magic smoke gets let out. Hence, in designing the protection, it is important to keep the MOV breakdown voltage greater than B+ and smaller than the insulation breakdown of the OPT.

 17th January 2019, 07:18 AM #102 eljefe diyAudio Member   Join Date: Dec 2018 I think I'm now ready to start designing a test rig to do the "knob twisting" that Tubelab_com mentioned in post #92. I think I need to acquire, at a minimum, a chassis, some quality sockets, at least one and possibly several pairs of KT-88s, an output transformer, some big load resistors, some means of providing B+ adjustable from X00 to Y00 volts, some means of providing adjustable screen voltage and grid bias, and an adjustable square wave signal for the grid. I also need some way to measure and/or display various voltages and currents (1 ohm resistors work well for current), protection for various bits (fuses and MOVs), maybe even a little fire extinguisher Just In Case. I'll probably come back and add the capability to put a signal on the screens before I'm done. What did I forget? Feel free to point me to a thread here or a reference elsewhere. I'm willing to wind my own transformers and similar things if that will save a buck here or there. So long as it doesn't compromise safety or the accuracy of the results, I'm probably open to it.
Kay Pirinha
diyAudio Member

Join Date: Oct 2011
Location: Somewhere in Germany
Quote:
 Originally Posted by Gnobuddy But I'm wondering what happens if the MOV starts conducting at the same time the corresponding output valve is also conducting hard. B+ is now clamped to ground directly via the MOV and valve in series. Will a fuse in the B+ line be enough to protect the output valve under these conditions?
Hi Gnobuddy,

at least for PP amps this issue can easily be avoided by placing the MOV, or a spark gap, across the primary, from plate to plate. I've seen this in many commercials amplifiers from the 1950ies and earlier that work at higher plate voltages of about 800 V (two EL34's or EL156's!) and more.

Best regards!
__________________
"Bless you, Sister. May all your sons be bishops." (Brendan Behan)

Gnobuddy
diyAudio Member

Join Date: Mar 2016
Quote:
 Originally Posted by eljefe I think there are two cases.
If transformers had no leakage inductance, and loudspeakers behaved like pure resistors, then I agree with you 100%.

But George mentioned having seen anode voltages well below zero, and also much higher than 2 B+. Neither of these conditions is possible if transformers had no leakage inductance, and speakers were purely resistive. So we have demonstrated evidence that in reality, the output stage can end up operating in regions that simply don't show up when we draw nice resistive load lines on a set of anode curves.

Part of the answer probably lies in the fact that a loudspeaker is a voltage generator once the cone is set in motion. Thump the speaker coil with a short hefty pulse, that gets the cone moving, and now the speaker will continue to generate a voltage long after the drive pulse ended, until the voice coil eventually stops moving. The cone probably executes damped oscillations before it stops moving, so probably it will be pumping an erratic AC voltage back into the output transformer.

Can this back-emf cause anode voltage to fly well above B+ while the associated output valve is still conducting? I don't know. But I wonder.

It's a bit like the fabled warnings on ancient maps: "Hic sunt dracones" - "Here be dragons." Should we worry, or is our dragon-proof ship's armour simply a waste of time, effort, money, and cargo capacity?

Perhaps your test-bench will yield some answers - but only if you end up testing with an actual loudspeaker instead of a resistive dummy load. And what sane person tests a horrendously loud guitar amp at full blast with an actual loudspeaker?

-Gnobuddy

Gnobuddy
diyAudio Member

Join Date: Mar 2016
Quote:
 Originally Posted by Kay Pirinha ... placing the MOV, or a spark gap, across the primary, from plate to plate.
This effectively shorts the two anodes (plates) together while the MOV is in conduction. I'm trying to imagine what happens to transformer and valve currents during that instant, and I realize I have no proper mental picture to explain what the heck is going on.

Perhaps part of the answer is that valves are very robust to extremely brief bursts of overcurrent, unlike semiconductors?

-Gnobuddy

 17th January 2019, 09:18 PM #106 Kay Pirinha   diyAudio Member     Join Date: Oct 2011 Location: Somewhere in Germany Yes, but the spark will become extinguished each time the signal crosses the zero line. Best regards! __________________ "Bless you, Sister. May all your sons be bishops." (Brendan Behan)
Tubelab_com
diyAudio Member

Join Date: Jul 2005
Location: West Virginia panhandle
And what sane person tests a horrendously loud guitar amp at full blast with an actual loudspeaker?

200 watts into a pair of 100dB speakers........Oh, wait, he said SANE PERSON, nevermind.
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Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......

Gnobuddy
diyAudio Member

Join Date: Mar 2016
Quote:
 Originally Posted by Tubelab_com 200 watts into a pair of 100dB speakers........
So nominally 123 dB SPL @ 1 metre.

Possibly 126 dB, as a pair of close-coupled 100 dB speakers will have twice the cone area of a single speaker, and therefore a 3 dB increase in efficiency (at least at frequencies where speaker diameter is much less than the wavelength.)

At 123 dB SPL, using the approximation that every 3 dB increase in SPL halves the tolerable exposure time before onset of hearing damage, we can expect hearing damage after approximately 4 seconds. Or 2 seconds at 126 dB.

Some relevant info here: Permissible Exposure Time for Noise SPL sound pressure level and duration Guidelines How long can a person endure a certain noise level before hearing damage occurs health sound level noise hearing ears impairment tinnitus damage - sengpielaudio Seng

Since SPL levels are inexact, and so is the assumption that our ears time-to-damage is logarithmic (halving for each additional 3 dB increase), we are essentially talking about virtually instantaneous hearing damage.

A second source ( Common Misconceptions about Hearing ) agrees with that opinion, stating "Recommended maximum allowable exposure times (by Nova Scotia Department of Labour) are.....0 min for above 115 dBA sound (there should be no exposure at this level!)"

If I had my life to live over, one change I would make would be to take better care of my hearing. I drove a noisy '73 Plymouth Road Runner for years, and that left me with some tinnitus in my left ear. Hearing damage is cumulative, and I found to my shock a few months ago that a short sit-in with a live band was enough to worsen my existing tinnitus. This wasn't a particularly loud band, either.

-Gnobuddy

eljefe
diyAudio Member

Join Date: Dec 2018
Quote:
 Originally Posted by Gnobuddy But George mentioned having seen anode voltages well below zero, and also much higher than 2 B+. Neither of these conditions is possible if transformers had no leakage inductance, and speakers were purely resistive. So we have demonstrated evidence that in reality, the output stage can end up operating in regions that simply don't show up when we draw nice resistive load lines on a set of anode curves.
But for my analysis to be correct, that doesn't matter. I totally expect the OPT to do all these horrible, nasty things. Note that in my example, the plate voltage got to 4 x B+ at the high end, and -2 x B+ at the low end.

It's not my assumptions about the OPT that I'm worried about, it's my assumptions about how MOVs work. Which is why I spent some quality time with MOV characteristics curves and reading papers about MOVs before I wrote that up. I could still be wrong, though.

My big worry is that even mil-spec NOS ceramic sockets are only rated for 1250V pin-to-pin, which is less than 2 x B+ for many commercial guitar amps. Just how exactly do I size my MOVs? If they are greater than the OPT winding breakdown voltage, my OPT does a great job of being a sacrificial fuse to save my MOVs. And if they are less than 2 x B+, the MOVs won't last past the first jam session, even if there is no arcing.

If B+ is at 600 V, the space between 2 x B+ at 1200 V and the point where my sockets could (theoretically) arc at 1250 V is a little small...which would mean my output tubes become sacrificial fuses to save my OPT.

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