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PostPosted: 10 Apr 2017, 15:40 
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If you send a 1khz sine wave to a single driver the impedance is constant and even by your chart bang on 8 ohms........that's the conditions under which the amp redplated


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PostPosted: 10 Apr 2017, 16:05 
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Juancho wrote:
If you send a 1khz sine wave to a single driver the impedance is constant and even by your chart bang on 8 ohms........that's the conditions under which the amp redplated


OK - I'll bite.

And if you send 38 watts of power at 1kHz into a single drive, you will slag, or partially slag the voice coil in only a very few seconds. How long does it take to red-plate? Next question: Are you dead-sure there are no stray frequencies and/or harmonics in that sine wave? I ask, because if you are putting out a real 38 watts (or so) for more than a few seconds, it *will* be at more than one frequency based on the continued survival of the driver. And why I like dummy loads for this sort of testing - no such worries.

I am guessing that you have tested voltages and signal, and what both look like at about every point in the unit, by now?

Have you tested for stray DC?


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PostPosted: 10 Apr 2017, 17:34 
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Peter W. wrote:
Juancho wrote:
I
OK - I'll bite.

And if you send 38 watts of power at 1kHz into a single drive, you will slag, or partially slag the voice coil in only a very few seconds.

Who mentioned 38 watts? The point is it redplates with idle current or with a sine wave. As is normal with Class A max dissipation is at idle

How long does it take to red-plate?

As previously stated 20-30 mins and the sine wave is pure

I am guessing that you have tested voltages and signal, and what both look like at about every point in the unit, by now?

Signal is perfect all the way up to the anode /screen where I'm told it should not look normal......All vs are correct at every point and do not waver

Have you tested for stray DC?


None present


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PostPosted: 10 Apr 2017, 18:44 
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What about a Zobel across the output?

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PostPosted: 10 Apr 2017, 20:22 
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Hi Everyone, I think we are losing sight of the issue. The KT120 amps have been built by many folks (both diy and kits) and have not had this issue before. Mods to the amps that are not part of the basic design may ultimately be necessary, but at present it is uncertain what is causing the red plating. My sense is that we should be cautious about making changes without knowing what is going on. I personally find it difficult to believe that the quantity of tubes tried can all be bad...but nothing else makes sense. I wonder if the main suppliers are now trying to get rid of quantities of tubes that don't meet spec by dumping them on us and hoping we won't catch on and keep sending them back. I don't have any here that exhibit the problem (tested in 4 different amps). Either there are a lot of bad tubes, the supplier is sending ones that others returned (as bad), or there is something very subtle going on in the amps in question.

Good listening
Bruce

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PostPosted: 11 Apr 2017, 00:50 
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I had a similar-ish problem with a PP-Class AB Fender Twin style build, B+ was 480V, biased ~60% dissipation
The amp worked fine until things warmed up, then the tubes would redplate and the OT was hot to touch.
After cooling down and repowering, it would be fine until it got warm again.
At cooler ambient temperatures, it often behaved OK

When the problem occured, there was a notable drop in volume, but the signal quality seemed the same (ie no extra distortion heard or visible on a scope)

DC & impedance measurements looked OK, but were always taken after things had cooled.

Long story short (No measureable oscillations, grid connections ok, no leaky bias caps etc),
I borrowed a friends amp that had the same transformer type, connected his OT to mine and it worked fine.
Also, I couldn't replicate the problem with my OT connected in his amp, but its B+ was 450V so not quite an apples with apples comparison.

Based on this
I bought and installed a new OT (same model & supplier) and its been working fine ever since.

I could never get to the bottom of this, but suspected there was some sort of thermal insulation breakdown occuring between the windings

Interestingly, the problem OT now resides in an amp with a 400V B+ without issues so far

In the case of these 4 x Oddwatts, and whats been tried so far, the transformers do seem to be the common albeit unlikely denominator.

Please keep us posted, and good luck with the progress
After being stalled by a few years of health issues, I've just returned to my KT120/150 Oddwatt build.
I've yet to source the tubes and my chassis' design will be long and thin, so I am watching this thread with interest.


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PostPosted: 12 Apr 2017, 09:09 
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I got some KT150 valves from the European distributor today. From the off they're a different valve. Much more open, scale and more bottom end than any from the previous two suppliers. I wait for dark to see whether there's any redplating.......... :evil: :evil:


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PostPosted: 12 Apr 2017, 16:28 
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Hi, There is hope after all. I wonder what the others were (are)?

Good listening
Bruce

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PostPosted: 12 Apr 2017, 17:41 
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:eek: :eek: :eek: One redplates badly........the others not at all at 123ma dissipation (approx 50 watts)


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PostPosted: 12 Apr 2017, 18:44 
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I have only seen it once in one of my amps when a NP bypass cap, Ck, shorted. Fortunately it was with some cheap 300Bs. Because the 300Bs which should have been in the amp were $1200 a pair I put fuses in the cathode path. It never happened again using the same caps.

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