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New build:  36LW6 integrated amp with phono avec SMPS/DC Boost.
New build:  36LW6 integrated amp with phono avec SMPS/DC Boost.
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Old 2nd April 2020, 07:21 AM   #111
multi is offline multi  Australia
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Even the British made KT88 could only handle 100k grid leaks the Russian, KT88/6550 can only handle 50k.
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Old 2nd April 2020, 10:59 AM   #112
jhstewart9 is offline jhstewart9  Canada
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Quote:
Originally Posted by kodabmx View Post
It was triode, and so far I've only had it happen with US tubes... the Soviet ones have been pretty well behaved. I was also using 330k grid leaks. Perhaps a lower value would have helped...
Looks like you were doing an accelerated life test, The manufacturer would be interested in the operating conditions & your results.
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Old 2nd April 2020, 12:18 PM   #113
kodabmx is offline kodabmx  Canada
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New build:  36LW6 integrated amp with phono avec SMPS/DC Boost.
Ya... The Soviet tubes list 1M or 2M2 usually. It's true though, these tubes worked for months with 100k grid leak. They were on for about 1 week 24/7 before both of those tubes ran away at the same time. I think there might have been an error with the biasing board because I've only ever seen one tube run away at a time but who knows...

The 6DQ6B that I'm switching to while I wait for the 6P31S lists 1M as the maximum grid leak, so it should be good.
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Last edited by kodabmx; 2nd April 2020 at 12:22 PM.
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Old 2nd April 2020, 12:38 PM   #114
wg_ski is offline wg_ski  United States
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The 330k could be the culprit. But even Tubelab George has had LW6’s run away single ended. I’ve seen bias drift up when running sweeps at a high bias during “experiments”. It seems like ALL devices, sand or vacuum, that are really designed for switching duty don’t like to sit there and dissipate DC at high voltages (remember the real anode is the screen grid). I think I would only do pentode with 150 on g2, and maybe 30mA, but that’s probably not consistent with your design goals. And I’d still drive it directly with a follower. Even 6550’s get that treatment in my current build in progress.

I was sort of in the process of collecting LW6’s (and a couple other 20-25W octal types), but I’ve put a complete halt to spending for the duration. Even at $11, unfortunately, no more electronics ordering. Only to resume when the crisis is over, if still gainfully employed.
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Old 2nd April 2020, 01:00 PM   #115
kodabmx is offline kodabmx  Canada
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New build:  36LW6 integrated amp with phono avec SMPS/DC Boost.
One thing is for sure... The tube can move current! I've never seen a 5W 10R resistor virtually catch fire along with it's 250R screen resistor.

I had one 6P45S run away but it just glowed orange hot and cracked the glass, the resistors in the amp were fine. Now it seems to make my design more robust, I need to make the resistors higher power. 5W screen and 10W cathode sense maybe...

I'd rather a bad tube just be replaced without having to repair the circuit, too.
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Old 2nd April 2020, 01:14 PM   #116
Lingwendil is offline Lingwendil  United States
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Maybe go for the aluminum-bodied style resistors? 25 watt units can be fairly compact.
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Old 2nd April 2020, 01:17 PM   #117
Kay Pirinha is offline Kay Pirinha  Germany
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Do you really expect cathode currents of about 1.6 A?
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Old 2nd April 2020, 01:25 PM   #118
kodabmx is offline kodabmx  Canada
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If the tube runs away, yes. Even in normal operation I expected peaks of about 800mA, but overengineering the resistors makes one less failure mode.

In reality, the bias board has an "open collector error output" that could be used to cut the power if the tubes can't be biased, but I don't understand that yet. I assume I could use a transistor to drive a relay. If I understand it right, it means that pin will be at ground if there's an error.

The aluminum resistors need heatsinks... I'm thinking MO types... I'm still using the 5W jobs, since this amp will now use 6DQ6B or 6P31S...

I wonder if running the screens through diodes would stop then from becoming an emitter in an overload situation...
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Old 2nd April 2020, 01:31 PM   #119
Tubelab_com is offline Tubelab_com  United States
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Quote:
even Tubelab George has had LW6ís run away single ended.
I killed 4 of them before ripping the amp up and reusing the parts for something else. It was a class A single ended design where the tubes were triode wired with a 100 ohm 2 watt resistor from grid to plate. All went into "slow cook" runaway at idle. The 2 watt resistor was burnt, but not open circuit in all cases, indicating excessive screen current.

Koda's tubes are Sylvania made. Mine were 2 X GE, and 2 X Sylvania. The amp was used for my computer speakers (Yamaha NS-10M's), and was turned on whenever the PC was on, and saw a lot of idle or very low volume time. This all happened about 10 years ago.

Quote:
The tube can move current!
It's peak current RATING is 1.4 Amps.....and that's the spec. It will do more, but my power supply limits at 1.7 amps. I took a rather crusty looking old 36LW6 tube and subjected it to some "testing" to draw it's curves at places not shown in the data sheet. It WILL dissipate over 600 watts for long enough to read the meters for a manual curve tracing (about 500 mS). The lines are grid voltages in 5 volt increments with +5V on the top. Screen voltage was fixed at 150 volts.
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Old 2nd April 2020, 01:56 PM   #120
Tubelab_com is offline Tubelab_com  United States
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Quote:
I wonder if running the screens through diodes would stop then from becoming an emitter in an overload situation
I found that to be useful in screen driven amps where the screen gets hot at the positive peak of the drive cycle where the screen is at +200 volts and the plate is at 30 volts. Then when the cycle reverses the screen is near zero and the plate is over 1KV resulting in a bright white flash!

I experimented with driving both G1 and G2 to mitigate these effects, but still blew up tubes. I have accumulated a lot of 6BQ6 and 25DN6 types at an average cost of 50 to 65 cents each, so I can afford to do these tests.

In triode where the screen and plate are at nearly the same potential, the glowing screen has no receptors for its excess electrons, at least not a high current path.

The death of 4 x 36LW6's put an end to my triode wired big sweep tube experiments.

I have however seriously abused plenty of 6AV5's in triode without a single failure over several years.

More recently I had two 6W6GT's fail from the same slow cook idle runaway. One was a crusty old tube, so I replaced it with a brand new tube and it died the same death after about a year. I got a big box full of 6DG6GT's for cheap. They are the same tube, so I plan to explore this phenomenon further, but tube death often takes a long time, so I haven't gone any further.

The 6W6 / 6DG6 is exactly the same tube as the 50L6, 25L6, 25W6, and 12L6 except for the heater ratings. These were used in millions of old radios and TV's for audio and vertical sweep, and often played for years continuously without failure at 150 volts or less. I have cranked 30 watts from a pair of 6W6's in push pull on 340 volts of B+ without a problem. That amp is still kicking with it's original tubes after almost 3 years. I have the screen grids at 125 volts.

The issue has to be related with screen grids at high voltages.
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