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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 18th January 2021, 11:58 PM   #71
Tubelab_com is offline Tubelab_com  United States
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I saw a 36LW6 hit 40 watts at 3% THD back in post #31 when fed with a certain BIG A$$ HP power supply set on 600 volts. The same board wired up as a single push pull amp has been to over 300 watts.

I am now trying to curve trace a couple 36LW6's and 26HU5's without blowing them up. I am still tinkering with this design. Despite working on this off and on for nearly 10 years, there is still more to learn about it.
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Old 19th January 2021, 06:48 AM   #72
dch53 is offline dch53  Australia
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I look forward to hearing whatever you learn. When I get a couple of projects out of the way I intend to reconfigure my Universal Driver Board crazy drive test set-up to play around with "push-pull UNSET".
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Old 19th January 2021, 03:05 PM   #73
ErikdeBest is offline ErikdeBest  Switzerland
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Subscribing here! Just found this thread by coincidence as it showed up on the initial page of Diyaudio.

very interesting information which I will surely be testing in the next weeks. Thank you and keep safe!
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Old 23rd January 2021, 06:35 PM   #74
TheGimp is offline TheGimp  United States
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Question George. Is there anything in particular we should look out for when choosing P-FETs besides SOA?

I am looking at using the TP0610T (multiple vendors, I don't know which I have) for the front end P-FET. It looks very similar to the BSS84 for Rdson, capacitance, breakdown, etc.

The big issue with the FET under the output tube is SOA, no? Along with low capacitance, and Vds with margin for drive voltage peak.
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Old 24th January 2021, 01:01 AM   #75
Tubelab_com is offline Tubelab_com  United States
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I guess that I should have made it clear that I don't always use the same part in my circuits that's in the simulation. I stuck the first part that LTspice had in it's library that looked like it would work into my sim. If it worked, I left it in the sim.

Once the sim was working, I put the characteristics that I wanted for that sim into Digikey's web site, picked out a page or two full of suitable parts with the right package, sorted by price, and bought 5 or so of each. The actual part I have been using in my input stages for everything from the 50C5 amp to the 250 watt 36LW6 monster is the Microchip VP0106 in a TO-92 package. Anything similar in a breadboard / PC board friendly package should work.

The phase inverter in the 50C5 amp is a Microchip LND150 in TO-92, and the output stage gets a Fairchild / ON Semi FQP3P50. There are probably better choices than the 3P50 in that output stage, but I had a bunch and they worked.

The big powered amp gets Fairchild FQPF9P25's since they are the only part that I haven't blown up, and you know how big my power supply is. Bad stuff happens when it gets hungry, especially when running at full crank. Yes, SOA is the big thing here, followed by the usual Crss and Gm. I pick a Vds spec with some margin above the voltage needed to fully cutoff the output tube under the expected operating conditions, and current capabilities beyond what the tube should handle.

It's the linear region SOA in between the extremes that's tricky, and poorly specified by the mosfet vendors, especially Fairchild/ONSemi. Beware of ANY mosfet where the DC SOA rating is equal to the rated dissipation, or a mosfet with no DC line on the graph. Secondary breakdown in mosfets wasn't well understood when I left Motorola in 2014, many vendors still believed that it didn't exist.
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Old 24th January 2021, 12:41 PM   #76
angrypat is offline angrypat  United States
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I love these Tubelab threads, sometimes they read like demolitions training classes! He blows stuff up so we don't have to- I think he really means that!
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Old 24th January 2021, 03:59 PM   #77
angrypat is offline angrypat  United States
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Is secondary breakdown a localized (on/within the die) thermal runaway? I was reading an application note from Toshiba and a Wikipedia entry, trying to understand a little. As mosfet temp increases the resistance decreases? Toshiba data sheet below.

https://www.google.com/url?sa=t&rct=...gpkKTWWvj7yB3p

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Old 24th January 2021, 05:12 PM   #78
TheGimp is offline TheGimp  United States
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Thanks George.

Interesting that the VP0106 is available in TO220, TO39, TO52, and TO92. I have them in the TO220 package.

I will probably try the TP0610T for the input tube. I have plenty in SOT23 and have adapter PCBs for them to go to through hole.

I don't have any FQPF9P25, but I do have a FQP3P50. It looks like they would be robust enough for my application. I do not plan on testing big sweep tubes.

I am considering purchasing a 1' section of 5" dia lexan pipe to place over the output tube during the test.
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Old 24th January 2021, 09:55 PM   #79
Tubelab_com is offline Tubelab_com  United States
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Are you using your big HP power supply for testing this stuff?

I place inverted coffee mugs over the mosfets on my line powered SMPS experiments.

I did have one 6HJ5 shatter, and another explode. Granted that these were being tested with the big HP turned up to 600 volts. A dumb math error led me to attempt to pass a 1/2 Amp pulse through the tube / mosfet combination at which point it all blew up, shattering the bottom of the tube.

How DUMM was that? Not as DUMM as trying it again expecting a different result. This time the tube exploded scattering glass all over the bench and floor. So, 600 volts times 1/2 amp is how many watts????? Even a dumm blonde should have realized this before pushing the button, but NOOOO, I had to do it twice!

I guess it didn't seem too far fetched since I actually tested a 6LW6 to higher power levels than this. Note the green line in this plot goes to 650 volts at 930 mA. That tube is still alive and well two years later.

The initial short circuit current on those power supplies is very large since there is 1000uf of capacitance across the output inside the supply, capable of delivering over 200 joules of energy to any unsuspecting part. I'm still not sure which blew first, the tube or mosfet since there was not much left of either. I'm setting up to curve trace some more big tubes. This time I will put my two big 8 ohm 500 watt resistors in series with the test subject to limit the explosions to minor levels.
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File Type: jpg 36LW6.jpg (121.9 KB, 38 views)
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Old 25th January 2021, 05:49 PM   #80
TheGimp is offline TheGimp  United States
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Hi George,

Yes, I am using the HP supply but don't expect to go north of 450V. I was thinking the same thing about putting some limiting resistance in series with the PS in case of a failure. Even at 450V that is a lot of stored energy, just over 100joules.

I went ahead and ordered the Lexan pipe to use as a shield, and will come up with something to cover the FETs as well.

I wonder if this 600V fuse would be worth inserting on the output of the PS.

https://www.allfuses.com/pub/media/d...use%20KLKD.pdf
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Last edited by TheGimp; 25th January 2021 at 06:02 PM.
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