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Old 10th July 2021, 07:10 PM   #261
Tubelab_com is offline Tubelab_com  United States
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I see 5 board orders so far. They will all go out Monday. I'll start the parts list and construction notes tonight.

Tomorrow is my youngest grandkid's 7th birthday. No useful work will occur tomorrow unless you count the clean up operation.

Three weeks ago it was the oldest grandkid's 16th birthday. I got him a cell phone and built him a better PC. He was using my old Core II Quad box still running Vista. Now he has a 3rd gen Core i5 with W10. Its still far cheaper than a car.

For now, its back outside.
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Old 13th July 2021, 09:08 AM   #262
zintolo is offline zintolo  Italy
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Thanks for shipping George,

I would like to try GU-50 on one of the two boards, so I've simulated GU50 curves with 10% (120k+150k a to g1, 33k g1 to gnd) and g2 at 180V, added the 44W (40W plate +10% pMosfet) max dissipation curve and the 4k loadline. I'll also raise g3 to +15V circa to sharpen the knee of the pentode.
Would you set B+ around 420V and something less than 100 mA at idle?

UNSET is coming?-gu50-10-g1-4kra-420vb-jpg

That would mean biasing around -45V, so a total swing of 90 + 80 (circa) so 170Vpp to reach full power, that will be around 17 Wrms.

Is it preferred to run it at lower voltages with higher g2 voltages and lower Ra?

If you prefer to switch all technical questions to another thread, it's perfectly fine for me.
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Old 16th July 2021, 05:32 PM   #263
w5jag is offline w5jag  United States
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The post office says I have an UNSET board waiting in the mailbox at my office, but I'm ninety miles away for the next few days, at least.
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Old 18th July 2021, 01:00 AM   #264
Tubelab_com is offline Tubelab_com  United States
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There were a total of 7 orders for the beta UNSET boards, so at this time there are some left. A couple potential builders expressed concern about parts availability in a world that can't even build cars right now for lack of chips.

Fortunately the UNSET design does not contain any "chips" but it does contain semiconductors and some are in short supply. Here is a list of what's needed, why it was chosen, and some possible alternatives.

The board has a place for a rectifier tube socket. The usual tube rectifier in a build like this is a 5AR4, but it is only good to a bit over 200 mA at 400 to 450 volts, and that's with silicon diodes in series with each plate. If a 5AR4 is used D1 and D2 can be 1N4007's, UF4007's or any of the larger diodes are used for D1 and D2.

For higher powered operation the rectifier tube is bypassed and larger diodes are installed for D1 and D2. The BY228-15 is good for B+ voltages approaching 500 volts. It is cheap and available. The DSA1-18D is good to about 600 volts of B+, but they are pricey, about $5 each and hard to find. Digikey and Mouser do have them now, they did not last month. That's what's currently in my board.

The power supply uses an N-channel mosfet / zener circuit to derive the screen voltage from main B+. This means that 400+ volts can be dropped across the fet in a 600 volt amp. Ordinary mosfets that were designed for SMPS circuits will die the first time the amp is driven to hard clipping even if operated well within the published ratings. This is due to a secondary breakdown effect not well known in mosfets until about 5 years ago.

Some manufacturers still deny its existence and continue to publish DC SOA graphs that will lead to blown fets ON Semi / Fairchild are the worst case here. Any mosfet that does not explicitly state applications that require operation in the LINEAR REGION are not to be trusted. Some mosfets mention "audio amplifier" in the possible uses, but this could be a class D amp. Any mosfet that has a published SOA where no DC curve is also not suitable here. I have a good sized collection of blown fets created simply by plugging my guitar preamp into the UNSET board and setting the knobs on 10.

It is believed that all DEPLETION mode mosfets do not have this failure mode, and indeed so far I have not blown up a 3 amp 1000 volt depletion mode mosfet. It is the IXYS IXTP3N100D2. Unfortunately Digikey has none in stock and Mouser is down to 166 of them. The IXTP3N50D2 should also work in an amp that runs less than 500 volts of B+ but I have not actually tested them. Mouser has 99 of them. The IXTP1R6N50D2 Is in stock, and should also work, but again I have not tested them.

I have some of these fets on order along with a few others, which will all be tested on arrival. These fets are not available in an isolated package so an insulator will be needed to avoid having B+ on the heat sink.

The output stage needs a P-channel fet that will also spend it's life in the linear region. The mosfet of choice here is the ON / Fairchild FQPF9P25 (isolated) or FQP9P25 (non isolated). The tab on this fet operates at less than 1 volt, so an insulator would only been needed in a situation where the heat sink is grounded. Mouser and Digikey have them. The non isolated fet will do a better job of transferring it's heat to the heat sink, so they are a batter choice if the 1 volt or so on the heat sink is not a problem. I have also ordered some lower powered mosfets with lower capacitance for testing, but it is believed that they are not needed.

The small mosfets and zener diodes all seem to be available in the 10's of thousands, so they are not an issue.
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Old 20th July 2021, 04:05 PM   #265
w5jag is offline w5jag  United States
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Got back into town and found the board in my mailbox - looks good!

Time to see what I have in stock, what I need to order ... I know I have the big pFET's in stock. I have some nFET's that should ( may ) work, although they are of dubious origin ( Ali Express ). But at least I have a lot of them!
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Old 20th July 2021, 04:31 PM   #266
Tubelab_com is offline Tubelab_com  United States
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Digikey shipped my order yesterday by Fedex two day. Fedex is pretty unreliable here, so I may get them this week sometime.

The N-channel mosfet is the screen supply, and the positive cathode bias reference. When mosfets fail they usually fail to a short. This puts full B+ on the screen grid which should make a sweep tube blow, fry, or maybe explode.

After blowing several mosfets in am amp with 600 volts of B+, yet not hurting a single tube, I realized that the cathode was yanked to about +125 volts fully cutting off the tube.

Mosfets blow pretty easily in a 600 volt amp, but not so much in a 350 volt amp. I plan to do some more testing once my part arrive and I have more than one working board.
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Old 20th July 2021, 04:54 PM   #267
w5jag is offline w5jag  United States
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Best I recall, they are "supposed" to be 800 and 900 volt parts, but apparently I left them at the lake with several other unopened mailers of parts. I'm two or three orders behind on opening received parts orders ...
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Old 21st July 2021, 03:13 PM   #268
w5jag is offline w5jag  United States
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Quote:
The board contains a 9 pin socket wired to accept any tube with the 9AQ, 9PM, 9NW, 9GK, 9BF, and 9AU pinout. Some type numbers are .... 6GK6 ....
I haven't been paying as close attention to this thread as I should have, I guess, but I've been going back through it to get a feel for the UNSET board and required parts and this caught my attention. Maybe I should not have given away my 4CX250 stuff?

To my untrained eye, these off shore PCB's look to be good quality. The layout around the power tubes looks very straightforward, and I understand the MOSFET flipping now that I have a board in hand.

I am leaning to building mine silk screen up as a breadboard to help understand and play with the circuit - it looks complicated.

Hot diggity dog - a case I thought would for sure go to trial Aug 2 looks to have settled, and my ordeal with the state of OK set for July 30 also looks it will be postponed, AGAIN, so that gives me time to do a quick wind down on the other thing and get back to tubes and parts with leads on them.
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Last edited by w5jag; 21st July 2021 at 03:28 PM.
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