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Old 18th April 2008, 04:12 PM   #1
w5jag is offline w5jag  United States
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Default TubelabSE - best MOSFET choice?

I got my TubelabSE pcb finished and powered up last night.

After about three hours, my left channel went dead. I swapped output tubes, and the channel remained dead. Fearing the worst, I measured the adjustable negative bias, and found it to be about (-) 187 volts.

I'd say that 45 was driven pretty well into cutoff.

At the time I made my parts buy, the MOSFET choice was unsettled due to the discontinuation of the 2SK2700. I chose to try the IXYS IXTP1R4N60P that was a suggested choice.

Is this still a good MOSFET choice and did I just get unlucky with a part that succumbed to infant mortality? Or is there a better choice now that some of these boards are seeing some field use?

The board seemed to work fine, this glitch notwithstanding. The only other glitch was that I could not get one of my 5842's to more than about 145 volts. I don't know if that's a tube issue, or another issue to look into - I just set them both to 140 volts and called it good.

I did have a bit of low level hum, but that was not unexpected at this point, given the amp is spread all over my bench, and I am using CRC rather than CLC to cut down on the clip leads during testing. I haven't measured it yet, but you have to put your ear close to the speaker to notice it.

For anyone that is interested, I am using a NOS Stancor 8404 transformer, 260-0-260v at 90 mils, 5v at 2A, 6.3vct at 3A for the power transformer, and I used a 33uf cap at C4. The tubes I tested with are 5AR4 and 45's. This puts my loaded B+ at 308 volts, measured across R4. haven't decided on OPT's yet.

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Old 18th April 2008, 04:29 PM   #2
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Aren't substitute MOSFET Fairchild FQP1N50 or Supertex DN2540N5? At least that's what I saw in Tubelab website.

R3 can affect the input tube voltage. You may be able to get 175 by changing the value but I would wait till Tubelab responds.
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Old 18th April 2008, 09:44 PM   #3
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Others that tried the IXYS IXTP1R4N60P had similar fate. I removed them from my reccomended substitutes list a while back.

The Fairchild part works well at the voltages that you are using, but has been known to oscillate when operated in the 400 volt range. This has zapped a fet while I was messing around with it.

I have 4 TubelabSE amps on my bench currently and I am testing several different mosfets. My current favorite is the Toshiba 2SK3563. It has the flattest "reverse transfer capacitance" VS voltage curve that I have ever seen. It is only good to 500 volts which should be plenty for all possible TubelabSE amps. I have another amp running with IR IRF820A fets in it. It works well also. I have about 5 more types of mosfets to test, which should get done this weekend.

The 5842 tubes seem to vary all ovet the place and I have met a few that wouldn't make more than 150 volts as well. You can adjust the value of the 510 ohm resistor in the cathode circuit to make your tube cooperate.

I have found that some 5842 tubes will oscillate if the input cable is grounded at one end only. The grid stopper resistor value may need to be increased to kill the oscillation. The current value is 1K ohm and I had to go to 5.1 K ohm for complete stability over about 15 different tubes. The resistor should be a carbon composition resistor.

The mosfets have always ran warm to hot on the TubelabSE. There have never been any failures with the Toshiba 2SK2700 parts, but the IXYS parts have failed at high B+ voltages. I have seen the Fairchild parts fail at voltages over 400V as well. To lower the dissipation in the mosfet the 20K source resistor should be increased if running at a B+ voltage over 325volts. I am currently running 30K resistors in an amp at 360 volts and 36K resistors in a 400V amp. With your 308 volts, 20K should work fine.

I will be performing a few more experiments on the TubelabSE amplifiers in the next week or two and I will update the parts list as soon as I have verified everything on several amps.
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Old 18th April 2008, 11:34 PM   #4
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I did a bit of shopping and found out that Toshiba 2SK2700 can still be had but it’s either from overseas or back ordered for at least 4 months.
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Old 19th April 2008, 01:33 AM   #5
w5jag is offline w5jag  United States
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Thanks, George.

The right channel is still soldiering on - I turned it on this morning, and it's apparently played all day without issue.

Is the 2SK3563 available from Digikey or Mouser?

You think the powerdrive circuit could drive an SV811 or SV572? Not that I would try anything like that ...

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Old 19th April 2008, 02:19 AM   #6
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I got my 2SK3563's from DigiKey.

The PowerDrive version that is on the TubelabSE board should have no problems with the SV811 or the SV572. I have not tried it since I don't have any of those tubes. I have experimented with some old 811A's. They could be made to work in the board, but I never could get them to deliver the sound that I wanted, and the bass was lousy without feedback due to the high plate resistance of those tubes.

The PowerDrive circuit that is on the TubelabSE PC board is limited by the amount of power that can be dissipated in the on board mosfets which limits the quiescent current. As with any follower circuit (tube, mosfet, or BJT) the PowerDrive can source far more current than it can sink. The sourcing ability is limited by the Rds ON of the mosfet and its peak current handling capacity, which even in this case is AMPS! Far more than the grid current rating of any mere mortal audio tube. Any grid current flow through the mosfet and adds to its dissipation, so tubes that require a lot of constant grid current may overheat the mosfet.

The sinking current is limited by the source resistor (20 to 30K) and the negative bias voltage. This is between 5 and 10 milliamps on the TubelabSE board. The sinking current is only needed to discharge the Miller capacitance of the output tube.

I needed a PowerDrive circuit to drive some big transmitting tubes well into A2, so I put the same circuit on a piece of perf board and mounted it to a big heat sink. No problem hitting 100 mA peak grid currents into an 833A.

http://www.tubelab.com/833SE.htm
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Old 19th April 2008, 11:12 AM   #7
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hey-Hey!!!,
The FQP family has D-S ratings up to 900V. Unfortunately the '1' series is done at the 500V mark and you'll need to go to the '2' level. FQP1N60 got discontinued a while ago, but the 2N60 is still low capacitance compared to its 600V competition...
cheers,
Douglas
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Old 19th April 2008, 03:55 PM   #8
John L is offline John L  United States
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George, where do you get most of your parts for these kits?
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Old 19th April 2008, 05:32 PM   #9
w5jag is offline w5jag  United States
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Thanks, George.

That sounds encouraging and discouraging - I know I have some SV811 but it's the same tube as 811A except for the plate connection and mu, so far as I know, so I guess I will stick with 45's for the immediate future .

I have the good channel wired up to a mono source, got out a better OPT (Hammond 125ESE) and even with a 45 that is barely better than a dud, it sounds really good. I expect I will have zero hum when I put a small choke in the power supply - it's barely audible now with an ear directly against the speaker.

Win W5JAG

One other - I thought it might lack bass response, but I have an AES 11BM8 P-P amp here in the shack, and the TubelabSE with a near dud 45 blows it away top to bottom. The bass response is very good with the 125ESE.
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Old 20th April 2008, 01:48 AM   #10
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George, where do you get most of your parts for these kits?
Resistors, caps, semiconductors, and connectors:

Digikey www.digikey.com
Mouser www.mouser.com

Tubes, sockets, transformers and chokes:

Antique Electronics Supply www.tubesandmore.com

The Allied 6K56VG and 6K7VG power transformers, and the cheap Triad chokes:

Allied Electronics www.alliedelec.com

Tubes and sockets:

Esrc www.esrcvacuumtubes.com

I also get some pieces from Ebay like motor run caps, and often find some parts surplus. All Electronics has the screw connectors, heat sinks and often some useful capacitors.
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