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Old 20th June 2011, 02:11 PM   #11
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Quote:
Originally Posted by tbie View Post
Could a blown Mosfet cause raise in voltage?
I fried mosfet (Q1 & Q2) before while dealing with a different problem. They do get damaged easily and it made the output tube grid voltage positive (it's supposed to be negative during that check out process). This reply by Tubelab explains it. Replace it with a new one and see what happens.
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Old 20th June 2011, 06:07 PM   #12
tbie is offline tbie  United States
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Thanks Everharmonics. I was reading the thread you alluded to, seems like u went through the same process. I think I know the problem now. By the way L 5842 burning brighter than the R also happened to me. This was immediately after the burnout. So, I am looking at changing both Q2 and U2. I will replace U2 with IXCP10M45S. But could you suggest a better replacement for 2SK2700, as I am using IRF830APBF vishay MOSFET rated at 500V.
Would 2SK3564 rated at 900v be suitable, CrSS is 13ohms though compared to 7 on 2Sk3563.
Another option is STP3NK90ZFP.

Thanks.
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Old 20th June 2011, 10:14 PM   #13
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I haven't been updated on the alternate mosfet options so I can't say but there was a discussion on it here. Mosfets for Tubelab SE?
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Old 21st June 2011, 11:13 AM   #14
tbie is offline tbie  United States
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Thanks for all your help. I am waiting for the Mosfets and current regulators 10M45S. I wish they were available locally.
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Old 23rd June 2011, 11:14 AM   #15
tbie is offline tbie  United States
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Changed Mosfet Q2 and current regulators U2 and U3. Its playing again. But problems still exist. R channel is still very very low, like I have to stick my ear right next to speaker to notice any sound coming out or it.
Volume pot, I am using 100K linear pot, and the sound is very very low. Speaker sensitivity is 96db, 8ohm. I am using GXSE 15-4-5k OPT to use with my other speakers but connected to 8Ohm for testing. Would this affect the amp.

With rectifier tube,
B+ = 464
B-=-265
Output grid voltage
L = -120
R = -84
With 5842
B+ = 454
B- = -246
Plate voltage
L = 130-140V
R = not able to change with cathode pot.
With 300B in
current across plate supply
L = -62.8mA
R = -69mA
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Old 23rd June 2011, 06:19 PM   #16
Green77 is offline Green77  Sweden
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Isn't your B+ at 454 V little to high? Nothing to do with your problem but...
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Old 23rd June 2011, 07:35 PM   #17
tbie is offline tbie  United States
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Socket extenders for 5842 have anything to do with it?
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Old 23rd June 2011, 10:36 PM   #18
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Quote:
Originally Posted by tbie View Post
Changed Mosfet Q2 and current regulators U2 and U3. Its playing again. But problems still exist. R channel is still very very low, like I have to stick my ear right next to speaker to notice any sound coming out or it.
Check to see if solders are all good. Check the wiring of output transformers.

Quote:
Plate voltage
L = 130-140V
R = not able to change with cathode pot.
This thread may be of help. Wiring the Tubelab SE without a volume pot
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Old 23rd June 2011, 10:37 PM   #19
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Originally Posted by Green77 View Post
Isn't your B+ at 454 V little to high?
That will come down when output tubes are in.
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Old 24th June 2011, 01:08 AM   #20
tbie is offline tbie  United States
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Quote:
Originally Posted by Evenharmonics View Post
That will come down when output tubes are in.
Even with output tubes B+ is hanging around 440V
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