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Old 18th November 2009, 09:16 PM   #1
btran is offline btran  United States
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Default Tubelab SE 2A3 _ Newbie question.

I just built the tubelab SE amp. and it runs 2A3 tubes. In the AC input I put the 3/4 A fuse. Is it suitable for the amp? The resaon I ask because I was running it for about 2 hr. and it blew fuse. I replaced another fuse and blew instantly. The power transformer hot but touchable. Please give me some advice.

Thanking you in advance!

Binh
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Old 19th November 2009, 12:39 AM   #2
tomchr is offline tomchr  United States
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I suggest the following:

Insert a new fuse. Unplug the tubes in one channel. Insert an AC ammeter in series with the secondary and measure the current. Then calculate the power drawn by one channel as Pchannel = Isecondary*Vsecondary, where Pchannel is the power drawn by that channel, Isecondary is the current in the secondary, and Vsecondary is the voltage across the secondary. Then calculate the current drawn by the primary as Iprimary = (2*Pchannel)/Vline, where Vline is the AC line voltage (115 V in the US, Canada - 230 V most other places). I would probably go with a slow-blow fuse with a rating that's 2-3x Iprimary. Remember that the resistance of the heaters is about one tenth at cold than at hot, so the heaters draw 10x the current when you turn them on cold. See tubelab.com SimpleSE for information on how to deal with this (inrush limiters).

Whatever you do, do >>>NOT<<< insert the ammeter in place of the fuse to measure the current. If something is indeed wrong with the amp, you can cause some serious damage both to the ammeter and the amp (been there, done that!)

~Tom
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Old 19th November 2009, 01:43 AM   #3
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I can't quite read the label on your power transformer, what number is it? The Tubelab SE with 2A3's needs a 1 amp or 1.25 amp fuse depending on which power transformer is being used (on US 120 volt power). A Hammond power transformer will get pretty hot, but if you can keep your hand on it for 20 seconds or so without using bad language, it is probably OK.
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Old 19th November 2009, 03:07 AM   #4
btran is offline btran  United States
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I use Hammond 270HX power transformer for the amp. I will buy some new fuse tomorrow and re-check the amp. Thank you Tom and George.

Regards,

Binh
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Old 19th November 2009, 03:48 AM   #5
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Are you using slow-blow fuses?
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Old 19th November 2009, 04:20 AM   #6
btran is offline btran  United States
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No, I didn't use slow-blow. I used the fast acting one 3/4A
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Old 19th November 2009, 05:56 AM   #7
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I'd pickup some 1A and 1.5A slo-blo fuses. Start with the 1A. I suspect they will suffice if it was working for a while on a 3/4A.
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Old 19th November 2009, 07:11 PM   #8
btran is offline btran  United States
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At the electronic store this morning, there was no 1A fuse, I took the 1.5A slow-blow. I replace fuse and the amp. sings for almost 4 hrs until now. It sounds beautiful on Klipsch Heresy . Thank you all for your help.

Best Regards,

Binh
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Old 20th November 2009, 02:20 AM   #9
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Nice combo. Using KLF-10s myself...they sound great.
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Old 20th November 2009, 04:17 AM   #10
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Nice. If you don't mind, what type of 2A3s are you using? From experience, take note that you cannot run a JJ 2A3 on this amp. It will hum like crazy. I once had the same set up as yours and forced to replace my 370hx with an Angela universal power tranny and convert the amp into a 300B. Godspeed.

John Revilla
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