bad 12au7?

STURMJ · 5201

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Offline STURMJ

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on: May 04, 2010, 03:24:56 PM
my resistance check is perfect, but I am blowing fuses on start-up. I suspect that the 12au7 is bad, there is a defect at the base.  Could this be the problem? I have another one but don't want to risk damaging it.  Should I give the other tube a try or should I go over everything again which I'll most likely do anyway)?
thanks in advance!


Online Doc B.

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Reply #1 on: May 04, 2010, 03:47:31 PM
pull the 12au7 and try turning it on.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Online Doc B.

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Reply #2 on: May 04, 2010, 03:53:09 PM
i'm posting from my phone, so forgive the brevity. fuse may be too small and evrything is OK. Should probably be 1A, we shipped some first run kits with 1/2A

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline STURMJ

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Reply #3 on: May 04, 2010, 03:57:11 PM
Cool and thanks!


Offline aristos_achaion

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Reply #4 on: May 10, 2010, 07:33:35 AM
Quote from: Doc B. on May 04, 2010, 03:53:09 PM
i'm posting from my phone, so forgive the brevity. fuse may be too small and evrything is OK. Should probably be 1A, we shipped some first run kits with 1/2A

Seriously? I've been through about five 1/2A fuses trying to figure out what's going on...


Offline Paul Birkeland

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Reply #5 on: May 10, 2010, 07:55:09 AM
Did you read up on the manual corrections?  At least one will cause the fuse to pop every time.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline aristos_achaion

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Reply #6 on: May 10, 2010, 08:01:39 AM
Quote from: Caucasian Blackplate on May 10, 2010, 07:55:09 AM
Did you read up on the manual corrections?  At least one will cause the fuse to pop every time.

I did, but it's possible that I still slipped up (I'm a total DIY noob). Do you know which correction would that be? I'd accidentally attached the wire from 4 on the B+ to 15U (per the typo in the original correction), but I changed that to 22L, which should be OK (though 14U should be better for the noise floor, supposedly).

The fuse only started popping after I moved that wire from 15U to 22L, BTW.


Offline aristos_achaion

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Reply #7 on: May 10, 2010, 11:54:17 AM
Never mind...I replaced the 1/2A fuse with a slow-blowing one (I'd accidentally picked up quick-blowing ones) and it works fine. I got some 1A ones as well just in case.

Beautiful sounds from this little thing with my Tung-sol Black Glass 12AU7...I'm definitely going to have to pick up the upcoming upgrade...thanks so much for the wonderful amp!


Online Doc B.

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Reply #8 on: May 10, 2010, 12:32:15 PM
Great, glad you got it up and running and you like it! Fast blo fuses just don't get along with our solid state power supplies too well. Slo blos will usually solve any start up transient issues, but sometimes folks do have to go one size larger than we spec, because we try to size the fuses so they will go very quickly if there is a fault in the circuit.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

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