Proper fusing with back-to-back xformers

LeftyStrat · 08-13-2007, 04:28 AM

I'm building a small 1 watt amp using a 12sn7 as the power tube (PP), with two 12ax7's for preamp and phase splitter. I am using a 12 volt wall wart back to back with a EH toroidal xformer for about 220 VAC, and plan on tapping the 12 VAC with regulator for DC filaments.

My question is what is the best method of using fuses to protect things. Would you just use a fuse on the wall wart input? Is there anything like maybe a 24 v slo blo around 2 amps?

Or would just using a more traditional value on the B+ be sufficient?

Thanks,
Thomas

LeftyStrat · 08-14-2007, 10:15 PM

Answering my own question here...

Looking at available schematics (Real McTube, Hoffman Amps, etc), looks like none of them use fuses.

Then I found this quote from Fred Nachbaur:

"If you want to improve your odds, you can put a 1/16 Ampere fuse in series with the AC input. Since these transformers are designed to self-destruct under fault conditions, the fuse isn't needed for safety reasons, but might well save you a transformer if you make a wiring error."

Thanks Fred (RIP)

08-13-2007, 04:28 AM	#1
LeftyStrat Member Join Date: May 2006 Location: Seattle Posts: 30	Proper fusing with back-to-back xformers I'm building a small 1 watt amp using a 12sn7 as the power tube (PP), with two 12ax7's for preamp and phase splitter. I am using a 12 volt wall wart back to back with a EH toroidal xformer for about 220 VAC, and plan on tapping the 12 VAC with regulator for DC filaments. My question is what is the best method of using fuses to protect things. Would you just use a fuse on the wall wart input? Is there anything like maybe a 24 v slo blo around 2 amps? Or would just using a more traditional value on the B+ be sufficient? Thanks, Thomas

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08-14-2007, 10:15 PM	#2
LeftyStrat Member Join Date: May 2006 Location: Seattle Posts: 30	Answering my own question here... Looking at available schematics (Real McTube, Hoffman Amps, etc), looks like none of them use fuses. Then I found this quote from Fred Nachbaur: "If you want to improve your odds, you can put a 1/16 Ampere fuse in series with the AC input. Since these transformers are designed to self-destruct under fault conditions, the fuse isn't needed for safety reasons, but might well save you a transformer if you make a wiring error." Thanks Fred (RIP)

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