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PostPosted: 17 May 2019, 01:52 
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Joined: 13 Jan 2018, 21:33
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Location: australia
When assembling the 300b set is it advisable install fuses in the internal power/signal circuit to protect tubes and transformers and speakers. Information on where best to place them and how to approach sizing the fuse if that is the case would be appreciated. Cheers!

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PostPosted: 18 May 2019, 14:50 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Woodo wrote:
... is it advisable install fuses in the internal power/signal circuit to protect tubes and transformers and speakers?
This is a contentious question in the amp building world. You can see some earlier comments in this very thread, here.

The key to using a a fuse in this manner is finding the proper value. Fuse are rated by their (I^2)*t value. This is a constant for the fuse so the greater the current, the shorter the time to fail. You really want the fuse in the B+ supply to the power stage so that it doesn't see the AC current variation. Some people put them in the cathode path but I'm not a fan of this arrangement.

Maybe some others will chime in with opinions.

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PostPosted: 19 May 2019, 22:25 
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Location: australia
Suncalc wrote:
mwhouston wrote:
So are you saying 1A fuse?
No. Actually what I'm saying is that a slow blow fuse of 250mA with an I^2*t rating of 0.15 to 0.25 should be acceptable. The equivalent 250mA fast acting fuse has an I^2*t value of only 0.036. which means that a large inrush could make them blow. I'm just giving options.

If the bypass cap shorts then the 300B will try to run at the intersection of its resistive load line and the Eg=0v plate characteristic curve. Because the DC resistance of the transformer is small, this will equate to many amps. However, the power supply is going to current limit because the transformer secondaries will bog down. So the current will probably be in the range of maybe half an amp to an amp for a very short time. The 300B should be fine with this surge so long as it's just a single shot. If you go with a 250mA fuse with an I^2*t value of 0.216 (i.e. a slow blow fuse) the half amp current spike will blow the fuse in ≈0.9s. If the current actually got to one amp, the fuse would blow in ≈216mS.

Personally, I would use the 250mA slow blow fuse in this application and not worry about it.

P.S. I would also check the ripple current rating on those non-polarized caps you're using. That may be the reason that they are blowing.


Thanks Matt. Reading further I saw you also wrote above about using the 250 mA fuse and the I^2t. Would the fuse need also to be rated above the 400 or so volts dc B+? The fuses I see at the local electronics shop are rated for 250v AC. Additionally I see Mark's fuse is on the plate connector of the 300b. Is that correct or is it preferred to precede the opt primary winding and after the final smoothing capacitor to also protect the opt. Apologies the questions are probably droll. Regards.

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