Based on Merlin's excellent article
http://www.valvewizard.co.uk/fuses.html
He says the best way to fuse the HT is to fuse the HV AC winding going to the rectifier.
Now I don't want to fuse both sides of the HV AC. I want to fuse one side.
For example we want our amp to blow it's fuse when the amp draws more than 200mA. My question is if only fusing one side of the HV AC do you half the max current to 100mA or keep it at 200mA?
I'm not totally sure if one side of the AC HV supplies 100mA and 100mA from the other or since it's AC one side supplies 200mA and the other 0mA and then they switch every 60Hz.
Thanks!
HT Fusing
Moderators: pompeiisneaks, Colossal
Re: HT Fusing
There's electrons flowing through the wires at all times - it's just the direction that's changing!
If you're fusing a HT with no center tap, just connect it as you would the DC from the rectifier. A T500mA / slow fuse for small amps (30-50W) and T1000mA for bigger amps (100W).
If your HT has a center tap, you must fuse both windings. Or else the unfused winding will still conduct if the other fuse blows - but in "half power mode". Can still toast the PT though.
There you'd theoretically want 2x350mA (500mA x 0,7) fuses for 50W amps and 2x700mA fuses for 100W amps. However, I find that these values pop sometimes under normal use, so I just put in 2x500mA or 2x1000mA fuses in these amps. That's sometimes a bit too high, but they've all popped very fast when I've simulated errors in the amp.
YMMV.
If you're fusing a HT with no center tap, just connect it as you would the DC from the rectifier. A T500mA / slow fuse for small amps (30-50W) and T1000mA for bigger amps (100W).
If your HT has a center tap, you must fuse both windings. Or else the unfused winding will still conduct if the other fuse blows - but in "half power mode". Can still toast the PT though.
There you'd theoretically want 2x350mA (500mA x 0,7) fuses for 50W amps and 2x700mA fuses for 100W amps. However, I find that these values pop sometimes under normal use, so I just put in 2x500mA or 2x1000mA fuses in these amps. That's sometimes a bit too high, but they've all popped very fast when I've simulated errors in the amp.
YMMV.
Re: HT Fusing
This is a dual 6V6 amp with a HV CT. A 5AR4 recto is being used.
I'm thinking because tube rectos rarely short compared to diodes maybe I should just fuse the CT or right after the recto. I really don't want to add two fuses, one for each HV AC line.
I'm thinking because tube rectos rarely short compared to diodes maybe I should just fuse the CT or right after the recto. I really don't want to add two fuses, one for each HV AC line.
Re: HT Fusing
That doesn't protect the PT from rectifier shorts.Structo wrote:Why not fuse the B+ right after the rectifier?
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Re: HT Fusing
Add diodes across the rectifier and fuse it after the rectifier. Then you have less enemies to battle.
Re: HT Fusing
If an amp already has a HT fuse, then I'll move it to the winding if FWB or the CT ground return if FW.
In the latter case, to mitigate for (tube) rectifier shorts, put Si diodes before the tube recifier, ie between the winding and rectifer plate.
Pete
In the latter case, to mitigate for (tube) rectifier shorts, put Si diodes before the tube recifier, ie between the winding and rectifer plate.
Pete
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Re: HT Fusing
Ha, ha, ha. I've seen some fireworks, let me tell you!ChrisM wrote:
I'm thinking because tube rectos rarely short compared to diodes maybe I should just fuse the CT or right after the recto.
You've got to fuse both sides to be safe; doesn't have to be user-accessible. Of course, plenty of amps that survived for 50 years, fused nothing on the HT side.