I've been working on a dead AC-30CC which turned out to have an open heater fuse. I have questions.
1) Is this typical for new amps for regulatory reasons?
2) If one blows but the replacement holds up for a while, is it probably just fatigue? (I don't have a direct way to check current draw)
3) One of the sockets is marginal...Rat Shack doesn't have parts and I wonder if I should just put in a jumper...opinions?
Fuse on heaters?
Moderators: pompeiisneaks, Colossal
Re: Fuse on heaters?
I don't think that such secondary fusing is a regulatory requirement.
I remember RG Keen being supportive of fusing all secondaries, and I think that it's been demonstrated that fault current in the heater winding may not cause a sufficient increase in primary current to blow the line fuse.
So if fuses are there, it may be a retrograde move to bypass them.
However, the low voltage, highish current nature of the circuit doesn't seem to work well with regular fuseholders, with connections degrading over time, resulting in heat build up and damage.
Just an idea, but would it be feasible to solder the fuses in place?
I remember RG Keen being supportive of fusing all secondaries, and I think that it's been demonstrated that fault current in the heater winding may not cause a sufficient increase in primary current to blow the line fuse.
So if fuses are there, it may be a retrograde move to bypass them.
However, the low voltage, highish current nature of the circuit doesn't seem to work well with regular fuseholders, with connections degrading over time, resulting in heat build up and damage.
Just an idea, but would it be feasible to solder the fuses in place?
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Stevem
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Re: Fuse on heaters?
fues for the heater and or a PT primary need to be of the slow blow type, is that what you are using?
If not than a fast blow type will hold for a while but blow after a given amount of turn on`s and turn off`s!
If not than a fast blow type will hold for a while but blow after a given amount of turn on`s and turn off`s!
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Re: Fuse on heaters?
You're right pdf64 - there are cases where a short on a heater cannot cause enough primary current to blow the mains fuse.
Even more importantly, the purpose of the AC mains fuse is to stop electrocution and fires, so it's sized differently than it would be for protection of the transformer. The AC mains fuse *may* protect the PT, but that's a happy side effect. It has other more important duties.
I suspect that in all commercial amps with secondary fuses, the secondary fuses are to limit the collateral damage from a load failure (i.e. bad rectifier, failing filter cap, etc.) and stop it also killing the PT. The PT is usually the single most expensive part in the amp.
And you're correct about fuse clips. Heater fuses need to be soldered in, or special high-current types with appropriate fuse holders. Actually, automotive fuses would be nearly ideal for the low voltage, high current and big surges of heaters.
I used soldered-in fuses in the Workhorse amplifiers.
Even more importantly, the purpose of the AC mains fuse is to stop electrocution and fires, so it's sized differently than it would be for protection of the transformer. The AC mains fuse *may* protect the PT, but that's a happy side effect. It has other more important duties.
I suspect that in all commercial amps with secondary fuses, the secondary fuses are to limit the collateral damage from a load failure (i.e. bad rectifier, failing filter cap, etc.) and stop it also killing the PT. The PT is usually the single most expensive part in the amp.
And you're correct about fuse clips. Heater fuses need to be soldered in, or special high-current types with appropriate fuse holders. Actually, automotive fuses would be nearly ideal for the low voltage, high current and big surges of heaters.
I used soldered-in fuses in the Workhorse amplifiers.
Re: Fuse on heaters?
I used to work on a line of medical devices which had what we called pig tail fuses I haven't seen them for the 5 x 20mm size....should look. The idiots have wiring coming across the tops of FS2 and FS3 and they are too taut.