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05-01-2024, 05:15 PM
(This post was last modified: 05-01-2024, 05:22 PM by physics.
Edit Reason: More details.
)
Ok, I'm putting together a shopping list for some electronics parts I need and would like to double-check with you guys the fuse values I've selected. I checked TUT3, and all I saw so far was current calculations. Useful, but I'm still wondering about margin factors.
Mains fuse is 1A, it's what the stock bogen was spec'd for. and even with the additional 12AX7 and the change to EL84 power tubes that only equates to an extra 38mA if I did the math right. The stock fuse is about double actual stock load if my calculations are correct, so I'm thinking there is enough headroom that the fuse doesn't need to change.
Heater draw is 2xEL84 + 3x12AX7 = 3.32A plus a few milliamps for the pilot light LED, so I'm thinking 3.6A minimum, possibly 4A. The heater is not center-tapped.
Stock it's a 20W amp, so peak audio output power would be 40W, add 20% per TUT3 for waste power and we're looking at 48W. Per the schematic the power transformer is designed for 400V plate voltage at 120V line voltage, so using that number I'd expect peak draw of about 120mA. This is where I start feeling funny. 120mA seems like an awful small fuse value, even if I'm rounding to a 200mA standard value fuse. Is that correct? Am I missing margin to keep the fuse from constantly blowing?
For the bias winding I'll have around 90VDC raw output across two parallel 25k pots in series with a 6k8 resistor to ground, so around 5mA assuming no grid current. I'm guessing just use the smallest fuse size I can, which is 200mA I think?
So, how does that all sound? Another option is to only use a mains fuse, like I think the Soma 84 does. Not quite sure why some of the projects are fused to the hilt and others aren't. I'm going to see if I can reverse engineer the Standard's fusing to back out margins.
Thanks for any input!
P.S. Forgot to mention. There are no center taps, and all the fuses I was talking about are assumed to be slo-blo.
Mains fuse is 1A, it's what the stock bogen was spec'd for. and even with the additional 12AX7 and the change to EL84 power tubes that only equates to an extra 38mA if I did the math right. The stock fuse is about double actual stock load if my calculations are correct, so I'm thinking there is enough headroom that the fuse doesn't need to change.
Heater draw is 2xEL84 + 3x12AX7 = 3.32A plus a few milliamps for the pilot light LED, so I'm thinking 3.6A minimum, possibly 4A. The heater is not center-tapped.
Stock it's a 20W amp, so peak audio output power would be 40W, add 20% per TUT3 for waste power and we're looking at 48W. Per the schematic the power transformer is designed for 400V plate voltage at 120V line voltage, so using that number I'd expect peak draw of about 120mA. This is where I start feeling funny. 120mA seems like an awful small fuse value, even if I'm rounding to a 200mA standard value fuse. Is that correct? Am I missing margin to keep the fuse from constantly blowing?
For the bias winding I'll have around 90VDC raw output across two parallel 25k pots in series with a 6k8 resistor to ground, so around 5mA assuming no grid current. I'm guessing just use the smallest fuse size I can, which is 200mA I think?
So, how does that all sound? Another option is to only use a mains fuse, like I think the Soma 84 does. Not quite sure why some of the projects are fused to the hilt and others aren't. I'm going to see if I can reverse engineer the Standard's fusing to back out margins.
Thanks for any input!
P.S. Forgot to mention. There are no center taps, and all the fuses I was talking about are assumed to be slo-blo.
