Proper Fusing

Discussion in 'Shock Brother's DIY Amps' started by XTRXTR, May 5, 2021.

  1. XTRXTR

    XTRXTR Tele-Meister

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    I was reading about fuses because I am planning on fusing my 6.3V winding on my amp that is being modified. I am modifying from a standard 2204 circuit to what I call a 2204 Vibe FX which will have a tremolo and a tube buffered FX loop

    I have already verified the standard Drake PT 1202-324 is used on a 2205 and 4210 both of which have two more 12AX7 tubes than a standard 2204. Bottom line I'm going from [email protected] 3.9A draw to a 4.5A draw on the 6.3V winding.

    So I read that one should oversize the fuse for nuisance blowing "Littelfuse suggests a “rerating” of 25% (for operation at room temperature); thus, a fuse with a rating of 10 amps would be used only if the circuit’s steady-state current will stay below 7.5 amps."

    Now my question: my [email protected] circuit should be fused at 6A...correct?
    4.5=x-.25x
    4.5=x(1-.25)
    4.5=x(.75)
    4.5/.75=x
    6=x

    Further I plan to add fuses to both sides of the winding - is this correct?

    Also I am raising the CT by 60Vdc due to high plate voltage exceeding 508Vdc and V2b cathode at 215Vdc so Vhk Max etc.

    My average wall outlet Voltage is 123Vac and occasionally gets to 125Vac and rarely at 120Vac I want to protect the winding if I get some spike in the AC line. I live about 15 miles north of St Vrain Power station which feeds Denver, Boulder, Loveland, Ft.Collins and Greeley. I am sort of in the center of those and I think the AC is higher here before it gets drawn down as it spreads to various substations to the other areas. I obviously want to protect my Drake PT against a short failure but I thought the context might be helpful as to why I want to get this right.

    I realize once two more tubes are in the B+ will drop a small bit, possibly the 6.3V winding which is actually 6.95V, more reason to be safe, I may want to put a 6A diode on the 6.3v winding if it does not drop with the extra tubes.

    Anyway I ramble its late, some direction is welcomed. :)
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  3. XTRXTR

    XTRXTR Tele-Meister

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    Really... that's a rhetorical really.
    That's Merlin Blencowe.

    Now I know why the Semko and CSA standards of the late '80s were abandoned for JCM 800s. At a partial reason. How many were brought to tech or warranty?

    So basically I have over thought this idea and need a zanax. If it's gonna go (6.3V winding) it's gonna go but most likely won't ever go...assuming the winding is rated for the job.

    Okay so onto the raising of the CT and Vhk management.

    Thanks @Tom Kamphuys
     
  4. Lynxtrap

    Lynxtrap Tele-Holic

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    Also, if a heater fuse blows with B+ on the plates, your tubes might be in trouble.
     
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  5. XTRXTR

    XTRXTR Tele-Meister

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    Good point I had not considered. If the heater fuse blows and you have fading heaters with B+ still at steady state then you have over current conditions on all of the tube plates.
     
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