Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on November 23, 2021, 12:59:46 am
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Hi
I always put a 220k bleeder resistor on the 1st B+ filter cap of my builds. If one has 4,5,6... filter caps.. is it good to have it on every filter cap or are you looking for trouble in that case?
The reason I'm asking is that on a JMP50 there are 2 can cap 50+50uf next to another. The first can has the positive sides directly linked and the it goes through the choke to the next filter cap, the dropping resistor to next cap...
I added that 220k from the 1st cap to ground, but also from the 3rd cap to ground. Is this problematic and why?
thanks as always ;)
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... I added that 220k from the 1st cap to ground, but also from the 3rd cap to ground. Is this problematic and why?
Not "a problem," just unnecessary.
All filter cap + terminals are ultimately connected to each other (albeit through resistors or a choke). A bleeder across any one cap will serve to drain all of them. Adding more bleeder resistors is just wasting current to create heat.
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... I added that 220k from the 1st cap to ground, but also from the 3rd cap to ground. Is this problematic and why?
Not "a problem," just unnecessary.
All filter cap + terminals are ultimately connected to each other (albeit through resistors or a choke). A bleeder across any one cap will serve to drain all of them. Adding more bleeder resistors is just wasting current to create heat.
Could this have caused my power transformer to generate the necessary heat to fry the HT winding, which happened BTW?
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Could this have caused my power transformer to generate the necessary heat to fry the HT winding, which happened BTW?
No. For example, let's say your B+ is 500V on each of those nodes. 500V across a 220K resistor is 2.27mA. Double that for 4.54mA. Not enough to start a fire. But the second bleeder is just wasting 2.27mA.
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Could this have caused my power transformer to generate the necessary heat to fry the HT winding, which happened BTW?
No. For example, let's say your B+ is 500V on each of those nodes. 500V across a 220K resistor is 2.27mA. Double that for 4.54mA. Not enough to start a fire. But the second bleeder is just wasting 2.27mA.
After I wrote that nonsense I figured that too. Thx for the confirmation...
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It is worth noting that any bleed resistor needs to be rated for the max working voltage it will be exposed to. It's likely that at least a 1-2W resistor would be needed for a 500V location - it depends on the spec of the resistor being used.
Sometimes a bleed resistor at the end of a string of rail dropper resistors can drop the voltage along that string such that filter and coupling caps see a noticeably lower peak voltage during power up (before any valves start conducting). That may allow say 350-400V caps to be used in some stages where otherwise 450V caps would be needed in all stages if the max B+ reached 450V.