Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: turtle441 on July 08, 2018, 02:37:15 pm
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Crazy question... I've been messing around building a Plexi SE using the pcb sold here: https://www.gapcb.com/products/plexi-se-pcb
C7 in their schematic is a standard cathode bypass cap, looks pretty electrolytic to me. https://www.gapcb.com/documents/Plexi-SE-PCB-Schematic-v11.pdf
Interestingly, their BOM lists it as a 100V film cap. Haven't ever seen a non-electrolytic used as a cathode bypass cap before. Does that work?? :dontknow:
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C7 in their schematic is a standard cathode bypass cap, looks pretty electrolytic to me.
What looks electrolytic about it?
You will have a hard time finding an electrolytic that is less than 1µF. All the old plexi amps used a .68µF cap in that position. Works very well.
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> What looks electrolytic about it?
On *that* drawing, the filter caps have a curved line, the tonestack caps are straight lines.
The 0.68u has a curved line.
OTOH, on the PCB, the known-electrolytic parts have + and - signs on the printing. The 0.68u does not.
We should also know that the difference Fender to Marshall is that Jim had boomier speakers and down-sized Leo's 10u-22u caps to under 1uFd, finding happiness around 0.68u. While you "can" get 0.68u in an e-cap it is very odd. And a film cap is better (won't rot in a decade). And Hoffmann stocks 0.68u film in lowest-available voltage *for* this purpose.
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You can "always" use a film cap in place of an electrolytic, if voltage and capacitance ratings are right.
You will have a hard time finding an electrolytic that is less than 1µF. All the old plexi amps used a .68µF cap in that position. Works very well.
I actually have 0.68uf electrolytics on hand but I don't remember exactly why or what I used them for. I assume I bought them because it was cheaper than a film cap at that size/voltage, but I doubt I saved more than $0.10 each.