Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cgiesel1 on August 25, 2012, 09:17:19 am
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Morning all,
I am getting ready to replace the electrolytic capacitors in my old tube amp.
I know it is ok to substitute for the nearest values --uf/v-- but is there a rule of thumb regarding how different the values can be? What is too different?
The substitutions in question are:
Power supply filter capacitor:
Original equipment:
Multisection
Sprague 270021-56 Substitution with capacitor pack
40uf 450v JJ 47uf 500v
30uf 450v F&T 30uf 500v
10uf 450v F&T 16uf 475v
20uf 25v Sprague Atom Aluminum 25uf 25v
The biggest issue I have is the speaker crossover. Can I sub the 16uf 25v caps for 25uf 25v or would that go badly? I am thinking a close match is a bit more critical in this case? Would I need to change out the resistors?
Speaker crossover:
Original Substitute with this?
16uf 25v 25uf 25v?
the crossover circuit looks like this: (r2 and r3 go to one terminal, transformer goes to a separate terminal)
[ R2 33ohm@20% |--> OUT
IN? -->[R1 10ohm@20%]-->[cap 16uf/25v]-->| R3 2.2 P 3v | [ Transformer pri 6000ohm sec 4-6 ohm ]--> OUT
Any advice would be appreciated
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All that should be fine. You might notice a difference with the crossover cap though, I'm not sure. Somebody will be along shortly that knows for sure. :smiley:
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The power DC filter caps are polarized, fine.
Speaker crossover caps *must* be bipolar/nonpolar (or not electrolytic, but in these values a fine film cap would be huge and costly(*)).
Look to DIY speaker stores for crossover caps.
25V on a LOUDspeaker crossover cap is pretty cheezy. OK for a 90-day warranty, but if you like the amp more than 90 months you oughta replace with >50V caps.
Try to stay within ~~20% of nominal crossover cap value. For "16u", aim at 13uFd-19uFd. In today's value-series, 15u is the new 16u.
PartsExpress has been around for decades:
http://www.parts-express.com/ (http://www.parts-express.com/)
http://www.parts-express.com/cat/non-polarized-electrolytic-capacitors/293 (http://www.parts-express.com/cat/non-polarized-electrolytic-capacitors/293)
(*) Actually, PE's price for 15u of fancy-film cap is not horrible, and it will last forever.
Metalized Polypropylene Capacitors (http://www.parts-express.com/cat/metalized-polypropylene-capacitors/294)
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Thanks for the reply. Parts express is one of my favorite sites. Good call on the non-polarized crossover cap. A closer examination of the cap showed that it is indeed non polarized.