Marcus0000 said:
thanks for the tips. I'm going to order from thetubestore. Couple questions:
I'm replacing all the electrolytics under the metal cover... and the tube store doesn't have exactly the same values of caps available. what was in there:
Mallory
1. 2x220 MFD 285 VDC
2. 3x20 MFD 500 VDC
3. 2x70 MFD 100 VDC
I can order:
1. Illinios 220 uF 300 VDC ( I assume uF is the same as MFD)
2. Atom 20 uF 600 VDC or Illinois 22 uF 500 VDC (way cheaper than the atom)
3. the closest are:
(atom or illinois) 80 uF 450 VDC or
atom 50 uF 50 VDC
What are my best choices? thanks
Interesting about the 220/300vdc. Is that the stock schematic value? It looks to me like someone had subbed them in over the years. Unless you've got the later "UltraLinear" Twin. I seem to remember those values in that one.
There were a LOT of Twins!:tongue:
Anyhow, you can ALWAYS go higher in voltage rating! It just means the cap has more safety factor available. So
[email protected] volt is fine.
You can also go a bit higher in capacitance value, to reach a standard available value. Those 2 pcs of 220 mfd actually will be in series in your power supply circuit, which will result in a combined value of half of one cap, or 110 mfd but at twice the voltage rating i.e. 600 volts. The actual voltage at that point in the circuit will be less than 450 vdc so there's no problem.
Either brand will be just fine. Some folks just feel more comfortable with a larger, old-fashioned style cap, that's all.
The last choices are for the cap(s) in the bias supply. You absolutely CANNOT use a 50 volt rated cap here! The bias voltage could exceed 50 volts and the cap would likely short out.
The Illinois is ok at 80 mfd but 450 volt is sheer overkill and likely kinda big. You might want to pick up something from that mainstream industrial distributor! A radial leaded cap is not too much trouble to mount in that spot and they likely have something over 70 vdc available, like 100 vdc. Much smaller!
If there's nothing close to 70 mfd then you could go for 100 mfd, which almost certainly is available. Or you might go a bit lower, down to 47 mfd. If it were my amp that's what I'd do. I don't like the idea of filters being too big a value in the bias supply. Bigger values take longer to charge up to the needed voltage and while the voltage is too low during warmup the output tubes will see too low a bias voltage and draw excessive current. All you need is a big enough value to keep the bias voltage hum free. Since there's no current to speak of in the bias supply yu can get away with MUCH lower cap values! If this were a Marshall you'd see 8-10 mfd instead of 70! This is another example of how the engineers that designed the Silver Face Fenders were kinda goofy...
Pay attention with that last cap! The bias supply is the only point in the amp where the negative lead is NOT tied to ground! All the other filter caps are tied negative to ground but in the bias supply you ground the positive!
If you make a mistake you'll find out pretty quickly...