I intend to replace the aged PA211 transformer of my PAS-2 dynaco pre-amp with a new one and would like to take the opportunity to mount it on the outside of the rear panel, hopefully to minimize hum and allow more cool air. I have come across some forum threads that a fuse could be put in place at the same time as a safety measure for the circuit. As I'm not good at electronics and circuitry (only capable of soldering and assembling PCB boards), I would appreciate if anyone could walk me through the way to connect the fuse. What kind of fuse and what wattage is required? Is the fuse going to be between the 120V inputs or other outputs? On the other hand, I've replaced the selenium rectifier and the multi cap with a multi cap board that has a fuse. With this fuse in place, is it still necessary to have another fuse for the transformer? Thanks in advance of any advices.
Follow Ups:
- RE: Fuse for PAS-2 Transformer Replacement - patknk939 12:43:57 05/12/15
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- In Reply to: RE: Fuse for PAS-2 Transformer Replacement posted by patknk939 on May 09, 2015 at 15:52:48
Hi 1973Shovel. I checked the bottom of the cap board. While the trace from "out to xfmr" runs the same way as you indicated, the trace from "120V in" is not. It runs to the small capacitor (right next to the fuse) that connects to the fuse. As for the half-amp fuse, I don't know why it is not the 300mA as the schematic shows. The fuse is there when I bought the board. In fact, the two caps (rectangular ones on left of the schematic) on the actual cap board are each 0.22uf, not 0.1uf showed.
- I'd say you're good to go - 1973shovel 14:04:01 05/12/15
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- In Reply to: RE: Fuse for PAS-2 Transformer Replacement posted by patknk939 on May 12, 2015 at 12:43:57
The cap is likely a high-voltage cap between hot and neutral AC. I'm confident your Audio Sound Lab board provides for the PAS to be fully fused as it is configured.
They likely upped the fuse value due to too many opening during power-up. 0.5A should be fine, and still offer you protection in the event of a short.
One question I didn't ask at the beginning, but have been curious about. Is there any particular reason you are replacing your PA211 transformer? Moving it externally, I understand. I did that on my Vorhis Last PAS, because I needed the room internally. I'm still using my original PAS transformer, and that's with somewhere around 80,000µF on the filaments. I just don't want to see you spending money on something you may not need.
Good luck with your project.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
- RE: I'd say you're good to go - patknk939 19:02:52 05/12/15
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- In Reply to: RE: I'd say you're good to go posted by 1973shovel on May 12, 2015 at 14:04:01
Replacing the PA211 transformer is not because it has problems. It`s that I thought the transformer is aged and the new PA211 is better constructed and is a little bit more powerful. So, hopefully it could be a little bit of upgrade for the PAS with the new transformer. Thanks for your time.
- If it were me - 1973shovel 04:43:49 05/13/15
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- In Reply to: RE: I'd say you're good to go posted by patknk939 on May 12, 2015 at 19:02:52
I would save my money.
The newer PA211 might have better specs, or whoever's selling it might claim it's better. But there are posts (you'll have to search) claiming some of the new production PA211s out there aren't as good as the original. I know, because I considered buying one to try building a dual-mono (one transformer per channel) power supply for my Last PAS. After reading what I read, I passed.
If you want an inexpensive upgrade, tweak, or whatever you'd like to call it, I would suggest adding capacitance to the filament supply. You could replace those two 2200µF caps on your board with the Nichicons I've linked below, but there's no need to. As long as you have room, you can add the extra caps, and simply wire them in parallel with the existing caps. Secure them with a bit of silicone, and you're good.
I'm not one of those "magic wire" guys, who claim they hear major improvements with every change. But I've never failed to hear an improvement when adding a significant amount of capacitance to a stock (or nearly stock, as yours is) PAS filament supply.
Good luck, and you're very welcome. Thanks for saying "thanks"!
"You won't come back from Fletcher-Munson curve"-Jan and Dean
- If it were me - 1973shovel 04:43:49 05/13/15
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- RE: I'd say you're good to go - patknk939 19:02:52 05/12/15
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- I'd say you're good to go - 1973shovel 14:04:01 05/12/15
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- Without knowing - 1973shovel 09:05:58 05/11/15
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- In Reply to: RE: Fuse for PAS-2 Transformer Replacement posted by patknk939 on May 09, 2015 at 15:52:48
which cap board you've used in your PAS, and where the fuse is in the circuit, it's only a guess as to whether or not all the components of your preamp are protected. Do you have a schematic of the board, or at least a brand?
Ideally, the fuse should be the first thing in the hot side of the AC line, prior to the on / off switch. Depending on how the board you have is wired, this may or may not be the case. If the fuse on the board is correctly placed, no second fuse will be required. What value of fuse is currently on the board?
More information, please!
"You won't come back from Fletcher-Munson curve"-Jan and Dean
- RE: Without knowing - patknk939 19:52:38 05/11/15
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- In Reply to: RE: Without knowing posted by 1973shovel on May 11, 2015 at 09:05:58
After going through my notes on upgrades to my PAS-2, I found that the cap board that replaced the selenium rectifier and the cap can has a 500mA/250V fuse in place. One of the 2 black 120V outputs from the PA211 is connected to "120V in" and the other output to "output to transformer" on the cap board (as shown in red circles on right side of the below image). So, with this fuse connected on the cap board, is it still necessary to have another one for the replacement transformer?
- RE: "is it still necessary" - 1973shovel 00:50:03 05/12/15
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- In Reply to: RE: Without knowing posted by patknk939 on May 11, 2015 at 19:52:38
If the traces on the bottom of the board run the same way as the two red lines I've added to your picture (and I'm betting they do), then no, your PAS is fused correctly, and there's no need to rework things.
Is there any reason you're using a half-amp fuse instead of the 300mA, slow-blow fuse called for?
"You won't come back from Fletcher-Munson curve"-Jan and Dean
- RE: "is it still necessary" - 1973shovel 00:50:03 05/12/15
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- RE: Without knowing - patknk939 19:52:38 05/11/15
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