doing some reading on the old pas's--a complaint i have come across is about the transformer being not as powerful as needed even tho adequate--is this a reality and does the large cap board mods compensate for this--to further my question has anyone ever built a more substantial transformer for the pas's and would there be a benefit in doing this--we mod/ upgrade the hell out of these things but never the transformer--howie
2 posters
pas2/3 transformer
howardnair- Posts : 21
Join date : 2010-05-31
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
The little PAS transformer is, as you say, "adequate". In normal use it will have no problems. Even adding larger filter capacitors does not cause problems, as once the new capacitors are charged, they don't take any more current to keep charged than to replace energy drawn off by the circuitry; probably less than an old, leaky quad capacitor.
However, there are some things you can do to reduce the stress on the transformer and give it a little bit of a helping hand:
1. Mount it outside the cabinet. The PAS transformer does heat up inside the sealed PAS cabinet. If you move it outside, mounting it on the back panel, it will be in open air, where it will keep cooler. If you are adding capacitance to the power supply, this will also give you some more room inside the cabinet. Yes, it looks butt-ugly but how much time do you spend looking at the beautiful styling on the back of your preamp? Besides, you might need more space behind the PAS anyway, to provide room for the stiff, firehose-sized megabuck interconnects with big RCA plugs, that some audiophiles like so much. IF you changed the input jacks on your PAS, the transformer mounted on the back panel will FORCE you to give those cables a little breathing (bending) room!
2. Replace the pilot light and amber lens with an LED and appropriate dropping resistor. That bulb draws about 0.15 amp, a small LED draws much less. You also have your choice of decorator colors when selecting an LED!
3. Don't add tubes to the circuit. Some have added a dual triode as a cathode follower to make the PAS immune to variations in input impedance of the equipment connected to it (I know Panor did it in the PAS-3 Series Two but I still think it's a BAD idea unless you're driving low impedance solid state stuff). Doing that should involve at least transferring the filament supply for the whole preamp to a separate filament transformer.
4. FUSE IT. Dynaco saved a few pennies by not including a fuse on the primary of the power supply. Of course, while they were in business, if a transformer went out due to a shorted power supply capacitor or rectifier, they could always go to the parts bin to get another one. But why worry about it when an appropriately sized fuse will protect the transformer? When you move the transformer to the outside of the back panel, you can add a terminal strip that will allow for a pigtail fuse to be soldered in, so you don't even have to drill a hole for a fuseholder. A 1/4 amp slow-blow fuse will work just fine.
However, there are some things you can do to reduce the stress on the transformer and give it a little bit of a helping hand:
1. Mount it outside the cabinet. The PAS transformer does heat up inside the sealed PAS cabinet. If you move it outside, mounting it on the back panel, it will be in open air, where it will keep cooler. If you are adding capacitance to the power supply, this will also give you some more room inside the cabinet. Yes, it looks butt-ugly but how much time do you spend looking at the beautiful styling on the back of your preamp? Besides, you might need more space behind the PAS anyway, to provide room for the stiff, firehose-sized megabuck interconnects with big RCA plugs, that some audiophiles like so much. IF you changed the input jacks on your PAS, the transformer mounted on the back panel will FORCE you to give those cables a little breathing (bending) room!
2. Replace the pilot light and amber lens with an LED and appropriate dropping resistor. That bulb draws about 0.15 amp, a small LED draws much less. You also have your choice of decorator colors when selecting an LED!
3. Don't add tubes to the circuit. Some have added a dual triode as a cathode follower to make the PAS immune to variations in input impedance of the equipment connected to it (I know Panor did it in the PAS-3 Series Two but I still think it's a BAD idea unless you're driving low impedance solid state stuff). Doing that should involve at least transferring the filament supply for the whole preamp to a separate filament transformer.
4. FUSE IT. Dynaco saved a few pennies by not including a fuse on the primary of the power supply. Of course, while they were in business, if a transformer went out due to a shorted power supply capacitor or rectifier, they could always go to the parts bin to get another one. But why worry about it when an appropriately sized fuse will protect the transformer? When you move the transformer to the outside of the back panel, you can add a terminal strip that will allow for a pigtail fuse to be soldered in, so you don't even have to drill a hole for a fuseholder. A 1/4 amp slow-blow fuse will work just fine.
Last edited by GP49 on Thu Jun 17, 2010 1:15 pm; edited 1 time in total
howardnair- Posts : 21
Join date : 2010-05-31
hi GP49(-well damn i was hoping on buying a 40 lb transformer with enough juice to supply a small city---actually thanks for satisfying my curiosity-as to the fuse --that is on the roy mottram cap board i have-i just may mount the transformer outside if not i usually build a wood case that is ventilated -i won't use the metal case --and the fm3 zone sells a led and resistor setup that mounts right where the old pilot light was installed-thanks for the info--howie