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    pas2/3 transformer

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    howardnair

    Posts : 21
    Join date : 2010-05-31

    pas2/3 transformer

    Post by howardnair on Wed Jun 16, 2010 7:36 am

    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

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: pas2/3 transformer

    Post by GP49 on Wed Jun 16, 2010 10:34 am

    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.


    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

    Re: pas2/3 transformer

    Post by howardnair on Thu Jun 17, 2010 8:10 am

    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

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: pas2/3 transformer

    Post by PeterCapo on Thu Jun 17, 2010 12:08 pm

    In the event someone is totally missing a transformer, here are a couple of sources for replacements, although I have some question as to how exactly they duplicate the operation of the original:
    http://www.triodeelectronics.com/pa2powtrandy.html
    http://cgi.ebay.com/POWER-TRANSFORMER-DYNACO-PAM1-PAS2-PAS3-PAS3X-/380142458596?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item58824196e4

    I am going to try an inline fuse holder that can be installed under the deck near where the power cord enters, in the hopes of keeping hum down:
    http://www.radioshack.com/product/index.jsp?productId=2102784

    I also got the fm3 zone LED module, but wired it according to Frank Van Alstine’s recommendation:
    http://www.avahifi.com/images/avahifi/root/audio_basics/ab_pdf/ab1983.pdf scroll down to volume two number four April, 1983.

    I’ve read a fair amount of debate on the usefulness of thermistors, but I will be trying a CL-140 thermistor in the transformer primary to ease any inrush, especially since I have increased capacitance in the power supply – including the fist capacitor after the 12X4:
    http://www.gesensing.com/downloads/datasheets/920_325a.pdf

    I haven’t got the preamp finished yet, but I hope to report back when it is completed (and working!). Any comments on the above appreciated.

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