PA100-D mods

General discussion

PA100-D mods

Postby TF3VS » 22 Aug 2013 14:33

Hi,
when building the PA100-D I notice that it is stongly suggested by the designers that the users make several modifications to the amplifyer.
It involves few capacitors and two ferrite rings.
These parts are not readily available in my country. I can not see that the Juma-shop is selling this as a mod kit (as I feel that they should).
Any recommendation where to buy it from, without spending big money (shipping can be expensive overseas for tiny things, hi)?

73 Villi
TF3VS
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Re: PA100-D mods

Postby 5B4AIY » 23 Aug 2013 07:43

Hi, Villi,
I obtained some of my capacitors from eBay suppliers, and they were relatively inexpensive, and the postage was free! These were from a supplier in Hong Kong. I have also obtained similar components from RS Components. Although they are a UK company, they do supply world-wide, and here in Cyprus I have had excellent service from them. Then there is Farnell Components, and they also supply world-wide. Unfortunately, unlike RS Components, Farnell imposes a 20 UK Pound (About 32 Euro) minimum charge. I have also dealt with mouser.com and they too will send world-wide.

I would recommend silver mica capacitors, with a working voltage of 500V. The amplifier is easily capable of 150W and the peak to peak RF voltage can be as high as 250V when working into a matched 50 ohm load. If the load impedance is higher, then naturally the p-p RF voltage will also be higher. The 500VWKG capacitors easily fit on the PC board without a problem.

You should at least consider replacing C27 in the 10m filter, but use two 100pf capacitors instead of a single 220pf. Similarly, if you are going the whole way, then replace C24/C21/C18/C15/C12 with a pair of capacitors instead of a single component. In my case I only detected excessive dissipation with C27, and have not had any trouble (yet) with any of the other capacitors, but I do have silva mica repacements available, just in case.

73, Adrian, 5B4AIY
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Re: PA100-D mods

Postby TF3VS » 10 Sep 2013 20:22

Thanks for your advise, Adrian.
I have now replaced the capacitors as per your suggestion (see http://tf3vs.ritmal.is/PA100D/). I did locate what I needed on ebay from a guy in French for a reasonable price and fair shipping. It seems that silver micas are considered to be something for audiophiles restoring their gear, hi. I also got FT50-61's and replaced the smaller rings as well.
All is FB. I noticed that the amplifier has gained some in the output, was that to be expected?

73 Villi
TF3VS
TF3VS
 
Posts: 5
Joined: 27 Nov 2011 20:34
Location: Iceland

Re: PA100-D mods

Postby 5B4AIY » 11 Sep 2013 06:05

Great! Took a look at your photographs, nice neat construction, and as you have found, the replacement capacitors fit quite easily. As for the slight increase in output, that is not unexpected, as the dissipation factor of silver mica capacitors is significantly less than the originals, and so there should be a few watts extra output available. For the ultimate in low-loss output filters, I would reccomend using all silver mica capacitors, but I also accept Juha and Matti's viewpoint that this would have significantly increased the cost of the amplifier with only a small improvement in efficiency which is hardly likely to be noticed in practice. All good engineering design involves intelligent compromises.
73 de Adrian, 5B4AIY
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