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|6/24/2003 2:05 AM|
What is considered an adequate vacuum level for potting pickups (Hg", etc.)? There are venturi units that claim as much as 28 Hg", which actually seems rather high compared to some of the reasonably priced A/C pumps.
|6/24/2003 5:34 PM|
Jeff G. wrote:
The confusion comes in where pump makers spec the vacuum as "inches below ambient pressure" instead of as an absolute pressure.
In any case, 15" Hg below ambient will get you started but 28" Hg will make things happen faster. Somewhere in between those values are vacuum generators that run off of compressed air -- inexpensive if you already have a compressor.
Otherwise, try and grab a small vacuum pump off of eBay.
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|6/24/2003 9:57 PM|
Thanks for the info Doc!! I was beginning to wonder if it was my breath or something viz-a-viz my posts.
I have a pretty big air compressor in the garage ... 30 gallon, 6HP or something like that. I reckon that should work, eh? Thanks again.
|6/25/2003 5:08 PM|
Jeff G. wrote
Since you have a compressor (and even a little 3 gallon hotdog tank compressor will do), check out GAST vacuum generators for starters.
A search on Yahoo will get you almost too many hits.
|6/25/2003 10:43 PM|
It's yer breath.
Here's something that occured to me today. I have a Food Saver vacuum sealer that has a Mason jar attachment. Basically, you put the metal cap on the jar and pull a vacuum ... no need to screw on the ring.
The machine pulls a pretty good vacuum, I'd say, based on what it does to a steak in the larger marinating container.
I'm thinking I might be able to immerse a larger mason jar in hot water to heat the wax and pull a vacuum with the Food Saver. Sound reasonable?
|6/25/2003 6:10 PM|
Well in another thread I was getting ready to vacuum pot a Squier pickup, there were some good suggestions about wax and equipment.
When I took out the PU from the guitar, it had already been at least dipped at the factory, since it was a problem performer when trying to play beyond a certain gain level, I went forward planning to fully melt the present wax before trying to add more.
Trying to get a good blend of other waxes with the paraffin, I had shopped at a crafts outlet with lots of candles and candlemaking supplies. Beeswax and microcrystalline wax are additives that will enhance the properties of the paraffin, basically I wanted a blend that was softer than the raw paraffin and would flake less. I was considering the type of polymer-fortified blend that was commonly used for Dixie cups when they were about the most common disposable container. All the ingredients are relatively expensive, even the paraffin in bulk is a few bucks if you get enough for some good sized candles. Actual candles themselves, on clearance, appeared to be below the wholesale cost of the wax, and there were plenty of them. I found a set of 8 round floating candles that would just dent from the pressure of a fingernail without any whitening or discoloration of the displaced wax. They were scented but were old and had lost most of the smell, I chose plain vanilla mainly for its natural color, plus it is a Squier so nothing fancy
Also there was a sticker about how part of the purchase price goes to help cure breast cancer, so for $1 I was able to further research in an area which I am not ordinarily capable.
This was before I saw the message about successful use of the toilet bowl rings, now that wax is a little sticky and resembles the kind I see on some NOS tube equipment transformers.
Anyway, put 5 of the candles in a pyrex bowl but the metal wick holders threw sparks in the microwave, so moved to the hot plate. Slow simmering & stirring got everything melted, pulled the wicks and determined congealing point by removing the mercury lab thermometer, the gram or two of mercury holds the heat and a nice sized drop of wax hangs on while the degrees slowly drop as they approach ambient room temperature of 99degreesF. Began to thicken and congeal about 58degreesC. I figured I needed about 10 degrees C above that to make it remain liquid while I removed the dish from the heat to do the vacuuming, so I let it sit on the hot plate and adjusted the setting until the wax eventually just stayed at 65C. Now wax can go a lot hotter than that without smoking and still look the same, so I thought it was a good idea to measure temperature carefully so the plastic bobbin would not melt. The liquid was plenty thin.
Putting the PU in the dish caused a buildup of solid wax around the pickup until it had about 5 minutes to warm up to the liquid temperature, then I let it stay in there another 15 minutes or so to give time for the original wax to melt within the coils of wire, plus I wanted the extra mass of the pickup to fully heat up and help maintain the temperature after I took the dish away from the heat. Didn't have a fully rigged crock pot so I just had to use an old pyrex laboratory vacuum chamber I've had sitting around for ages. Good visibility though so I could observe the process in detail.
The vacuum pump is the mechanical/oil kind, this one was a Welch from the '50s and I had restored it a few years ago, they are still making the same model today but they cost over $1000 new. Took over $100 worth of oil to fill it. A/C & refrigeration servicemen use smaller pumps that give nearly the same laboratory vacuum and cost about half as much plus require less of the special vacuum oil. Sometimes see them at pawnshops for $150 to $200. Anyway, this level of vacuum leaves the chamber containing orders of magnitude less gas than would result from an aspirator suction device.
So I put the wax dish in the chamber, shut the lid and sucked out the air. After the chamber was largely evacuated, fine bubbles continued to emerge from deep within the coils of the PU for a number of minutes, but they only appeared when maximum vacuum was being applied. This was a vacuum engineer's offgassing nightmare, where deeply soaked gases have a tortuous path from their source to the vacuum outlet, with no significant pressure remaining to push molecules toward the lowest vacuum zone at the chamber outlet, you just have to wait for molecular motion to randomly find the occasional molecule in the vicinity of the *black hole* before it can be sucked away. With this pump turned off, the chamber was evacuated well enough to exceed what an aspirator could do while the aspirator was running, but the constant stream of fine bubbles only came out when the pump was running to maintain the laboratory vacuum inside.
After the guitar was reassembled (I waited for the pickup to cool down and the wax to solidify ), it was obvious that no miracles had occurred, on the modified JCM800 you can turn the pre gain up to 4 where it could only handle 3 before, the difference is significant but not dramatic.
I now suspect the factory may only be dipping the pickups, or is not using as high a vacuum as I did or taking as much time for the re-potting.
|6/25/2003 10:06 PM|
Mike thanks for you entertaining details...
A question I have always had regarding this process is:
After you successfully vacuum out the air of the pickup, when you remove the pickup from the hot wax and vaccum chamber doesn't most of the hot wax just run back out with the displaced wax being replaced by air?
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