AB Carbon Comp resistor readings higher than nominal

[leonard.gulyas]

Dear Members,

I would like to know are these resistors any good for an amp build (68'-69' spec. JMP50) considering the actual higher value readings (none of them are within tolerance)?

(Resistor nominal value in bold, measured value in italic, please note that multiple measured values represent multiple resistors of the same kind):

33k Ohm 5% - 35.55k Ohm
470 Ohm 5% - 500 Ohm
56k Ohm 5% - 60.2k Ohm
68k Ohm 5% - 72.7; 73.0; 73.6; 74.7k Ohm
470k Ohm 5% - 507; 508k Ohm
100k Ohm 10% - 111.2; 112.0; 116.7k Ohm
1M Ohm 5% - 1.118; 1.131; 1.159; 1.161M Ohm

Please enlighten me.

I read this on the hificollective.co.uk website:

Allen Bradley RESISTORS special treatment method
As stated Allen Bradley resistors are carbon composite, being made of an organic material they do have the tendency to absorb water from the atmosphere. This absorption can increase the actual resistance of the component, hence the slightly higher than normal tolerance published on this page. To fix this simply remove the water from the component and seal it. The best method is to place the resistor/s on a baking tray set, your oven to 80 degrees centigrade and leave in the oven for 8 hours. The secret is to remove the mositure slowly if you rush it it will become noisy in circuit. Once cooked seal the body of the component in Shellack (preferably the flakes) to stock any further water absorption. At the end of this procedure you will have a much lower noise, more closely tolerance resistors. Special thanks to Colin Cornish for this excellent tip.

Has anybody tried it yet and what was the outcome of this method?

Regards

[Roe]

ABs typically read high as did some of the resistors and pots used in original marshalls.

[leonard.gulyas]

So Roe can I use it in a build like this or should I try out the "baking" process?

[Reeltarded]

Use them with confidence. CC resistors drift up in value. It's common to have 20% drift on some values, and that is magic.

[lprman]

I built a metro JTM45 a year and a half ago, and all the ABs were higher than stated. Many of them were off by as much as 20% if I remember correctly. I put them in anyway and they work fine. The amp sounds great and I couldn't be happier. Besides, as the amp gets used, the resistors should dry out from being in a hot environment.

[leonard.gulyas]

I thought I let you know I went ahead with the procedure on hificollective.co.uk.

Here are the result after the procedure (Please note that I added some more resistors. So once again the legend: resistor nominal values in bold, measured values in italic new values in red after baked in an oven for 8 hours at 80C/176F and sealed in shellack):

33k Ohm 5% - 35.55k 35.62k
470 Ohm 5% - 500 501
56k Ohm 5% - 60.2k 60.4k
68k Ohm 5% - 70.5k; 72.7k; 73.0k; 73.6k; 74.7k 71.4k; 72.1k; 72.3k; 72.7k; 73.8k
470k Ohm 5% - 481k; 486k; 487k; 507k; 508k 488k; 493k; 494k; 513k; 513k
100k Ohm 10% - 111.2k; 112.0k; 116.7k 111.1k; 112.4k; 116.7k
1M Ohm 5% - 1.118M; 1.131M; 1.159M; 1.161M 1.120M; 1.130M; 1.158M; 1.160M
820 Ohm 5% - 817; 841 820; 843
10k Ohm 5% - 10.2k 10.32k

So we can call the process pretty much useless but maybe the shellack seal will prevent the drifting of the resistor values in the future.

(On the side note: resistors ordered from hificollective.co.uk are further away from the nominal value than the ones from valvestorm so keep in mind folks from Europe next time you order)

Regards

[demonufo]

I don't know as I'd bother with shellac. Modern acrylic PCB sprays (conformal coatings) should do the same just fine.

[revolver1]

I read that bit on the Hi fi Collective site as well back when I was doing my build. My flat has had problems with humidity over the last few years. Another factor was that eventually I'll probably ship the amp back to Australia so 6wks in a cargo container crossing the equator probably aint gonna do those CC's any favours.

The Hi Fi CC's were all way, way outa spec. Valve storms wern't bad and also I found Watts Tube Audio was pretty good. Watts was the only place I could get an 820ohm one in spec the rest were miles off.

Anyway I did the bakeing thing, quite a few different batches and it made no differance at all. It was very time consuming and pretty much a huge pain in the butt with no reward.
I also found that after bakeing they became more fragile in that the heat from the soldering iron wrecked them and threw them way,way outa spec If I wasn't carefull.

So in the end I just ordered a whole selection from VS and WTA and used the ones that were within an ohm or so from spec. I then did the Shelack thing wich again is a pain to do and I recon the sugestion of the PCB spray might be a better way to go since Shelacking is a bit of a performance ie; getting it, mixing it, applying it then waiting for it to dry and it dosen't look so great. Also I had to use heat sinks when soldering because the shelack melts with the heat. (heat sinks are probably a good idea with CC's any way from my experience).

I've checked the values many times since I finished the amp about 9 months back and they are all just as when I put 'em in. So i would say seriously don't bother with bakeing 'em. Just select the ones that are in spec or maybe you want them outa spec but if moisture is an issue and you want them to stay where they are Shelack works. Although there may be better products out there for the perpose of sealing. Also get a selection from a few different sources since I noticed trends in values getting them all from the same batch.

[demonufo]

Anyway I did the bakeing thing, quite a few different batches and it made no differance at all. It was very time consuming and pretty much a huge pain in the butt with no reward.
I also found that after bakeing they became more fragile in that the heat from the soldering iron wrecked them and threw them way,way outa spec If I wasn't carefull.

The trouble with domestic ovens is the accuracy is absolutely non-existant at those lower temperatures. I discovered this years ago when baking magnetic tapes. I used to have to take them to work and pop them in the commercial ovens that were predominantly used for baking paint and coatings, and were pretty super accurate at all ranges.