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I like this site very much. I did not make that kind of thing. Now it's all learned.
Klaus Heyne's Mic Lab / Re: consequences of incorrect plate voltage
« Last post by klaus on June 24, 2018, 05:45:03 pm »
Plate and B+ voltages are not necessarily interrelated. Bad tubes will cause low plate voltages, even if B+ is correct.

On the other hand, extreme B+ variations-too much or too little- will cause capsule distortion or noise, because capsule polarization voltage is usually divided off the B+ supply voltage. Here, even just a few more volts deviating in either direction will cause audible artifacts.

None of this answers your question. I have not experienced frequency deviations, because the other effects of too litte or too much B+ and plate voltages are overwhelming.
Klaus Heyne's Mic Lab / consequences of incorrect plate voltage
« Last post by Brian Campbell on June 24, 2018, 05:39:26 pm »
What are the effects of incorrect B+/plate voltages with regards to frequency response?
Is there a general guideline of tolerances?
Sometimes you will want to remove stains or residue from adhesive badges or labels on mic bodies. The art: to remove the stain without irreversibly altering or removing the micís surface plating.

Here is the general rule when choosing chemicals or mechanical means to remove foreign objects or dried, staining film from mic bodies: start with the lowest impact chemical and work up from there. The ultimate cure, mechanical abrasion, will usually also cause ultimate and irreparable damage to the surfaces they are applied to.

Here is how I usually proceed, in the order from mild to aggressive:

1. VM&P Naphtha - the same stuff luthiers use when they need to remove pickguard adhesive from even the most expensive acoustic guitars: it will not harm the surface of your mic but in many cases will dissolve the adhesive.

2. 100% or 99% Isopropyl Alcohol. Again, in this case it will not affect the nickel-plated surface of your mic.

If that still does not remove the yellow stains, and if you are sure that the letters embossed into the surface are anodized black, rather than painted black, choose:

3. Acetone. It's the most aggressive of chemicals I would use on a mic body. It does not affect or bite into metal, but if the lettering is painted, it will affect or dissolve the paint. If the lettering was black-anodized, acetone will not affect the anodization.

Test the acetone on a section of the lettering that is hidden or not in the center of viewing: put a small amount of acetone on a Q-Tip, and lightly rub it across a lettered area. If the Q-Tip turns black, the acetone dissolved paint. Even with painted letters, you can still use acetone on this mic, if you are very careful: remove the yellow film around lettering, and don't rub the acetone into it.

I would stay away from abrasive cleaning pastes: at minimum they will later the sheen of the affected area and make it brighter, at worst, they will leave scratches that cannot be removed without causing ugly shiny spots, if you try to buffer them out.
Klaus Heyne's Mic Lab / Re: KM84 Low output suddenly
« Last post by boz6906 on June 17, 2018, 01:14:36 pm »
High static charges can puncture the gate insulation on some FETs, but usually not JFETs like the 2N3819.

With a close strike (little delay between flash and bang) and 100meV discharge anything is possible.
Klaus Heyne's Mic Lab / Re: KM84 Low output suddenly
« Last post by BluegrassDan on June 13, 2018, 10:50:24 pm »
Thanks guy! I really appreciate the help.
Klaus Heyne's Mic Lab / Re: KM84 Low output suddenly
« Last post by klaus on June 13, 2018, 06:22:29 pm »
Nichicon, Panasonic, Elna... all fine and, at 63V rating, will fit.
Klaus Heyne's Mic Lab / Re: KM84 Low output suddenly
« Last post by BluegrassDan on June 13, 2018, 04:42:08 pm »
One more question. Is there a preferred type of capacitor to replace the tantalums? Would be hard to find any film capacitors small enough to fit. Would small Nichicon or similar electrolytic be okay?
Klaus Heyne's Mic Lab / Re: KM84 Low output suddenly
« Last post by klaus on June 13, 2018, 01:42:28 pm »
You can obtain the original 2N3819, now made by Fairchild, from Neumann or any well-stocked parts house.
But you must properly bias it, otherwise the mic will have low or no output, will distort or be noisy.
Klaus Heyne's Mic Lab / Re: U67: Add More Top End?
« Last post by klaus on June 13, 2018, 01:38:21 pm »
A clerical mistake, not a design change: the second value on the schematic, a bit closer to the tube filament, shows the correct negative voltage.
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