Refurbing an old valve mic. I want to replace a couple of high R presumably Carbons with the quietest available. Presumably Vishay Metal Film? There is no current flowing but 1/2W would be a suitable physical size. The values are the problem. And shipping to Ireland kinda makes the UK and USA a nuisance. 60 Megohm and 100 Megohm. Help!
DD
DD
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Why not series 6 - 10Meg film they come in almost any of the assortment kits? At no current there is no excess noise only sqrt(4KTR) which is the same for all materials.
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RS? Farnell?
RS has a handful of "metal glaze" resistors in the 50-100 MΩ range. Would those work?
Tom
RS has a handful of "metal glaze" resistors in the 50-100 MΩ range. Would those work?
Tom
Special Valve Mic U47
A good idea, perhaps two 30s or such. I should have been more specific. The Mic is a Neumann U47. I would like to use the best possible resistors, and a couple of caps.These things go for 10-15,000 Euros, so appearance will be a thing when I go to sell it in 5 or 10 years time. So ideally a pair of nice looking metal films of the same type. RS and Farnell don't have 60M.
Tx DD
A good idea, perhaps two 30s or such. I should have been more specific. The Mic is a Neumann U47. I would like to use the best possible resistors, and a couple of caps.These things go for 10-15,000 Euros, so appearance will be a thing when I go to sell it in 5 or 10 years time. So ideally a pair of nice looking metal films of the same type. RS and Farnell don't have 60M.
Tx DD
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Noise
My Fluke and friends meters don't go to 100Meg so I can't check the resistance. I strongly suspect it has drifted in 60 years or so though! Re the noise, the 60 Meg is at the grid so if there were any current at all flowing, it would be amplified. Would that be correct? Thanks for posting the Schematic.
My Fluke and friends meters don't go to 100Meg so I can't check the resistance. I strongly suspect it has drifted in 60 years or so though! Re the noise, the 60 Meg is at the grid so if there were any current at all flowing, it would be amplified. Would that be correct? Thanks for posting the Schematic.
If the mike works, I would NOT! 'refurb' a Neumann U47.
I'd rather touch-up the Mona Lisa than mess with a U47.
I'd rather touch-up the Mona Lisa than mess with a U47.
If you intend to sell this, the value will certainly drop because of anything that you do to it.
Leave it as it is. Anyone who would want this mic, will not want you to mess with it.
Leave it as it is. Anyone who would want this mic, will not want you to mess with it.
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did you check doenberg?
Dönberg Electronics - Electronic Components and Equipment
btw, such hi-ohmic circuitry is quite sensitive to any dirt. I recommend to clean it.
I know such electronics have a hard time at West Cork
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Well, I am in Cork City, so minus the salty air issues! I will contact Donberg.
I know well that these mics have a collector aspect, so I have been moving slowly and carefully. But it has been serviced and repaired before so no red seals on the screws etc. But I do want to use it in my studio soundsound.ie and at the moment it is too noisy. I am in hurry so I spend way more time asking questions..... I have cleaned the silver plating of the valve socket. Also hoovered out dust and actually cleaned anything safe. The VF14 looks..... new! The capsule looks quite clean, but I will probably deox the silver pattern switch blades. It may be a bit tricky measuring those High Rs. Given the age it is a betting certainty that the Carbon resistors will have drifted. Disc ceramic on the 64V...... Output film cap...... Weirdly when I was extracting the valve socket, two of the wires just popped off. Not sure if it was dreadful soldering during some servicing or that the valve bouncing around slightly in it's very light resilient suspension caused the wires to fray and crack at the attached point.
I know well that these mics have a collector aspect, so I have been moving slowly and carefully. But it has been serviced and repaired before so no red seals on the screws etc. But I do want to use it in my studio soundsound.ie and at the moment it is too noisy. I am in hurry so I spend way more time asking questions..... I have cleaned the silver plating of the valve socket. Also hoovered out dust and actually cleaned anything safe. The VF14 looks..... new! The capsule looks quite clean, but I will probably deox the silver pattern switch blades. It may be a bit tricky measuring those High Rs. Given the age it is a betting certainty that the Carbon resistors will have drifted. Disc ceramic on the 64V...... Output film cap...... Weirdly when I was extracting the valve socket, two of the wires just popped off. Not sure if it was dreadful soldering during some servicing or that the valve bouncing around slightly in it's very light resilient suspension caused the wires to fray and crack at the attached point.
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You can check the resistance of the high-value resistors by putting them one at a time in parallel with a lower value resistor that can be measured by your meter. Then using the parallel resistor formula and the change in resistance measurement, calculate the resistance of the high-value resistor.
Take care,
Doug
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I've never tried a 15000 € mic, but I agree with Rayma.
Don't fix it if it isn't broken.
I wouldn't mess with the patina too, but would perhaps just clean the grilles.
This affects the value.
Don't fix it if it isn't broken.
I wouldn't mess with the patina too, but would perhaps just clean the grilles.
This affects the value.
The noise level you are hearing may not be due to the resistors, have you tried another tube?
Always keep all the original parts for an item like this, even if they have been removed.
Patina is considered to be highly desirable, being difficult to fake.
Always keep all the original parts for an item like this, even if they have been removed.
Patina is considered to be highly desirable, being difficult to fake.
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Good idea re parallel resistors. But a friend here suggested placing a 1 or 2 Meg close tolerance in series, apply a high voltage. The voltage drop differential should tell the High R value.
I doubt that RS have stock of such odd resistors. It's been a while but they may have a minimum order and even postage here is nuts.
A few have mentioned this collectors aspect which is good advice. But the mic is not working properly, it is excessively noisy. I have cleaned it as advised by Klaus Hayne, a well known specialist. Washing up liquid, warm water. It came up looking very nice. Obviously very old but also obviously clean, no PVC tape residue etc. etc. The patina if anything is showing better now that a layer of handling dirt is gone. I don't have another tube, they are very hard to find with no guarantee of functioning. Thousands of bucks. I will of course keep any parts I change.
I doubt that RS have stock of such odd resistors. It's been a while but they may have a minimum order and even postage here is nuts.
A few have mentioned this collectors aspect which is good advice. But the mic is not working properly, it is excessively noisy. I have cleaned it as advised by Klaus Hayne, a well known specialist. Washing up liquid, warm water. It came up looking very nice. Obviously very old but also obviously clean, no PVC tape residue etc. etc. The patina if anything is showing better now that a layer of handling dirt is gone. I don't have another tube, they are very hard to find with no guarantee of functioning. Thousands of bucks. I will of course keep any parts I change.
If you can't/won't afford the postage on a resistor shipped from RS, perhaps selling the $15k microphone as-is and buying something that works would be the better option. Leave it for someone with deeper pockets to restore it.
I find it unlikely that a resistor would have gone noisy. Would it be possible to clean those high-resistance resistors (gently!) to remove surface contaminants? It wouldn't take much of a leakage path to reduce a 100 MΩ resistor to something much lower.
There's a VF14 tube on eBay with a buy-it-now price well below the multi $k you mention. Supposedly tested. Located in Italy.
Tom
I find it unlikely that a resistor would have gone noisy. Would it be possible to clean those high-resistance resistors (gently!) to remove surface contaminants? It wouldn't take much of a leakage path to reduce a 100 MΩ resistor to something much lower.
There's a VF14 tube on eBay with a buy-it-now price well below the multi $k you mention. Supposedly tested. Located in Italy.
Tom
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Not sure metal film go that high as metals are good conductors - this might be a case for metal-oxide. Vishay only seem to do upto 50M in metal film.