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A Question About Cables (really...) - Tube Mics & 7-Pin XLRs

SuicideSquid

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I'm currently working on a couple of microphone mod projects involving a 47-style large diaphragm tube mic, and a C12-style large diaphragm tube mic.

For those not super familiar with microphone tech, tube microphones house the tube in the microphone body. The standard 3-pin XLR cable is connected from the microphone preamp to microphone power supply, and then a seven-pin cable, consisting of the 3-pin signal/ground/shield connections of a standard XLR cable, plus low-and high- voltage power connections and grounds, are run from the power supply, through a cable typically ten feet in length to the tube mic. The low-voltage power connection typically carries around 150mA of current at 6-12volts, and the high-voltage connection flips that, carrying just a couple of mA at 120-200 volts.

I am broadly skeptical of claims that cables make a difference - I use standard XLRs in my recording setup, and I've never paid more than about $25 for a cable for my home theatre or headphone setups either. However, I've read from a couple of otherwise-reliable sources that the quality of the 7-pin XLR cable can make a meaningful difference in the microphone's output - that poor-quality cables such as those included with budget tube mics like the Apex 460 can cause a sag in the voltage to the heater (the low-voltage, "high" amperage line) which will alter the performance of the tube (presumably due to poor soldering on the connections and improper impedance or high resistance on the cable itself?).

This seems plausible to me, but as most of us know from the home theatre world, there's a massive difference between a plausible theoretical difference and measurable real-world difference.

Does anyone know of any objective testing showing a measurable difference between a cheap 7-pin cable and a "high-end" 7-pin cable? The "high-end" ones are typically in the $50-$100 range so we're not talking about a crazy amount of money here but I'd still rather not throw my money away on something that's just pseudoscientific nonsense.

Here's Microphone Parts's blurb on their Gotham cable. https://microphone-parts.com/products/gotham-gac7-neutrik-xlr7-tube-microphone-cable - Matt McGlynn, who runs mic parts, is a straight shooter and a good engineer, and his mic kits are great so I'm inclined to take his word on this, but the reference to "burn-in" is a big red flag for me, so I don't know what to think.
 

Speedskater

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Almost no activity with regards to 7-pin cables. (or are you thinking about CAT cable?)
There is a difference between budget cables and cheap cables.
But even budget cables might have problems with a 300 foot long on location run in a harsh electrical environment.
 

Blumlein 88

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I'd say what truth there is to this would be down to good connectors and not trying to make a super thin cable with overly small conductor sizes. So any reasonable cable with Neutrik or Amphenol connectors should do the trick. Nothing esoteric involved here. I have used tube mics a few times and never had any issue with the supplied 7 pin cables.


 

sam_adams

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Gotham GAC-7 is quality cable for this intended purpose (Specs)(Pricing at Redco). Another high quality tube mic cable is Mogami W3172(Specs)(Pricing at Redco). You probably won't go wrong with either. Just remember that custom made cables made with quality-sourced parts are only as good as the quality of the workmanship of the cable assembler.
 

AnalogSteph

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However, I've read from a couple of otherwise-reliable sources that the quality of the 7-pin XLR cable can make a meaningful difference in the microphone's output - that poor-quality cables such as those included with budget tube mics like the Apex 460 can cause a sag in the voltage to the heater (the low-voltage, "high" amperage line) which will alter the performance of the tube (presumably due to poor soldering on the connections and improper impedance or high resistance on the cable itself?).
That is certainly a possibility. If in doubt, consult a wire gauge / resistance calculator. For example, 22AWG of pure copper is good for 54.8 mOhm/m, times 2 is 109.6 mOhm/m. That is a drop of 16.44 mV/m at 150 mA. If we've got a 5% tolerence for the heater voltage which seems to be recommended, that would mean a permissible length of less than 20 m / 60' at 6.3 V / 150 mA for sure. Even less for a 6Z1P (6.3 V / 175 mA), and less than 10 m for an MSC-2 ( 2 V / 105 mA). More - up to 30 m / 90' - for a 6072A (12.6 V / 175 mA, UNLESS it's parallel wired for 6.3 V which obviously doubles current to 350 mA, in which case the maximal length drops to a quarter or about 7.5 m; the Micparts Tube mic power supply has a 6-6.8 V heater output, and their cable is 6 m long).

The legendary VF14 tube (of U47 fame) used a 60 V / 50 mA heater, a smart choice. It would take 500 m of cable for a critical voltage drop to occur!

GAC-7 uses 0.5 mm² conductors for the heaters at 37 mOhm/m each (20-ish AWG). Expect +50% in permissible length over the previous calculations. W3172 uses 22AWG. (My guess really wasn't too bad!)
 
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SuicideSquid

SuicideSquid

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That is certainly a possibility. If in doubt, consult a wire gauge / resistance calculator. For example, 22AWG of pure copper is good for 54.8 mOhm/m, times 2 is 109.6 mOhm/m. That is a drop of 16.44 mV/m at 150 mA. If we've got a 5% tolerence for the heater voltage which seems to be recommended, that would mean a permissible length of less than 20 m / 60' at 6.3 V / 150 mA for sure. Even less for a 6Z1P (6.3 V / 175 mA), and less than 10 m for an MSC-2 ( 2 V / 105 mA). More - up to 30 m / 90' - for a 6072A (12.6 V / 175 mA, UNLESS it's parallel wired for 6.3 V which obviously doubles current to 350 mA, in which case the maximal length drops to a quarter or about 7.5 m; the Micparts Tube mic power supply has a 6-6.8 V heater output, and their cable is 6 m long).

The legendary VF14 tube (of U47 fame) used a 60 V / 50 mA heater, a smart choice. It would take 500 m of cable for a critical voltage drop to occur!

GAC-7 uses 0.5 mm² conductors for the heaters at 37 mOhm/m each (20-ish AWG). Expect +50% in permissible length over the previous calculations. W3172 uses 22AWG. (My guess really wasn't too bad!)
Amazing, thank you!

I'm using a 3 metre cable so it sounds like unless there's something defective in the cable construction, I've got nothing to worry about.
 
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SuicideSquid

SuicideSquid

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Almost no activity with regards to 7-pin cables. (or are you thinking about CAT cable?)
There is a difference between budget cables and cheap cables.
But even budget cables might have problems with a 300 foot long on location run in a harsh electrical environment.
These are not standard home audio cables or CAT cables. They're 7-pin XLR cables for tube mics that carry power alongside the standard XLR signal.
xlr7.jpg


Gotham GAC-7 is quality cable for this intended purpose (Specs)(Pricing at Redco). Another high quality tube mic cable is Mogami W3172(Specs)(Pricing at Redco). You probably won't go wrong with either. Just remember that custom made cables made with quality-sourced parts are only as good as the quality of the workmanship of the cable assembler.
Gotham with Neutrix connectors is what's being sold for around $80-$100 for a 20' cable.
 
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SuicideSquid

SuicideSquid

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Quality parts and cable. The price is not unreasonable if the workmanship is top notch.
That's not the question. The question is will it make any difference over a basic 7-pin XLR over a typical run length.
 

thecheapseats

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just ran across this thread - been busy and haven't logged on in a while...

as for 7pin (some cases 9pin) and other oddball multi-core cable used for tube/new/vintage mics for combined power suppy/mic(s)-signal(s) - gotham has been my go-to for decades... one of the few applications when I use gotham over mogami every time...

gotham historically gets the cable's outer jacket o.d. 'right' (as well as the wire gauges) - for the many times you're soldering to old or new unobtainium/nutty-money vintage connectors (tuchel etc.) that need wiring replaced... if you're buying cables pre-assembled or someone else does the assembly - I'd be damn sure they got the gauge/pinouts right (powerdelivery and signal)...

been there done that on telefunken elam(s), m&u neumann(s) and a few other oddball mic/powersupply combinations over the years... those mic classes demand the interconnects are wired correctly with the proper gauges for power, shielding and signal... years ago I caught a few mistakes when buying/testing used vintage mics and studio rework techs didn't get it right...
 
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