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bockaudio

Platinum Blonde

Posts: 1,516 Member Since: 02/02/2011

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Apr 9 12 2:03 PM

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To the hardworking and or lucky owners of VF14 U47's (nuvistor and otherwise modded 47's need not read further), a reminder to check the B+ voltage regularly. Now, if your first question is "what's B+?", PLEASE have someone else, who is qualified with high voltages, to do this for you.
As real, original U47 power supplies age, the B+ often creeps up, resulting in a U47 that "sounds better than ever", but, reduces the life of the already elderly VF14. I have seen as high as 130v (supposed to be 105v) before the tube finally gave out. If you have an original psu you will likely be affected by this. There are various repairs to counteract this climb, ultimately leading to replacement of the power transformer or the entire psu.

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jpaul bordon

Silverado

Posts: 176 Member Since:29/01/2011

#1 [url]

Apr 9 12 5:05 PM

Thanks for the reminder David. I check it every 2 months if I dont go on the road. But on the road I check it every time I strike it up.   Probably over kill but better safe then sorry. I have an Oliver PSU same design as the Neumann NG's..
Quick question...

I was told to measure the B+ at the tube pin.  I bring just a fluke handheld VOM on the road and see maybe .1 volt difference between the reading at the tube verses the PSU.   Does it matter where you take the reading?

They are listening to us... stop it...

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klaus

Gold Finger

Posts: 385 Member Since:19/02/2012

#2 [url]

Apr 9 12 5:51 PM


There is no B+ at any of the tube pins. And there is no discernible voltage drop between measuring B+ at the power supply or measuring it at the mic's connector. 

If you keep the power supply's B+ adjusted, so that the heater voltage as measured at the tube pin remains at 36VDC ± 2 volts, you will be fine.

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jpaul bordon

Silverado

Posts: 176 Member Since:29/01/2011

#3 [url]

Apr 9 12 6:06 PM




I should have said B+ in the mic ... sorry not tube pin..  brain fart.  I took out my cheat sheet and thats what I have to check 35v... 36+- 2 is what you are saying  AT the tube. 105v psu out


They are listening to us... stop it...

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ben sneesby

Silverado

Posts: 120 Member Since:01/02/2011

#5 [url]

Apr 15 12 7:47 PM

We had one here just yesterday at 120v. Customer had no idea of the consequences.

-bockaudio

isn't that the hot rod mod?  hehe

Ben Sneesby www.beesneezmicrophones.com.au ben@beesneezmicrophones.com.au All Australian made microphones like you've NEVER heard before!!

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radiovinheta

Silverado

Posts: 86 Member Since:05/02/2011

#6 [url]

Jun 6 12 5:02 PM


There's any trick to ressurect vf14 tubes? I have a u47 Telefunken Badge vintage mic. I'll buy cable and psu and will test the vf14m. But it if fails, can i try any trick to make this tube works again?

Luis Fernando - Brazil

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oliver archut

Gold Finger

Posts: 566 Member Since:14/01/2011

#9 [url]

Jun 20 12 9:17 AM

Depending on primary (the voltage gets higher) or secondary shorting (the voltage gets lower) the x-former might work for quite a while after getting compromised and destroying the electronics.

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kats

Platinum Blonde

Posts: 1,098 Member Since:22/01/2011

#10 [url]

Jun 22 12 9:23 AM

Checking the voltage at the heater as Klaus suggests is the smart way to do it. 105v at the psu may not be optimum for an aging mic. In the case of my mic I needed 107v at the PSU for a solid 36vdc at the heater.

Tony K. [url]empirerecording.ca[/url] Entertainment is a bore, communication is where it's at! - Brian Jones 1967

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meverylame

Tin Man

Posts: 48 Member Since:04/02/2011

#11 [url]

Aug 14 12 4:16 PM

After reading this I took a gander at the modded peluso power supply that was being used with the u47 at a studio I'm working at and was a little shocked to find the power supply output was hanging around a solid 305 volts. This is clearly not right.. I opened the supply and found a really shoddy mod where someone had previously modded the psu by removing a pair of zener diodes right at the output of the power supply, I suppose they are to act as regulators. I am putting in a digi key order to replace a couple of bad components, and also getting some appropriate zeners to regulate the power to 110 and then place a trimmer after to be able to tweak in the voltage. Any thing I should be wary of?

Jason Kingsland

www.JasonKingsland.com

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bockaudio

Platinum Blonde

Posts: 1,516 Member Since:02/02/2011

#12 [url]

Aug 14 12 9:00 PM

After reading this I took a gander at the modded peluso power supply that was being used with the u47 at a studio I'm working at and was a little shocked to find the power supply output was hanging around a solid 305 volts. This is clearly not right.. I opened the supply and found a really shoddy mod where someone had previously modded the psu by removing a pair of zener diodes right at the output of the power supply, I suppose they are to act as regulators. I am putting in a digi key order to replace a couple of bad components, and also getting some appropriate zeners to regulate the power to 110 and then place a trimmer after to be able to tweak in the voltage. Any thing I should be wary of?

-meverylame


300v sounds like the mic is not connected to the psu- please correct me if I am wrong.
Though zeners can be good regulators, they were not part of the original u47 psu, a simple RC network will suffice. If you wanted to use the zeners as absolute overvoltage protectors, it's not a bad idea. So if your target is 105v with simple RC network, the zeners set for 120v will only conduct under an overvoltage (failure) condition. But this was not in the original U47.
More IMPORTANTLY: if this supply is feeding a standard VF14 U47 heater arrangement the zeners would need to be very high wattage, so they don't burn out or short. I can't do the math now but 5W won't cut it.

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studiochap

Gold Finger

Posts: 254 Member Since:05/02/2011

#13 [url]

Apr 18 14 9:59 AM

Here's another contributing factor for over-voltage in tube 47s and 48s in the UK...Many of the supplies have been left at the stock European 220v without any modification. Although the UK is nominally now at 230v to match Europe, there is a permitted tolerance, so we are usually at 240v ( just measured an outlet in my kitchen at 240v bang on...).
Hence there's almost 10% over-voltage from the AC power straightaway, before we dial-in shorted trafo windings.

Gwyn Mathias Mixer Madness in one form or another since 1972... Happy Sequoia user!

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ampman1961

Silverado

Posts: 90 Member Since:29/01/2011

#14 [url]

Aug 19 14 3:21 PM

There are two ways to do this safely, and here's why. Our mains voltages have crept up over the years since the norm being 110-115 back in the 50's,
to today, where our typical mains voltage can be as high as 130 volts. Not good. Seeing that maintaining proper heater voltage is of vital importance,
I would use a filament transformer voltage bucking method to reduce the secondary voltage of the mains transformer. Being careful to observe both
plate voltage and filament voltage. The other way is to use a Variable transformer set so that the filament voltage is right on target, then compare the
B+ reading. Under load, that B+ should be good unless there is already a problem with the tube, or power supply.
David, a couple of questions for you: What is the usual no load resting B+ on the 47 supply? And, is there already good filament voltage regulation
in a healthy U-47 supply? It's been a few years since I repaired one so I am bit out of touch with that. I have several clients who own and
operate late 50's Vox AC-15's & 30's that exhibit this same problem. The post war spares being used to build that equipment wasn't designed
to operate at 120-130 volts, and they simply cook away. On one amplifier, I measured the filament voltage at almost 7.5 volts on 6 volt tube.
Cured the problem with a Variac. This also cured the over voltage problem on the rectifier tube filament which uses 5 volts (5AR4, AC-30). This
correction caused the amp to run cooler and not chew through tubes. This is a corrective measure aimed reducing voltage for protection,
not to induce voltage starvation for effect. Different gear, same result.

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bockaudio

Platinum Blonde

Posts: 1,516 Member Since:02/02/2011

#15 [url]

Aug 21 14 11:18 PM

The heater and B+ are from the same source on a U47, there is no separate B+ and heater, except in a few cases where users modified the mics to be so.
Fixing the voltage on a U47 psu is usually as simple as changing the value of one of the 1k 6W resistors, variacs and external transformers aren't necessary.
Sometimes those old transformers need to be replaced, but rarely, in my experience.

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bockaudio

Platinum Blonde

Posts: 1,516 Member Since:02/02/2011

#16 [url]

Aug 28 14 1:07 PM

I should add, that there are aftermarket regulated supplies available for U47's, but their topology is always so different from the original Neumann design, there is always a radical sonic change in the mic when using them.

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