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Variac on vintage power supplies Condenser Microphones
Old 2nd April 2015
Here for the gear

Variac on vintage power supplies

Is anyone using variacs for general day to day use on their vintage tube microphone power supplies?

I don't have regulated AC in my studio and I also tend to take gear to other locations. I had originally purchased a variac for use with some of my vintage Fenders that also travel. Those amps are usually biased at their original spec'd line voltage of 117V and whenever I do session with them I meter the variac and bring them up to their proper line feed.

I have some power supplies that I'm certain are regulated (A Hamptone on my U47 and newer German made one for my M269c), however I'm not really sure about the original supplies on my M49s or U67s.

Two of the primary questions I'm throwing out there is if I run these older non-regulated supplies at their originally spec'd voltage I'm not likely to cause harm am I? Should I also have all of them adjusted to reflect my intention to use them at their intended voltage as well (i.e. adjusting plate and bias voltages accordingly)- or is this a recipe for disaster on that day it gets plugged back into a regular outlet by mistake?

I'm just trying to treat these things with kid gloves they demand but I don't want to cause inadvertent harm trying to do so! Nobody ever told me owning something with VF14 and AC701 tubes would be a constant source of worry.....they just sound soooo good!

Dalton Brand
Waveburner Recording

Last edited by Waveburner32; 2nd April 2015 at 04:24 AM.. Reason: added signature
Old 2nd April 2015
Lives for gear

Im not an expert so I would suggest that you investigate farther.

That being said, as long as you are using the variac transformer to feed the appropriate line voltage to each respective unit, shouldn't be any problem. Actually it sounds like a good idea. You never know what kind of line voltage you will get at other places.

Before I finished reading your post I was thinking Van Halen and using a variac to purposefully feed low voltage to your vintage units. That would probably be a no no.
Old 2nd April 2015
Here for the gear

Just to be clear- the intention is NOT to run the line voltage at anything other than the original voltage for which the power supply was designed.
Old 2nd April 2015
Lives for gear

All gear is designed to work correctly and safely with a mains supply tolerance of (usually) plus and minus 10 percent. When the gear was originally made the tolerances were 'skewed' in some countries (+6, -10 percent IIRC) so unless you are encountering mains outside this specification then there is no real problem.
If you often encounter particularly high or low voltage then rather than a variac I would suggest getting an 'autotransformer' which would give you one or more 'tappings' so for a nominal 110 Volts in you can get 100, 110 and 120 Volts out (for example). Using a variac is open to greater 'misuse' and it is possible that some gear could be harmed if left with particularly low or high voltage for an extended period. Example if it were knocked to a low value before you go out for a lunch break.
The 'tolerance' for voltages within valve gear is often specified as 'within 20 percent' so anything closer to 'nominal' values is a bonus.
Depending on the 'vintage valve mic supplies' these often have their own internal regulation for the voltages that matter. Exact HT values are not necessarily that important but heater current is (within plus and minus 5 percent).
Matt S
Old 2nd April 2015
Lives for gear

Originally Posted by Matt Syson View Post
If you often encounter particularly high or low voltage then rather than a variac I would suggest getting an 'autotransformer'
Is this the same as a rack mount voltage regulator?
Old 2nd April 2015
Lives for gear

No, it is a transformer with essentially 1 'winding' but there are tapping points to give various voltages.
If you think of it as say a 130 Volt transformer which has connections at 110, 120 and the full 130 Volts.
If your mains is 120, you connect it to the 120 volt terminal then the other 2 terminals will give you 110 and 130 volts, which you then connect to your gear as needed. If you go to a place where the mains is 110, you go in on the 110 connection and the other 2 connections remain at 120 and 130 Volts.
The Variac is a similar idea but is far more expensive (for the power rating) and probably provides more 'variation' than you really need.
Matt S
Old 2nd April 2015
Lives for gear
Klaus's Avatar

The discussion of unregulated power supplies for tube mics should include a couple of core issues that need to be addressed.

1. Tube mic supplies of the 1950s and 1960s were designed with a then common line voltage of 220/110VAC, as basis to set up specific DC supply voltages required by each mic model. That line voltage has been chanced since to 230/115VAC nominally, and in many areas in the U.S. the line voltage now hovers around 120VAC. That is the AC voltage I use to set up tube mic supplies, unless I am given differing instructions.

2. As a conclusion of no. 1 above, and due to power supply DC-output voltage drift over time, ALL power supply voltages should be calibrated to comply with the nominal voltages the microphone manufacturers prescribe. For example, all Telefunken AC701-equipped mics need 120VDC B+ and maximum of 4VDC heater supply voltage. (Examples: A 10% over-supply of the heater voltage cuts the AC701's tube life in half or worse. So I aim for staying within 2% of the nominal voltages. A 10% under-supply of B+ voltage can in some mic models result in a 2-3dB noise floor reduction, due to insufficient polarization voltage to the capsule.)

Because some microphone types are highly sensitive to even modest over- or under-supply of voltages, I recommend the following process, regardless of whether regulated or unregulated AC-power gets to the supply:

a. measure the AC voltage on the wall socket(s) most likely to be used for the power supply, and at times of the day when the mic is most likely to be used.

b. using an average AC voltage from (a.), calibrate the power supply's DC voltages for the specific mic, then label and assign the power supply for that mic with the specific tube installed (no mixing or substitutions of mics, tubes and power supplies, once a supply's voltage has been mated to a specific mic with a specific tube!)

c. if you have the money, buy a regulated AC supply and pre-set it to the AC voltage that you used to calibrate the mics, if that was already one earlier.

Last edited by Klaus; 5th April 2015 at 04:27 PM..
Old 7th May 2015
Lives for gear

Thanks for your reply Klaus. Informative as always.
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