AKG C60 PSU questions (N60A) and tube mic PSU's in general

I've got four AKG C60's with PSU's, and I decided to run some tests on the PSU's last night and noticed some things and wanted to ask some questions to those of you that know more about tube mic PSU's and particularly these old AKG PSU's. For those that don't know, the C60 uses the AC701(K) which is a very well known and expensive tube used in the ELA M 251 and 250 "euro" release, the Schoeps 221 series, as well as the Neumann M-series tube mics.

I measured the heater and plate voltages on the PSU and got an initial set of voltage results. The gist of it was that the heater voltage stayed pretty consistent on three of the PSU's over time (pretty much between 29-30 VDC). On one of the PSU's it was around 23.55 volts.

Should I be alarmed at that variation between those PSU's? The mic sounds fine when used with the PSU with the 23.55 voltage measurement.

Now the plate voltages were the strange story and the main cause for my questions. I tested the plate voltages and initially got something around 130VDC measurement, but I noticed that the voltage was steadily and slowly rising, even a few minutes after powering up. So I decided to wait a bit and let it stabilize. I tried it after about 10-15 minutes or so, and the voltage was up around say 135-136VDC, yet still climbing. So I thought, "Hmmmmmmmmmm... Maybe I'll leave these on overnight and measure in the morning". So I did that, and the voltages had finally stabilized (i.e. no more rising voltage). Here were the measurements that I ended with. The numbers 2 and 4 reflect the pin numbers on the Tuchel connector. 2 is plate, 4 is heater filament voltage:


PSU 1
2. 149.6
4. 23.57

PSU 2
2. 140.5
4. 30.03

PSU 3
2. 142.4
4. 28.75

PSU 4
2. 143.3
4. 29.15

My main questions are just about the tolerance level of a typical AC701k and tube mics in general and whether this sort of variation is normal, and if it is OK. What are the real world effects of having these variances in voltage as compared to performance from mic to mic (i.e. will one have higher output than the other or different response because of this variance)???

Also, is it normal for vintage mic PSU's to fluctuate that much over time? 7-10VDC on the plate voltage seems like a lot over the course of at least 20 minutes (was more like 8 hours, but I don't know at what time point it actually stabilized). Should I be concerned? Do I need to let these PSU's "warm up" for that long before I use the mics?


Cheers,

T

:shock: your heater voltages seems very strange....the C60A schematic shows 4V for the heater - also AC701 datasheet says 4V @100mA .

i recently bought one without PSU and did just that ,and it worked fine for me....so..... :shock:

are the values given with the mic connected?

No the mics were not connected- purposely too. I didn't feel like poking around inside the mic with a probe when the tube costs around $300-$500. I'd rather not do that unless I must.

Yes you are correct mich about the ratings on the C60 and the heater voltage. I saw that on the C60 spec sheet before I even measured anything. At this point, I'm more concerned at the differences in the voltage readings between the PSU's rather than the actual heater voltage with the mic attached. Do I have to attach the mic and measure in order to trust the differences in these measurements?

T

You should test them under load. Make a load using resistors

http://www.radiomuseum.org/tubes/tube_ac701.html

heater current of .1 amp at 4VDC

4VDC / .1amp = 40 ohms for the heater load

DC power = voltage x current = 4VDC x .1 amps = .4 watt

Do you have a link for the schematic of your C60 so we can calculate the resistor needed as a load. Looking at the C60 PDF I have it does not have voltages marked at the microphone section I would guess 60VDC at the capsule across 82K about .7ma maybe try a 150K load resistor for testing

Here is the schematic:

http://www.vintageaudiogearbox.com/ebaypics/AKG_C60-N60Schematic.pdf

T

One thing I learnt at great expense with my C60 was that the AC701K is not at all tolerant to over voltage. So be very careful if you're fiddling with the heater supply voltage.

Thanks for that notice Rob. I'll be very sure to be careful with my voltage measurement before I hookup the mic.

Thanks for the help and the lesson Gus. Ohms law to the rescue again! That schematic is there, but it's a bit much for me to calculate from looking at a schematic.

T