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
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