under or over powering tube mics

[klem]

In 101 terms, what are the consequences of running a tube mic at the low or high end of recommended voltages?
Let's say an M49 circuit:
Heater 6 to 12V
B+ 100 to 120V
I imagine shortened lifespan if heater is elevated, but how about the sonic impacts? Is there a change in how they respond to transients, or amount of headroom in the circuit? (Drums are always at front of mind for every build for me.)

I didn't find the topic though it's surely been covered here... welcome links or pdfs that directly address this. Thanks

 

[rock soderstrom]

In 101 terms, what are the consequences of running a tube mic at the low or high end of recommended voltages?
Let's say an M49 circuit:
Heater 6 to 12V
B+ 100 to 120V

As far as I know, M49 microphones first used MSC2 and then AC701 tubes, both of which do not work with 6.3V but with 3.8V and 4V (+/-0.2V) respectively.

Even if you operate an M49 "clone" with e.g. a 5840 like O.Archut did, doubling the heating voltage from 6V to 12V would extremely shorten the lifetime, absolutely not recommended! There is also no good reason for small increases in the heating voltage outside the tolerances. You mentioned it yourself, shortening the life time for no appreciable improvement. The designers of the tubes knew what they were doing.

Most M49 schematics I have seen have a B+ of about 120V. Slight changes to this voltage, even from 20V, make almost no difference in my experience. You should not lose sight of the polarisation voltage obtained from B+ in larger experiments, otherwise you can experiment with it. More B+ results in more headroom of the mics impedance converter. In my experiments I found the other direction, i.e. less voltage, soundwise more interesting. YMMV.

 

[Tubetec]

In certain cases you can underrun the heaters a bit , but you wouldnt intentionally over volt them ever as far as I can see , if you do change it the characteristics of the tube change a bit from the datasheet .

The Ht voltage isnt all that critical , although increasing it might give slightly more headroom the higher you go might also have other effects on the longer term stabillity and life of the tube ,
Many tube mics have around 70 volts at the anode , in that case you may undervolt the heaters a bit without detriment .You'll probably notice a shift of the anode voltage as you adjust the heater supply ,

 

[klem]

More B+ results in more headroom of the mics impedance converter. In my experiments I found the other direction, i.e. less voltage, soundwise more interesting. YMMV.

Could you elaborate on this, "soundwise more interesting"?

For context, my question stems out of looking for a single toroidal that will output the V listed in the OP, to be used in a circuit that is similar to M49 (so I'm told... unfortunately I was asked to keep the schematic private, but that's peripheral to the more general idea anyhow...) Say there was 'off the shelf' tx that had two secondaries for H and B, and one model had 100V for B+, another 120V, another 140V... That was what got my gears turning about the potential consequences of going either under or over the expected range. So do tell of your experiences and impressions of mic performance at various V! Cheers

 

[rock soderstrom]

Could you elaborate on this, "soundwise more interesting"?

For my application, voice and acoustic guitar, I usually found less headroom more suitable, as more harmonic distortion occurred. ("tooby" sound") However, this is purely a matter of taste and this can be completely different when recording loud sources (drums etc...) or other styles of music. Again YMMV.

You don't have to decide on a fixed B+, you can solve this more flexibly.

One of my power supplies can cover a wide range of B+ voltages. As I said before, pay attention to your polarization voltage with such experiments.

 

[Delta Sigma]

I think I used 5.9V for 5840. Can't remember exactly. Heater should be at or under what it's spec'd for. I think I did 6.1V for 6072 and EF86.

 

[Delta Sigma]

Are you looking to find an ideal tube mic transformer?

It would be nice to find a suitable toroid with dual outputs for 6V H+ and 105-120V B+ supplies.

 

[klem]

Yes, but the reason for this thread was my wondering about the effects of 'out of spec' values, i.e., see post #4 above.

But, since you bring up ideal transformer, I've found a couple that may work for the mic

https://www.antekinc.com/as-05t120-50va-120v-transformer/(overkill i'm told)

or
AN-0206 - 25VA 6V Transformer (or AN-0210), using one primary for the B+. That would halve the amperage for secondaries, which I *think* is still ok for this application, but also, using a supply that doesn't go through the transformer for B+, are there safety issues with that, or just general more potential damage to the PSU PCB components since it's getting whatever is coming from wall outlet?

 

[Delta Sigma]

That's a nice transformer, but it might be a bit big for available cases.

The heater output is low for the passive designs out there. Could likely still do a regulated H+ from 6.3V.