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[Chris Moore]

When I purchased my C37a, the original owner said he'd had it modified by Tracy Korby for lower noise. Not sure exactly what was done, but the noise floor on mine is comparable to most Neumann or AKG tube mics I've used. It also seems to have less noise than I recall from using other C37a's, but I've never directly compared mine to another C37.

[J.J. Blair]

Maybe it's the 3 micron diaphragm?  

Anyway, a friend called Sony and the C800 capsules are available for $590, each, which is about the price of a K47, no?

I just received some Sprague PP and foil caps in the mail today.  I'll replace the the coupling caps in the PSU and tell you guys if the noise gets any better.

[J.J. Blair]

So, I just tried the Sprague PP and foil caps.  The noise was better by 1.6dB.  I don't know how you quantify this in terms of noise weight, but going straight into an API 512b and then into the Apogee AD16Xs, I set the level so my voice was peaking at -5dB, with an RMS average of -25dB.  The noise was at -71.9dB.  If that tells you anything useful, I have no idea.

Going back to these .47

[Tim Campbell]

J.J.,
On another forum we found Russian 6au6's actually suited these mics best. You may have to audition a few of them to find some with the lowest noise but they're dirt cheap.

[Dale Ulan]

Thanks, Tim. I'll have to track a few down for my Altec's and the C37's and try them out.

[Syborg Studios]

Here are a couple pics of the capsule of my C-800....

[Syborg Studios]

And the back of the capsule....

[J.J. Blair]

Thanks, Scott!  It does indeed look like a direct replacement.  Maybe before I spent $1,200 on a pair of new capsules, I'll try to electronically balance the two mics by adding a capacitor in parallel to the coupling cap of the one whose bass response is shy.  I'll let you know what happens.  

[Gustav]

What is your heater voltage in the microphone?

[Silvertone]

Hi JJ,
Like I said when Scott and I shot these things out they sounded identical (except for the noise floor). Better to save that money if you can.. but at least you'll have a solution no matter what.
best,
Larry

[maxdimario]

With a cathode follower tube amp, the mic's transformer can be placed in the psu box, therefore reducing the size of the mic.I think this was their prime concern.

The cathode follower, as mentioned, makes it possible to do so because it lowers impedance electronically.

As Oliver mentioned, the cable makes a difference, because although a cathode follower has a lower impedance than normal for a tube circuit, it is actually subject to capacitance issues with aged cables, and interference.

The output transformer has a 10k resistor on the input winding, which means that it probably won't be subject to too much 60-cycle hum from the mains transformer.

If those are the original Electrolytic caps, you may try and place a new 10 or 20 uF cap in parallel to the last cap connected to the b+ power supply with alligator-clip connectors (mind the plus and minus if you don't want to blow up anything).

even though cathode-follower circuits are self-regulating, you may find a decrease in noise and a slight improvement in highs.

if you do, solder it in or go ahead and change the others, as they have probably never been replaced.

the E cap you place in parallel with the other will keep its charge for several hours if it's not discharged with a resistor or something so watch out! Don't touch the leads.

P.S.: Did you try to swap mics with psu's to see if the bass problem is in the mic or the psu? The psu holds all the coupling caps and output transformers, so if it's not the psu it has to be the capsule.

[J.J. Blair]

Max, if you go back and read the thread, the answer is yes.  I eliminated every possible variable, and concluded it is without a doubt the capsule.  Thanks.