radardoug
Well-known member
You have no grid resistor, wont work at all!
Sony C-37A questions
- Thread starter dukasound
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radardoug said:
The Altec M11 "coke bottle" doesn't have a grid resistor, and it works just fine. I'm using a similar principle here.
radardoug
Well-known member
Ah well, you know so much more than me, I've only been mending mikes for 50 years. Do you want to put money on it?
radardoug
Well-known member
The M11 uses an inbuilt resistance of 10,000 megohms, according to the manual. So although no resistance is shown, there is a resistance there.
I haven't been building mics for nearly 50 years, but this design has been around for at least that long. It uses grid leak bias. Check out David Royer's SDC mod (kind of hard to find, but you can see it here: https://web.archive.org/web/20080721215458/http://www.diyfactory.com/projects/royerproject/royermod_2.pdf). Just like in the M11, the "inbuilt" resistance is the resistance between the elements of the tube itself...no additional resistor needed.
radardoug
Well-known member
Relying on undefined leakage resistance for the grid resistor is not likely to be satisfactory. In addition, it is the operation of a tube way out of its comfort zone with high value grid resistors that causes more noise in the circuit. So relying on a resistor that isn't even a resistor will end in tears.
radardoug said:
This is well-tread territory. Here's yet another mic (Sela T12) that operates this way:
If that's not enough, here's a discussion from this very forum on the topic where folks start off skeptical and then come around:
https://groupdiy.com/index.php?topic=62601.0
The Royer mod ("Royer mod 2") has been done by dozens of people who are happily using their mics with no tears.
To be clear, all I did was make the PSU regulated so it could be used with multiple mics, and ask a question about where to locate the bias resistor. The gird leak biasing has been thoroughly vetted by others for some time.
Sony really seemed to like grid leak bias inputs back in the day. I was asked to modify an SRA-2 (consumer reel to reel recorder electronics unit) a while back for use as a mic preamp and direct interface. That unit used grid leak bias. I ended up modifying it to use good ol cathode bias as the grid leak config they had in there as stock overloaded too easily with modern mics, even without a step up input transformer in front of it.
Irrelevant for this discussion. It’s just interesting to me that Sony seemed to like that bias scheme for these applications for a time.
BT
Irrelevant for this discussion. It’s just interesting to me that Sony seemed to like that bias scheme for these applications for a time.
BT
contact bias
https://www.w8ji.com/fusing_and_floating_grids.htm
https://www.w8ji.com/fusing_and_floating_grids.htm
Thanks Gus! That page has a lot of good info on it. It makes sense that this is distinct from "standard" grid leak bias.
Any insight on my initial question (post #38)?
Any insight on my initial question (post #38)?
radardoug
Well-known member
I think you will still find your schematic will not work. You are trying to build a cathode follower type output. In this, the cathode will sit at about half the B+. Even if you did generate grid bias, it would be a volt at max, and so you wont have the cathode at half of B+. Why dont you build it and see?
radardoug said:
I plan to build it, I'd just like an answer to my original question first.
Again, this circuit has proven to work time and time again. I didn't come up with it. Here's David Royer's schematic:
Here's a discussion of yet another mic, the Thompson Standford-Omega that also works this way.
https://groupdiy.com/index.php?topic=50750.0
Both of them are cathode followers using contact bias. There is no question that this circuit works.
What I'm asking is if there will be a difference in noise if I house the 39K bias resistor in the PSU vs in the mic itself.
I like the way Bruel and Kjaer do the cathode follower , basically all you have in the mic is the capsule , grid resistor ,coupling cap and polariastion resistors , the feedback loop of the cathode follower encompases the cable so output isnt affected by cable capacitance as much , they quote 200khz as the bandwidth of the cathode follower part of the circuit , of course that figure will be limited further by the capsule dimensions also . The B&K also doesnt feed a transformer direct from the cathode follower , you can either take output direct from the cathode at the psu end into high Z or it goes to a single transistor stage and through a balance transformer .
I have heard some complain about Sony tube mics saying the very high grid resistance (or complete lack of it in the C37a) means you really have to work on selecting a tube , others seem to think even the lifespan of the tube itself might be adversly affected by how its biased .
Im a big fan of the articles Dave Royer wrote on converting tube mics , it provided lots of inspiration to me to get into tube mic building , it didnt cost much and brought a big improvement over transistor to me ears .
I have heard some complain about Sony tube mics saying the very high grid resistance (or complete lack of it in the C37a) means you really have to work on selecting a tube , others seem to think even the lifespan of the tube itself might be adversly affected by how its biased .
Im a big fan of the articles Dave Royer wrote on converting tube mics , it provided lots of inspiration to me to get into tube mic building , it didnt cost much and brought a big improvement over transistor to me ears .
Recording Engineer
Well-known member
Funny you mention this... 5840 works great in my C37a type mics that Chris at Barbaric built for me! And isn’t it David Royer’s favorite choice for his designs?
All those sub mini tubes are really high quality , 5899 is whats used in the B&K , the small size is a bonus in mic building too , means it even easy to fit to a pencil type mic ,
I kinda fancy the idea of building one similar to the B&K circuit ,but instead adding a pre output stage built into the PSu , matching the Cf into hi Z seems to allow it handle bigger swings with less distortion , the B& K can drive 20 volts into high impedence with around 1% distortion ,but as you load it down % dist rises and output voltage drops a bit .
I kinda fancy the idea of building one similar to the B&K circuit ,but instead adding a pre output stage built into the PSu , matching the Cf into hi Z seems to allow it handle bigger swings with less distortion , the B& K can drive 20 volts into high impedence with around 1% distortion ,but as you load it down % dist rises and output voltage drops a bit .
