Al's pseudo-C800/G7 mic idea...

Hey gang,

So I got these chinese capsules gathering dust here, and decided to try them out in this circuit:



The gain section comes straight out of the C800 schematic that's been floating around, but the capsule biasing arrangement looks more like the G7's, except I've implemented a continiously variable pattern pot...

I'm planning on using a NOS 6AU6WA, since I got five of them floating around as well.

As for transformers, I'll try the 6.45:1 OEP first... When I get some cash, I'm thinking of getting a Lundahl and a Cinemag and popping them in, but that's still a long way from where I'm at right now. I'll build the basic circuit first, then worry about trying out different transformers. I also have to wait for the 1G groupbuy resistors to show up...

So what do you all think? Comments, advice, mistakes?

Thanks!

Peace,
Al.

do NOT use the OEP A262A1E step-down trafo - it hasn't the needed primary inductance.

Use OEP A262A3E in reverse instead...

Jakob E.

[quote author="gyraf"]Use OEP A262A3E in reverse instead...[/quote]

Yeah, that's the one I'm using :thumb:.

I have a mic body waiting for electronics... I'm itchin'! :green:

Peace,
Al.

The nos 6au6 stuff did not have good noise levels if you want to try them run the fil at 6 to 6.3VDC I have tried a few., the 800 thread has some hints.

Look at the vm1 thread for a better capsule pattern control circuit.

Look at the square for an even better capsule pattern control circuit.

I used a ll7903 look up the spec and SIZE. AMI and cinemag at > 7:1 and < 10:1 should be fine.

If you look close alot of microphones are the same basic designs

[quote author="Gus"]The nos 6au6 stuff did not have good noise levels if you want to try them run the fil at 6 to 6.3VDC[/quote]

Hmmm... The C800 schematic specifies 5.7V for the heater (lower noise?). I tried burning the crap off the cathode of two of these GE 6AU6WA and tested for noise, and in one of them the noise actually dropped a little bit (-2dB). The other I don't know, because it was already low enough that the tube's noise was getting swamped by my computer's noise... What brand 6AU6 did you use that were so noisy, Gus?

[quote author="Gus"]Look at the square for an even better capsule pattern control circuit.[/quote]

Interesting! What makes it better, exactly? I'm not sure I understand the DPSTswitch in the square mic that's labeled "cardioid only", could you please explain? I like the fact that this arrangement does away with the coupling cap between the capsule and the grid...

[quote author="Gus"]I used a ll7903 look up the spec and SIZE. AMI and cinemag at > 7:1 and < 10:1 should be fine.[/quote]

Yeah, I was looking at a couple of the Cinemags. I'll see what AMI/TAB have there... Thanks!

Peace,
Al.

I tried a mullard and a bunch of GEs. The noise was not bad with the nos it was that the right tube has even less noise. It is not a real 6au6

I believe it is in the oxides

its all in the threads.

Some of your cap values are to high it will take a long time for the pattern to change.

[quote author="Gus"]I tried a mullard and a bunch of GEs. The noise was not bad with the nos it was that the right tube has even less noise. It is not a real 6au6[/quote]

Hmmm... I'll keep an eye out for the Russian tube. I just haven't seen it for sale! :?

[quote author="Gus"]I believe it is in the oxides[/quote]

You're probably right.

[quote author="Gus"]Some of your cap values are to high it will take a long time for the pattern to change.[/quote]

Absolutely. The 10µF caps should be 1µF. Pattern change should happen in about .3 seconds.

Peace,
Al.

By the way, if anyone knows where I can get the Russian (6J4P?) tube, I'd appreciate it. Thanks!

Peace,
Al.

[quote author="alk509"][quote author="Gus"]I tried a mullard and a bunch of GEs. The noise was not bad with the nos it was that the right tube has even less noise. It is not a real 6au6[/quote]

Hmmm... I'll keep an eye out for the Russian tube. I just haven't seen it for sale! :?
[/quote]

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=14947&item=3864739144&rd=1&ssPageName=WD1V

[quote author="Marik"]http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=14947&item=3864739144&rd=1&ssPageName=WD1V[/quote]

Ha! eBay! The one place I, for some reason, neglected to look !

I just ordered 8 of these puppies. Thanks a lot Marik, you're a legend! :thumb:

Peace,
Al.

Still toying with this, here's a power supply:



...And here's circuit boards for both the mic and the PSU:





I still haven't tried any of this, so don't jump in and expect it to work... I'll keep you all posted.

Peace,
Al.

I don't like the power supply, unneeded complexity IMO, a 4 or 5 stage RC might be better. Remember its a tube circuit.

I am not a fan of back to back transformers you should be able to find an EI one if you look around.

Use a bigger output cap on the fil supply even some of the china stuff has >1000uf

compare uf400xs to 1n400xs price and specs.

Perhaps I should mention that some of my parts choices are based on what I have on hand... With that in mind:

[quote author="Gus"]I don't like the power supply, unneeded complexity IMO, a 4 or 5 stage RC might be better. Remember its a tube circuit.[/quote]

I don't like unregulated supplies on mics, tube or solid state... call me crazy! :green:

I'm looking at the PSUs, and there's really nothing I would change (except maybe the transformer arrangement - see next point). How exactly is the PSU complex? It's the basic circuit lifted almost verbatim from the regulators' datasheets! :?

[quote author="Gus"] I am not a fan of back to back transformers you should be able to find an EI one if you look around.[/quote]

Yeah, that has me worried... I still have to build it and let it run for a couple of hours to see what kind of heat buildup I get, but if that's not a problem, it will work 100% fine, AND with the advantages of toroids, AND for cheaper than an equivalent E core.

[quote author="Gus"]Use a bigger output cap on the fil supply even some of the china stuff has >1000uf[/quote]

Not necessary. The LM317 gets 80dB of ripple rejection with the 10uF cap from the adjust pin to ground. Pulling .35A out of the 1000uF cap, I get 2.9Vp-p of ripple, before the regulator... That's like 2.9E-40 (two-point-nine times ten-to-the-minus-forty!) Vp-p at the output of the regulator... For a heater supply, I'd say that's pretty damn close to perfect! (certainly much better than a sectioned RC filter of any sensible size).

[quote author="Gus"]compare uf400xs to 1n400xs price and specs.[/quote]

Cool, I'll look it up. Thanks, Gus! :thumb:

Peace,
Al.

If you look you might find the transformer I use for my tube microphones <$20.00 USA.

I was just giving hints, what do I know about tube microphones anyway?

[quote author="alk509"]

I don't like unregulated supplies on mics, tube or solid state... call me crazy! :green:
[/quote]

Al, officially--YOU ARE CRAZY! :grin:

Seriously, I don't see the reason of building tube mic with B+ regulated, unless you want to sound it like a... solid state. At least build both--regulated and unregulated, and then compare them. You will see what I mean.

For B+ rectifier I'd use Hexfreds, available from Kevin Carter, or Antique Supply--they are much more expensive, but sound much better than 4007s.

[quote author="Gus"]If you look you might find the transformer I use for my tube microphones <$20.00 USA.[/quote]

That's cool, but I don't have one. Again, I'm using what i have on hand...

I actually have an E core transformer that would probably work, but I want to try out the back-to-back trick that a lot of people here use.

[quote author="Gus"]I was just giving hints, what do I know about tube microphones anyway?[/quote]

I'm not quite sure what to make of this... Did I offend you somehow? Please exlain.

Thanks!

Peace,
Al.

[quote author="Marik"]Al, officially--YOU ARE CRAZY! :grin:

Seriously, I don't see the reason of building tube mic with B+ regulated, unless you want to sound it like a... solid state.[/quote]

See, I'm a little conflicted with this, precisely because of what you say about SS-vs.-tubes. But the thing is I just LOVE these TL783 ICs. I've replaced the RC power supplies on three different boxes (an LA2A-type compressor, and two mic pres), and the sound just comes alive!

Having said that, now that I think about it I can think of at least two mics that I absolutely love that have unregulated supplies... :?

[quote author="Marik"] At least build both--regulated and unregulated, and then compare them. You will see what I mean.[/quote]

That sounds like a sensible solution! :grin:

[quote author="Marik"]For B+ rectifier I'd use Hexfreds, available from Kevin Carter, or Antique Supply--they are much more expensive, but sound much better than 4007s.[/quote]

Yeah, I've heard about those... Are those the ones Gus recommended in his post? I don't have any, but I'll look into them and maybe order some the next time I get parts. Thanks!

Peace,
Al.

OK, I just checked both the FREDs and the UF400x diodes... I'm definitely getting some next time I order stuff!

FWIW, Digi-Key actually has some FREDs, but they're kind of expensive compared to AES and others

Thanks again, guys! :thumb:

Peace,
Al.

Without wading into the reg vs unreg debate, I have to point out that Al's slide rule slipped here: "Not necessary. The LM317 gets 80dB of ripple rejection with the 10uF cap from the adjust pin to ground. Pulling .35A out of the 1000uF cap, I get 2.9Vp-p of ripple, before the regulator... That's like 2.9E-40 (two-point-nine times ten-to-the-minus-forty!) Vp-p at the output of the regulator... For a heater supply, I'd say that's pretty damn close to perfect! (certainly much better than a sectioned RC filter of any sensible size). "

80dB of ripple reduction is a factor in voltage of ten to the 4, not 40, so you will see 290uV p-p by that reckoning. Still good.

Another handy trick is to bias the filament a bit positive with respect to the cathode to reduce coupling.

As far as ideal diodes, the SiC ones are expensive, high voltage drop, but have unmeasureable reverse recovery times. I wish they would make some little ones too. Pease points out that small-signal schottkies have a bit of a conventional junction in their structure which makes their behavior a lot less ideal.

[quote author="bcarso"]290uV p-p[/quote]

Yeah, I just saw that... Goddamn non-RPN calculator messes with my mojo!

Thanks, Brad! :thumb:

Peace,
Al.