Tube mic preamp/mixer

[Sir Trefor]

This is a schematic of a circuit in RCA's "Receiving Tube Manuel" from 1959- "Low Distortion input amplifier stage"----
would it be suitable for a microphone preamp?

7199 characteristics

7199 (continued)

 

[Geek]

You'll need more gain if you use a dynamic mike. Try a 470k resistor in the pentode plate and adjust the cathode resistor accordingly for bias.

 

[SY]

Unavailable and pretty noisy.

 

[Geek]

She's not unobtanium 😉

http://shop.store.yahoo.com/thetubestore/sovtek7199.html

 

[SY]

But she's still a noisy (for this app) pentode.

Thanks for the correction- owners of stock ST70s will be delighted.

 

[fdegrove]

Hi,


..........And why use a triode/penthode where a single LOW noise penthode would do nicely already?

Isn't there a circuit using the 5879 in one of the RCA Receiving Tube Manuals designed for micpreamp use.......?

Or an EF86/6267?
At least that one's still around in abundance and likely to stay with us for another twenty years.
Plenty of low level circuits available to pick from as well.

No idea about the US but in Europe at least there are still some absolutely amazing small signal penthodes around that are hardly used by anyone anymore.
Quite a few of those could be just the ticket you need.

Cheers,😉

 

[Sir Trefor]

other circuits?

I can either try Geek's Idea(which I don't have a problem with), or go with fdegrove's idea, but there are no other circuits like this in my manuel.

Does anyone in this thread hav a schematic of a circuit that would work better? A single pentode?... What about phantom power?

Thanks

 

[mcs]

There's a good mic preamp here: http://gyraf.dk/gy_pd/g9/g9pd.htm

But it's not exactly simple and cheap...

Best regards,

Mikkel C. Simonsen

 

[Geek]

Phantom power injector 😀

http://www.diyaudio.com/forums/attachment.php?s=&postid=472164

 

[Sir Trefor]

Thanks Geek!

Cool! Phantom power 😀!

-Trevor

PS- I was thinking...could I replace the one 7199 tube in my circuit with 2 tubes (a triode and a pentode) that would provide similar gain characteristics? Before I even posted this thread I had heard unfavorable comments about the 7199...

 

[Geek]

Less tubes = less noise. A dual triode will be less noisy than a triode/pentode and a single pentode (like a 6AU6) can be less noisy than a dual triode......

 

[Sir Trefor]

how about...

what about a 12AX7?

-Trevor

PS- Geek, is the phantom power circuit you posted supposed to be it's own console, seperate from the preamp? And I assume that I am reading the schematic from left to right---- with the input being the jack on the left side of the drawing, and the right being the output. correct?

 

[Geek]

Re: how about...

what about a 12AX7?

12AX7 is an excellent tube. Using both sections will give you plenty of gain, but without NFB, will be noisier than a single high-gain pentode.

PS- Geek, is the phantom power circuit you posted supposed to be it's own console, seperate from the preamp? And I assume that I am reading the schematic from left to right---- with the input being the jack on the left side of the drawing, and the right being the output. correct?

You can place it anywhere you want. In the preamp or mahe an inline coupler from a plumbing pipe and some connectors.

Yes, I forgot to add that in the circuit, the DC blocked side to the preamp, the mic to the other. Reverse it and your preamp frontend may go POOF!

 

[fdegrove]

Hi,

Using both sections will give you plenty of gain, but without NFB, will be noisier than a single high-gain pentode.

One of the most amazing small penthodes I know of is the D3a.
Even trioded it still has a mu of 77 and a Req of 65 Ohm.

I have no idea it's ever been used in a mic pre but I'm sure it could.

Cheers,😉

 

[Geek]

Hey Frank,

Sounds like a candidate for a phantom powered, in-mic pre, eh? 🙂

 

[fdegrove]

Hi,

Sounds like a candidate for a phantom powered, in-mic pre, eh?

Ah...Guess you had a look at the datasheet.

Among other apps, yeah...Would be great.

You can still find original Siemens NOS for very reasonable money.

Cheers,😉

 

[moamps]

Phantom power injector 😀

Hi,
Standardized value for polarised resistors in balanced phantom power supply is 6k8 (not 10k). Manufacturers design mic's electronic using this value. Also, serial caps are to small; input mic preamp's impedance is few kohms. Usually, serial caps are good quality elcos 47uF/63V blocked with small (100nF) MKP.
Regards
Milan

 

[Geek]

Indeed, you are correct!

I couldn't remember the exact values when I ran that off and mine is glued shut.

 

[Sir Trefor]

too bad...

I will now have to go back to the electronics store to purchase the nessasary parts; as I am already in the process of building this thing---- thanks for the warning; at least I didn't solder anything together yet!

-Trevor

PS- thanks, everyone, you've given me me great info on what tubes I should use--- but I need a circuit schematic to use the tubes in!

 

[Sir Trefor]

Hi,
Standardized value for polarised resistors in balanced phantom power supply is 6k8 (not 10k). Manufacturers design mic's electronic using this value. Also, serial caps are to small; input mic preamp's impedance is few kohms. Usually, serial caps are good quality elcos 47uF/63V blocked with small (100nF) MKP.
Regards
Milan

Do you mean 6 to 8k..., or... 6k?
which caps are you referring to? (what are the serial caps)? And, when you refer to "blocking" the serial caps with smaller caps, does that just mean to connect the 2 caps in series?