Preamp?

[k7iou]

Anyone know what kind of preamp this is?

[KL7OF]

what is the tube?

[Pete, WA2CWA]

Stamped on the chassis 7025 (12AX7).  It's a phono-mic-tape preamp

[k7iou]

Do you think the output would be high enough for "Line" In on a Shure SR107 equalizer? I believe it's is 1A.

[Pete, WA2CWA]

I have no clue. Try it with and without it. Experimentation can be fun.

[W3RSW]

 I see only a phono/ mike switch. Look inside to see if a NARTB network really is modified in what was the mike circuit along with what most probably is an elementary, two stage RIAA phono feedback network.

Neat little preamp. Solder those rivets to chassis and reform or replace the filter caps. Is the rectifier a selenium rectifier requiring changeout, etc.?  Probably will have to recap the coupling, RIAA and bypass caps too.

I imagine the mike capabilities were originally for a crystal, high impedance, high output type if still there, unaltered.

[w1vtp]

I have something like that that I put in my entertainment console.  50's / 60's turntables used ceramic cartridges.  This preamp was used to put in the necessary gain / eq to make a magnetic cartridge  (Pickering, GE RP50) work in these systems.  Probably could bypass the EQ circuit, install a XLR socket or a 1/4" RTS socket and use it to juice up a low level mic for line output.

Would take a bit of work.  Would be easier to use a Beringer MIC100 that's already to use

2C. Al

[Opcom]

that is exactly what it is. lets one use a magnetic cartridge in place of a ceramic.

[W1AEX]

It looks functionally similar to a Lafayette phono preamp I used with my stereo system back in the 60's when I upgraded to a magnetic cartridge. The one you have looks like it may have pre-dated the stereo version I had but the idea is the same. I loved the look of all that chrome back then...

Rob W1AEX

[KA2DZT]

I agree, it's a magnetic cartridge preamp.  Probably of little use as a mike amp without mods or complete rebuild.

Fred

[W3GMS]

The RIAA equalization is quite easy to remove.  If when in the mic position you have any gain you may have nothing to do since the mic channel should be flat.  If you have a scope and signal generator you can tell in short order what needs to be done.  I did not look up the tube but have heard of the number before.  If its a dual triode similar to the 12AX7, AU and AT series you will have plenty of gain.  If your new to preamp design, look in the back of some of the RCA receiving tube manuals and they have a lot of circuits for preamp and such.   Not sure what your mic will be but if its a hi z crystal make sure the first grid resistor is around 5 meg or so. 

Its fun if you have the time to re-purpose things.  With a little work and a lot of fun I think you could easily accomplish what you want to do with it.

Joe, W3GMS   

[W3RSW]

***Long Live the GE VRII***

--The standard in Garrard changers for a quarter century.
Well it seemed like it back before we all hit dog year multipliers. 

[k7iou]

The microphone is an American D22 currently fed into a rad shack preamp and then into a Shure SR107 which I am using with my Ranger.
I need alittle more gain. I may pick up the preamp as he is asking $10 for it. But the MIC100 looks like it would do the trick too.
Decisions decisions......

[AB2EZ]

Make certain that you have the "linkage bars" behind the name plate correctly set to the high impedance position. It appears that the output voltage is 34dB higher in the high impedance position... and the high impedance position should be fine in conjunction with the preamp you are already using.

http://www.coutant.org/amerd22/

Specifications

•Frequency Response: From 100-8,000 cps, plus or minus 5.0 dB
•Output Level: −86 dB at low impedance. −52 dB at high impedance
(0 dB = 1 volt/dyne/cm²)
•Impedance: May be easily and quickly changed from low (30-50) or high (40,000) ohms impedance by removing name plate and changing linkage bars to impedance desired


Based on the above specification, and noting that 1 dyne/cm**2 = 0.1 pascal, one can compare the output of this microphone to an RE-27 (2.5mV/pascal), or an SM-58 (1.85mV/pascal).

This microphone has about 14dB less output voltage than those other microphones when in the low impedance setting. This microphone has about 20dB more output voltage than those other microphones, when in the high impedance setting.

Also, make sure that you are using the "line" output of your Shure equalizer. The "line" output's level is 27dB higher than the "auxiliary" output's level, and 50dB higher than the "microphone" (level) output's level.

http://cdn.shure.com/user_guide/upload/1602/us_pro_sr107_ug.pdf

The combination of a dynamic microphone, a plain vanilla microphone preamplifier, and the equalizer (which can provide 20dB of line input-to-line output gain) should work fine (more than enough output to drive a Ranger) if all of the pieces are configured correctly, and working as they were designed.

For example, an ART Tube MP is a preamplifier that provides (among other things): a high impedance (840k ohm) 1/4" TS (unbalanced) phone jack input; a low impedance, balanced XLR output or a 1/4" unbalanced TS phone jack output; and more than 50dB of voltage gain.  You can find the technical specifications in the owner's manual... which can be downloaded from the ART web site.

It costs $49.00 new at Sweetwater, with free shipping.

Stu

[k7iou]

Ok, I picked it up and would like to modify the 12AX7 tube preamp for use with the D22 dynamic mic & Shure SR107. The amp works but I need to control gain and sounds flat & muffled.
Ideas, design where to start? I don't have a RCA tube manual for preamp designs

[KD6VXI]

http://www.tubebooks.org/technical_books_online.htm

Go get what you need.   This website should be in any vintage radio buffs favorites.   He does his best to ensure no copyright issues will come up,  too.

--Shane
KD6VXI

[Pete, WA2CWA]

Ok, I picked it up and would like to modify the 12AX7 tube preamp for use with the D22 dynamic mic & Shure SR107. The amp works but I need to control gain and sounds flat & muffled.
Ideas, design where to start? I don't have a RCA tube manual for preamp designs

Replace the caps; check the tube; check the resistor values; replace the selenium rectifier with a silicon rectifier; replace the power cord with a 3-wire cord. I don't see a power switch; might want to add one.

12AX7 mono preamp circuits: https://www.google.com/search?q=12axt+mono+preamp&client=firefox-a&hs=6yO&rls=org.mozilla:en-US:official&channel=sb&source=lnms&tbm=isch&sa=X&ei=HeO9U7mlCJCXyAS264HYCQ&ved=0CAkQ_AUoAg&biw=1291&bih=828#channel=sb&q=12ax7+mono+preamp&rls=org.mozilla:en-US:official&tbm=isch&imgdii=_

[k7iou]

http://www.tubebooks.org/technical_books_online.htm

Go get what you need.   This website should be in any vintage radio buffs favorites.   He does his best to ensure no copyright issues will come up,  too.

--Shane
KD6VXI

Wow I was thirsty and you gave me a fire hose 
Allot of information here. I will continue looking but haven't found it yet.
73

[k7iou]

The RIAA equalization is quite easy to remove.  If when in the mic position you have any gain you may have nothing to do since the mic channel should be flat.  If you have a scope and signal generator you can tell in short order what needs to be done.  I did not look up the tube but have heard of the number before.  If its a dual triode similar to the 12AX7, AU and AT series you will have plenty of gain.  If your new to preamp design, look in the back of some of the RCA receiving tube manuals and they have a lot of circuits for preamp and such.   Not sure what your mic will be but if its a hi z crystal make sure the first grid resistor is around 5 meg or so.  

Its fun if you have the time to re-purpose things.  With a little work and a lot of fun I think you could easily accomplish what you want to do with it.

Joe, W3GMS  

Well I found this, http://diyaudioprojects.com/Schematics/7025-12AX7A-Tube-Phono-Preamplifier-Schematic.htm
I found this article. Now I need to figure out what part of the circuit the RIAA is.

[AB2EZ]

The RIAA equalization can be removed by removing C6, C7, and R10. Removing these components will not only remove the undesired RIAA equalization, but will also increase the voltage gain of the preamplifier (at 1000 Hz) by around 25dB.

http://en.wikipedia.org/wiki/RIAA_equalization

You should add a 10,000 ohm resistor across the "AF Output" terminals of the pre-amplifier to reduce the gain (to keep the preamplifier from becoming unstable... and breaking into oscillation)

With this pre-amplifier, modified as above,  providing the input to the Shure equalizer, the voltage gain provided by the pre-amplifier should be (much) more than enough, in conjunction with the available 20dB line input-to-line output voltage gain of the equalizer, to drive the Ranger's audio input.

Some details that you may not want to get into:

This pre-amplifier (before modification, as above) is designed to feed into a very high impedance load ("220,000 ohms minimum"). Your Shure equalizer's input impedance is designed to work in conjunction with a source whose output impedance is "10,000 ohms or less".

Adding the 10,000 ohm resistor across the "AF Output" terminals of the pre-amplifier (as described above) will make the pre-amplifier act as if it has a 8000 ohm source impedance. This will result in a 13.7dB reduction in the voltage gain provided by the pre-amplifier*. However, the removal of the RIAA equalization components will more than make up for this.

*I.e. the second 12AX7 stage presently has a plate resistance of 62,500 ohms in parallel with a 100,000 ohm load resistor (R5). The net equivalent load is 38,461 ohms. The voltage gain of the second stage is: the transconductance x 38,461 ohms. With a 10,000 ohm load resistor added across the "AF OUTPUT" terminals, the net equivalent audio frequency load will be reduced to 7,936 ohms. The voltage gain will be reduced to: the transconductance x 7,936 ohms. 20 log [7,936 / 38, 461] = -13.7dB

Many all-tube 12AX7-based pre-amplifier designs, that I see on-line, employ one more tube... usually in a cathode follower configuration... to provide an output capable of driving a lower impedance load.

With the removal of the RIAA components and the addition of the 10,000 ohm resistor across the "AF Output" terminals of the pre-amplifier, this pre-amplifier can feed into a lower impedance load. You could convert the second half of the 12AX7 tube to a cathode follower configuration... but that would further reduce the available input-to-output voltage gain of this pre-amplifier... if it is looking into a 10,000 load.

Stu  

[k7iou]

Hi Stu, my Elmer came by and we removed the equalization & you are correct it was hotter than a pistol. So we put the 18k resistor back between pin 6 and 1 of the 12ax7. It seems to work well. I had to back the gain off to unity & level -6 on the SR107.
I changed out the 18k to 10k so the gain would be the same thru the SR107 in bypass mode. Thanks to Jim k7jeb for the schematic and circuit analysis .
de k7iou