WE 407A Push-Pull Mic Preamp

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Larrchild

Well-known member
Joined
Jul 15, 2005
Messages
806
Location
Fort Lauderdale, Florida
I'm sick of my Counterpoint 6DJ8-based home preamp. I want to build a new smaller, better sounding line stage. So I went into the tube wine cellar and fished out 6 Western Electric 407a's. They seem to be a medium-mu animal, but if WE built them, I bet they are dead-quiet.

Here's a schematic for a line stage I found on the net using them:
407a.jpg

http://www.the-planet.org/linestage.html at the bottom.

And a big, honkin 1mb PDF on the 407a.

Should I Build a circuit using these? My FET input Acoustat amp is 2 feet away and hi-z in, as they go, so the follower's hi-z mite sqeak by.
 
Go for it!
I have a few of those too, so you do the work.
Is that a dual triode?
MAybe something in a push-pull might fit better?

Heck, you have six of the lil guys?
But that means iron.
Try SE first.
6 resistors, 2 caps, I want this thing done by 9.
 
If I put iron in it, I'd remote the supply.
But a cool PP circuit with minimal F2 and a nice lil F3 thd nub would sound silky.

'course a mic xfmr could be attached, with 2 plate coupled stages. (standing on their grounded cathodes, thank-you :razz:)

But that would blur brand-recognition here in an already crowded market.
Poor Dave, at the beginning of the one-bottle thread, every second person is referring to your WE 417 pre. ha!

"We are not the Peoples Front of Judea, We are the Judean People's Front!"

No thanks. And yeah CJ it's a dual medium-mu triode. Grid to Plate cap= 1.3 pf. And 20/40v fils!
 
Lar', the circuit you posted looks competent. By all means, give it a whirl. I do wonder what the designer was trying to swamp out with that 1K series output resistor, though. It partly defeats the purpose of the cathode follower.
 
[quote author="Larrchild"]I'm sick of my Counterpoint 6DJ8-based home preamp. [/quote]

Yeah, I had that one once. Got from ebay broken for $120, fixed it, and after a couple months got so tired of it that put back on ebay and trippled the money.

I am with CJ--PP with iron.
With right layout you won't need remote PSU. The PP will cancel any residual noise.

But ultimately, it really depends what kind of sound you are after. The direct coupled CF lill sucker is proved with time. Literally, hundreds of high end preamps were built with this arrangement. It actually is so simple that you can build it and see if you like it. You still have 1hr 15min left till 9 :razz:

I am sure it will be at least much more musical than Counterpoint.
 
kick out the 0.47uF at the grid, lower the value of the 1Meg resistor (unless it's a DI) to something more like 221K, also a 200-300 ohm grid stopper on both grids for grid current and Mhz damping?
Different designer and different techniques, yeah that's what it is.
icon_wink.gif

Crank up that soldering iron and let her rip.

analag
 
Ok, Ok , I'm on it. Last 1/2 hr, I made this "Dave-Plate" inspired by his lunch hour project. I will use it to build these various circuits, including PP.
cam.jpg

It has a copper ground point that floats with a transistor insulator on one end. (or not, depending on rf characteristics)

Alright, I have 10 more minutes, lemme populate this!

Lar'
 
> Western Electric 407

Or 6BQ7. Different heater needs and of course not WE quality, but the 6BQ7 is essentially the same tube made by the millions for TV tuners.

From experience: 6BQ7 will not live forever at 300V 10mA. This is outside the rating, so no shock there. In your proposed circuit with maybe 250V B+, they would probably last many years.

Noise? There is a limit how low it can be. ~90% of 6BQ7 will be very near that limit. WE407 may have a better yield, but at 6BQ7 prices you can sort.
 
Alrighty then. Here's the circuit with some measurements and listening.
(I assumed you meant 9AM, CJ).

407a.33.jpg


250 volts B+, 10ma current, fil = 20.0v DC, 20 db gain exactly!, THD =.27%
Sounded good driving my alesis shop amp into a Westlake BBSM-6.
407a22.jpg

No funny rf oscillations Noise seemed fine, but I'll crank an accurate number out tommorow. The blue-green line is freq response. Plenty of f2 on the spectrum, thats for sure. Maybe PP next.=)
 
[quote author="Larrchild"] Maybe PP next.=)[/quote]

Tomorrow, first thing in the morning. You still have an empty socket and plenty of space on your breadboard.
 
Tonite's "Chef on a Shoestring" topology for this will be a fully differential 2-stage, iron in and out, poosh-pool preamp with hum balancing pot and tertiary differential feedback using 2 407a's.

:guinness: onward!
 
[quote author="Larrchild"]Tonite's "Chef on a Shoestring" topology for this will be a fully differential 2-stage, iron in and out, poosh-pool preamp with hum balancing pot and tertiary differential feedback using 2 407a's.
[/quote]

:shock: :shock: :shock:

Why wouldn't you keep it simple? 1:4 in, 4:1 out, 1 stage?
 
Thats what I'm talkin bout.
Voltage, up..up...up .up up....up..up......up........up.....voltage up;up....up up.........up....up.upp...up.up.upup......up.voltage up...up up......Voltage, up..up...up .up up....up..up......up........up.....voltage up;up....up up.........up....up.upp...up.up.upup......up.voltage up...up up......Voltage, up..up...up .up up....up..up......up........up.....voltage up;up....up up.........up....up.upp...up.up.upup......up.voltage up...up up......Voltage, up..up...up .up up....up..up......up........up.....voltage up;up....up up.........up....up.upp...up.up.upup......up.voltage up...up up......Voltage, up..up...up .up up....up..up......up........up.....voltage up;up....up up.........up....up.upp...up.up.upup......up.voltage up...up up......Voltage, up..up...up .up up....up..up......up........up.....voltage up;up....up

But be careful with that 250 B+. Tube voltages can kill you in a heartbeat. If you are unsure of your work, GET A PROFESSIONAL TO HELP YOU!
there are way too many innocent kids doing DIY that are getting killed everyday. I would rather have my son sweep streets in Bagdad that work on a tube project.
Please stick to solid state, Larry, we don't want anything bad to happen to you! Always keep one hand in your pocket. Quit smoking, never play with matches., and always wear a seatbelt. Make sure you have the right size fuse. Take your vitamins. Install a GFI on all house outlets. Always wear a life jacket. Do not operate moving machinery. Never J walk. J running is OK. Get a complete physical twice a year. This includes a Class 5 colonostomy. Unplug your wallet befor you go to bed. :?:


Hah! I just figured out Dan Torres. He's a local Fender fixer, who hustles wanna be rockers for repair work. He used to have this column in Vintage Guitar where he would tell you about a useless amp mod that everybody knew about ten tears ago. It was usually a paragraph or two long, with a whole other page that had nothing but a long winded warning on the dangers of tube circuits. He figured it was better to scare people into bringing their amp to him rather than do the mod themselves. Pretty clever, considering I use to think he was really lame.

Did this come off as being negatory?
Darn, pin 2 instead of 3. Sorry, rail got inverted.
 
Ok, even though this started out as a home hi-fi line stage, I didn't want to use transformers in the home playback setup that were not super-transparent, so when I get some, I'll get back to that.

In the meantime, I had plenty of "Character" sounding iron here, so I built a 50db Push-Pull mic pre with dc gain control. I noticed that the distortion in these variable mu designs didn't increase that badly throughout the gain span, so I'm trying this DC control as a mic trim. At 250 vdc, I am getting 50db of gain with -5 on the grid. .05%THD with zero-feedback on the 1st stage and only local FB on the 2nd.

Before I pulled apart the single ended stage I did 2 days ago, I connected a mic xfmr to it and listened on the trusty '57. It was good , but I like this better. 3rd harmonic leads, near total cancellation of 2nd. (see earlier pic above) Anyway, I didn't find the ideal tertiary-equipped output xfmr, (nor one to pass dc), so I have not added global feedback yet. Any suggestions in that area would be happily recieved. :green:


I'll post an mp3 against a pedestrian 002r pre soon. =)
dual407.jpg

p_p2.jpg

dooldif.jpg
 
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