Please help me with a ROG style adaptation of the REDD47

[tcobretti]

The Beatles used a simple preamp called a REDD47 on most of the their recordings.  This was the amp they daisy-chained to get the distortion on "Revolution", and I'd like to try it as a mic preamp as well.  Here's the schematic (sorry about the huge image but when shrunk it became illegible for some reason):



So I have a few questions.  I've been hangning around here a while, but am still pretty stupid on this stuff.

1.) How would you ROGify the first tube?  Since it isn't a triode can it be done?

2.) Will the same transformer rations work at 9v?

3.) In the "Recording the Beatles" I got for X-mas (it's awesome) they say all the inputs at Abbey Road were wired for 200 ohms impedance.  Do you think the input and output impedances of this should be adjusted to work with modern gear?

Happy Holidays everybody!

[frequencycentral]

1.) How would you ROGify the first tube?  Since it isn't a triode can it be done?

That's just a pentode gain stage. You should be able to replace it with a FET, just ignore grids 2 and 3, so you are just focusing on the input by the 100pf cap (grid 1), the plate (at the top), and the cathode (at the bottom). I think???

[rnfr]

i'll be following this thread.  looks like it could be interesting.

are those two triodes some kind of differential amplifier?

[sean k]

The triodes are simply wired in parallel which tends to double the current output and so the impedance is lower.

I'm interested in what voltage the B+ was at the top of the E88CC 'cause I've got all of those tubes and just picked up an ol' radio chassis with an SE 6AQ5 output tube with a 6AV6 driver and I'm sure I'd have a spare 19mA.
That'd make a nice little amp 

[sean k]

Incidentally the E88CC was just a very highly spec'd version of the 12AX7. Much more linear for use in audio signal equipment.

EF86 is the tube used in the jangley amps so you want something linear for bass and mids but then adds harmonics to the treble end.

Notice also that the feedback from the plates of the E88CC to the cathode of the EF86 is positive so alot of the tone shaping will be about that. 1M in and 100k out should be fine.

[tcobretti]

Here's a modded version that has some voltages. for Sean.

http://www.aronnelson.com/gallery/main.php/v/TCobrettis-Stuff/album160/reddish.gif.html?g2_imageViewsIndex=1

So the input tube problem may be solved, does anybody have any ideas about questions 2 and 3?

[rnfr]

i did some reading, and i believe the triode pair are called a cathode coupled pair.

as far as qu2 goes, do you really need a transformer if it is going to be stompbox based?  is the transformer for balanced signals, or is there something else going on there?

[sean k]

The Xfrmr's are just for low impedance sources like dynamic mics but it is also a not so much balanced as floating input as there is no earth connection just a negative and positive. The 200 ohm signal travels longer distances without losses with much greater succes than a higher impedance and the floating aspect is about keeping earth potentials of equipment apart. Alot of that is just studio normal where lots of stuff is interlinked so a proprietary method is used... then, I suppose, it was 200 ohm floating for signals.

Matching mosfets to valves is kinda beyond me simply because I've got the valves but they all, the valves, have tonal characteristics which you'd find in the data sheets. Theres the curve at specific plate voltages and the capacitance between cathode and grid which may be the most important stuff to look at.

With my totally limited experience I'd simply build the circuit with transistor sockets then try all sorts of mosfets and jfets until you found something you liked. Oh, and use conventional inputs and outputs, 22 nano with a 1M to ground for input and 100 nano and a 100kA pot for out... then start tweaking.

[tcobretti]

Ok, thanks for the tips, I'll let you know if I get it working.  I'm gonna have to order some parts so it may be a little while.

Let me know if you get the valve version working; I'll be curious as to how it sounds.

Thanks again!

[rnfr]

yeah, didn't think you'd need the transormers.  good luck tco.  let us know how it goes!

[Gus]

A few things

A lot of tube preamp sound has to do to the complex transformer, circuit, input, and output loading and freq response, and the core size, winding  and interleaving, alloy (Si steel m4 m6 m whatever, mu 49 or 80  or.......) , lam shape, signal level ......................  Chapter 5 of the big red book 4th ed. and the jensen site and other sites have good writeup you can find about transformers. 

The transformer interface is what most of tube preamp sound is about.

How it loads the microphone some sound good at lower input Z.
What the output is connected to.
Tube/solid state transformer out condensers can have their gain and response changed by the reflected load to the drain or plate loading.  Even transformerless out microphones can be affected by the input transformer

A ec88 is a 6dj8,6922 frame grid tube, a different tube than a 12ax7.  The tube is "doubled up" for more output drive, note the plate current and anode resistor value.

Mosfets tend to have more input cap than tubes. 

Also how will you scale the stages signal swing and gain for the lower 9v solid state voltage?

[rnfr]

you'll most likely have to rebias all the stages.