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Want help - RCA Tube Manual RIAA Phono Stage

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Good day all.
I am building the RIAA pre-amp in the RCA tube manual. I have layed out a
PC board so I'm commited to this design. I do not have a 7025 tube so I'm going to use either a 12AX7 or Russian 6N6P. I would like comments on the
value of R3 / R5 and especially R2 / R7. Thanks..
 

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Hello all,
Thanks for your input.

DF96 I am very confused by your comments. Where in my request did you
get the idea that I was asking for a re-design. I clearly stated that I was
going to build it as shown in the RCA tube manual and that I had drawn up
a PC board for it. I only asked for comments on the values of a few resistors
as I do not know how changing from a 7025 to a 12AX7 tube would play. I
mentioned that I had a 6N6P and might consider using it as it is pin compatible
(except for heater). IMHO it was a simple request, not a complete make-over.
 
Bruce,

The difference between the 7025 and the 12AX7 lies in the heater construction. The 7025 has a spiral wound, hum bucking, heater. Way back when, non battery low voltage DC supplies were HARD to execute. The hum bucking construction made AC heating tolerable. Since building a regulated 12 VDC supply for the heaters is very easy, you can use any 12AX7 that tickles your fancy. I highly recommend the Sovtek 12AX7LPS, as it's clean, quiet, and inexpensive. BTW, it just so happens that the 'LPS is a genuine 7025 equivalent, with the spiral wound, hum bucking, heater. :D LPS stands for long plate spiral.
 
brucetassin said:
Where in my request did you
get the idea that I was asking for a re-design.
When you said you were thinking of using a 6N6P and TheGimp pointed out that this is a different valve with different characteristics. I assumed you already knew that 7025 and 12AX7 are identical, apart from heater arrangements, so would need no change in resistor values.

I clearly stated that I was
going to build it as shown in the RCA tube manual
No you didn't, you said you were thinking of using a different valve and asked what resistor changes that would mean. Perhaps you did not realise that changing the valve will upset the RIAA network and require a redesign, which is why TheGimp and I alerted you to this. We were trying to be helpful.
 
Bruce,

I gather you are building a stand alone phono preamp. The original RCA setup can't fight its way out of a wet paper bag. It has zero load driving capability. :( Look at the tweaked RCA setup, with its source follower buffer that can drive a load.
 

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Last edited:
Thank you all for your input. As I stated in my first post, I have a PC board drawn for the original RCA design. I will etch it this weekend and mount parts
now that I know the 7025 is a modified 12AX7 and does not need parts values changed, and the Russian tube will not work in it. It may not be much of a
pre-amp but it will be what it is. I just might build the "Tone and volumn" stage
from the same RCA tube manual. Alot of this exercise is to see if I can design
a PC board and make it work, to whatever extent.
Thanks again.
 
I got the PCB etched and most of the parts mounted. It turns out that I
was missing 1 resistor and 2 caps. I will have to order the parts and then
I can fire it up. Don't be confused with the 12AU7 in the picture, It's just
there for the picture. I will put a 12AX7 in when it's done.
 

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I see ceramic caps in what appears to be the equalizer network, these tend to be both microphonic (depending on value) and pretty non-linear as well. I'd recommend their replacement with small polystyrene film caps. (REL amongst others makes some very inexpensive but decent ones.)

With good parts, fairly high output MM or MC, and an appropriately high input impedance in the following amplifier this design can sound quite good. It will have trouble driving any sort of significant cable capacitance so keep the cable short.

Some of my early designs were not too different.
 
About 15 years ago, there was a commercially available circuit board for the RCA RIAA preamp (from Handmade Electronics, I believe). I stuffed one of those, put it in a nice chassis with power supply, and added another tube socket for a 12AT7 cathode follower, to help with the output impedance problem. The result sounded quite pleasant.

--
 
Bruce, I'm not sure I'd say it "sucks to the max" I personally really enjoy this stage and have built a number of them over the years. My favorite uses 6SL7s (or 12SL7s) instead of the 12AX7, but this would not work on your board since the SL7 is an octal tube. At any rate, if you use it within the limits give (like next stage needs 250K or greater input impedance) you may be surprised just how much it doesn't suck.
 
About 15 years ago, there was a commercially available circuit board for the RCA RIAA preamp (from Handmade Electronics, I believe). I stuffed one of those, put it in a nice chassis with power supply, and added another tube socket for a 12AT7 cathode follower, to help with the output impedance problem. The result sounded quite pleasant.

--


I also built the Handmade kit. Designed by Gordon Rankin, it was a nice implementation. Mine is on the sideline right now while I come up with a new power supply for it.
 
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TUBECAD PHONO AMP, MOD RCA PRE-AMP

Bruce:

Here the layout of a modded RCA clone. My great grandfather copied it from Tubecad. I listen to it for years now. It sounds outstanding.

I include the article and the pcb layout.

I hope this will help. I ask you great grandfather and he stated that good metal poly caps can be used for the RIAA.


Joe
 

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Without getting to in depth.... Here are a few basic to strive for to make this simple RIAA stage pleasurable to listen to...

1. RIAA ACCURACY....this is pretty much trivial transfer function...just basic math to figure the proper values for POLES and ZEROES.... The bigger question is if you even care...since a sloppy RIAA curve will make some audiophiles go balastic while other listeners can be very happy and don't mind.... Keep in mind that MANY pressing were cut with sloppy RIAA curves to begin with....

2. HEADROOM.... are the tubes biased nicely??? Will a loud passages cause clipping and go to the rail??? The worst part of clipping is that the RIAA caps will get charged up to the rail, then they need "time" to recover...not good...
In the old days they used "1812 Overture" as the transient overload test record... then you ask yourself...is it my pre-amp??...or is it the cartridge not aligned properly?? Oh maybe the recording was distorted??? Or maybe all the above??

3. NOISE FLOOR......What you can do to minimize noise.....carbon comps for the added warmth...or metal films for the cold and sterile truth.....

4. VOLTAGE REGULATION.... I prefer good voltage regulation for linearity....but with that comes NOISE from the regulator/ reference.... A reg with a low noise floor is key to a good RIAA stage and this RCA phone stage is no exception...it will make it come alive...

Have fun with this phono stage....It may not be the best design...but having good supporting circuitry can make all the difference....

Chris
 
To all who got involved, Thanks for your input.
The last of the missing components showed up in todays mail.
I finished stuffing the board, constructed a crude voltage doubler power supply and fired up the pre-amp. It worked without a single problem, and sounded much better than I expected. It is very quiet considering the power supply I used for testing. Now I will build anothr board, get a chassis and
construct a real power supply to make a stereo unit.
Just for fun, I tried several tubes to hear how it would sound.
First was a 12AU7, (not very loud but worked). Next was a 12AT7, (sounded better and just a little more volumn), then a 12AX7 (loudest and best sound).
I also put the Russia 6n6P It worked pretty nicely also. I'm staying with the
12AX7. If anyone would like to built a pre-amp, you can use my PC board layout, it works well.
 
Yes, definitely stay with 12AX7!

An RIAA preamp is no place for "tube rolling." The 12AU7 and 6N6P are radically different from 12AX7. Nowhere near the same amplification factor, plate resistance or transconductance. The frequency response of the preamp will be greatly altered by putting in such different tubes. It would deviate widely from "flat" response (adherence to the RIAA characteristic). It's no surprise that you wound up preferring the 12AX7 in this preamp. That's what it's designed for, so of course that is what will work best in that circuit.
 
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