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Old 24th May 2012, 12:38 PM   #1
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Default Want help - RCA Tube Manual RIAA Phono Stage

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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Old 24th May 2012, 02:26 PM   #2
TheGimp is offline TheGimp  United States
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6N2P(-EV) is close to 12AX7 with 6V filament.

6N6P has much lower mu, and will not give sufficient gain. Also, Rp is different, so it will throw off the RIAA equalization.
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Old 24th May 2012, 02:43 PM   #3
DF96 is offline DF96  England
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Build it as it is, or use one of the modified versions which others have done. Don't try to redesign it, or ask us to redesign it for you. Yes, changing to any valve other than 12AX7/ECC83/7025 or equivalents requires a redesign. Minor exception: you could use two 6AV6 as their triode is equivalent to half a 12AX7.
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Old 24th May 2012, 11:05 PM   #4
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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.
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Old 25th May 2012, 02:18 AM   #5
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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. LPS stands for long plate spiral.
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Old 25th May 2012, 10:35 AM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by brucetassin
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.

Quote:
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.
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Old 26th May 2012, 01:14 AM   #7
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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.
Attached Images
File Type: gif Original RCA Phono Preamp Schematic.gif (55.3 KB, 895 views)
File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 916 views)
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Last edited by Eli Duttman; 26th May 2012 at 01:16 AM. Reason: added schematics
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Old 26th May 2012, 03:50 AM   #8
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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.
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Old 30th May 2012, 12:49 AM   #9
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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.
Attached Images
File Type: jpg TPA1.JPG (72.4 KB, 846 views)
File Type: jpg TPA2.JPG (74.4 KB, 827 views)
File Type: jpg TPA3.JPG (79.1 KB, 804 views)
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Old 30th May 2012, 05:16 PM   #10
kevinkr is offline kevinkr  United States
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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.
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