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Old 23rd August 2015, 07:36 PM   #1
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Default Someone, please check my plot.

I have decided to built the Phone pre-amp from the RCA tube manual.
But I want to make some changes, mainly the tubes. I have 12AV6's on hand
so I will use them. I will build a regulated power supply using the LR8N3-G
with a pass transistor. The max voltage my power supply can provide is 200vdc.
I will regulate this down to 185vdc.
I made the calculation seen on the load line chart.

This is a learning exercise for me so Please don't suggest other circuits.

Are my lines and cathode resistor value correct?

Thanks...
Attached Images
File Type: jpg rcaPhono_12AV6-question.JPG (36.9 KB, 147 views)
File Type: jpg 6AV6 LOAD LINES question.JPG (138.0 KB, 146 views)
File Type: jpg REGSUP.JPG (8.8 KB, 145 views)
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Old 23rd August 2015, 08:52 PM   #2
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Quote:
Originally Posted by brucetassin View Post
Are my lines and cathode resistor value correct?
Looks fine to me. Real world results will be a little different because the tubes will pull a little grid current so near zero bias. But your method is correct!

All good fortune,
Chris
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Old 23rd August 2015, 09:02 PM   #3
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.....seems correct, try and don't wory....
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Old 23rd August 2015, 09:18 PM   #4
Ketje is online now Ketje  Belgium
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You can check it here Triode/Pentode Loadline Calculator
Mona
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Old 23rd August 2015, 10:39 PM   #5
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I agree with chris, phono cartridges output is so low that there really isnīt any problem running the tube nearer to cutoff, say 0,7ma (Vg -1V), on that loadline.
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Old 24th August 2015, 12:33 AM   #6
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Hi Bruce

Sure, use ~500 ohm to bias your cathode. If my quick calculation is correct, you should at the very least use 40uF to bypass that cathode resistor though. I would suggest a 100uF cap. Please do the calculations yourself though to be certain - I always need to triple check my own values.... Morgan Jones is an excellent reference for this!

F = 1/(2 * pi * C * R) or more conveniently C = 1/(2*pi*F*R) where F is your corner frequency in Hz and C will be in Farads - you will likely want to convert it to uF

R is equal to Rk in parallel with (Ra + rp)/(mu + 1) I get something like R = 380 Ohm in your proposed circuit...

Also, I would put the follower resistor before the coupling capacitor. I didn't check your RIAA values. According to the RCA specs sheet, the plate resistance is 80k Ohm - this needs to be included in your passive RIAA correction as well.

Not sure how you wish to bias the second triode. Your output will not likely have a fairly high impedance, so I hope the connection to the next stage is short. Long cables won't work well here...

Kind regards
Ian

Last edited by soulmerchant; 24th August 2015 at 12:37 AM.
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Old 24th August 2015, 03:11 AM   #7
rongon is offline rongon  United States
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I've been listening to a version of this circuit today. I hope you don't mind me making a few suggestions...

First, I'll second marcelop's suggestion to run the tubes at a bit lower plate current. This should allow you to get the grid-cathode voltage high enough to keep the 12AV6s from drawing grid current. I think you might want to shoot for marcelop's suggestion of a -1V grid bias rather than something like -0.5V. I remember reading that moving magnet cartridges do not work well into an input stage with grid current.

From your loadline, it looks like you could get Vp = 115V, Vk = 1V, Ip = 0.7mA. I think a cathode resistor of 1.5k would get you pretty close, as 1V/1500R = 0.67mA.

Interestingly, Fender guitar amps used those same plate and cathode load resistor values, 100k in the plate, 1.5k in the cathode. With a 47uF cathode bypass cap, I get F = approx 3.5Hz.

Regarding soulmerchant's point that the output impedance will be really high, I have a question: Are you making a standalone phono preamp, or is this going straight to a line stage with gain, inside a 'control preamp'? If it's a standalone phono preamp, I'd look at adding a cathode follower or an FET follower on the output. The high input impedance of the follower will make life much easier for the second stage, and its much lower output impedance will have a better chance of driving cable capacitance and any potentiometer losses. If the phono preamp without a follower tries to drive a a couple feet of cable and a 100k pot, I'd expect there would be a loss of gain and an increase in distortion.

In the version I made, I used the stock RCA circuit using 12AX7s but with a +300V plate supply, and DC-coupled the plate of the second stage to a 12AV7 cathode follower (mostly because I already had a sleeve of those tubes and it was easy to get it working in this particular circuit). I really like the result.

To help you figure out more accurate R and C values for your particular operating conditions, you might want to take a look at John Broskie's TCJ RIAA app. However, I modeled the original RCA EQ values in LTspice and it came out pretty accurate. There's a lift of about 0.5dB in the mid-bass and 0.2dB around 10kHz, but that sounds 'good.'

Have fun with this. You might find that you really like it.

RonG
--

Last edited by rongon; 24th August 2015 at 03:33 AM.
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Old 24th August 2015, 03:26 AM   #8
Koonw is offline Koonw  Malaysia
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Quote:
Originally Posted by brucetassin View Post
I have decided to built the Phone pre-amp from the RCA tube manual.
But I want to make some changes, mainly the tubes. I have 12AV6's on hand
so I will use them. I will build a regulated power supply using the LR8N3-G
with a pass transistor. The max voltage my power supply can provide is 200vdc.
I will regulate this down to 185vdc.
I made the calculation seen on the load line chart.

This is a learning exercise for me so Please don't suggest other circuits.

Are my lines and cathode resistor value correct?

Thanks...
I plug in 490 ohm, distortion wise OK, but it's does not look like RIAA curve of +-20db over 10hz-20Khz. Are all other components value correct? The response should go down slope near 10hz, not 1Khz.
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File Type: png 12AV7 phono.png (245.5 KB, 95 views)
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Old 24th August 2015, 03:42 AM   #9
rongon is offline rongon  United States
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Hi Koonw,

I think your model has 12AV7 tubes in it. The circuit in question will be using 12AV6, which is pretty much identical to a 12AX7, but in a 7-pin mini bottle. That might be why the EQ in your model comes out so far off.

--
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Old 24th August 2015, 04:24 AM   #10
Koonw is offline Koonw  Malaysia
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Quote:
Originally Posted by rongon View Post
Hi Koonw,

I think your model has 12AV7 tubes in it. The circuit in question will be using 12AV6, which is pretty much identical to a 12AX7, but in a 7-pin mini bottle. That might be why the EQ in your model comes out so far off.

--
I tried 12ax7 which has nearly same mu and rp is the same slope. So if I increase C4 by 100 time (1000/10=100) from 3300p to 330000p, this is getting closer to RIAA curve.
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