Phono pre amp RF problem
i've been reading these pages for a long time but this is my first post.
I have searched the archives (and the rest of the net) extensively and i have not been able to find a solution.
I am an experienced tube-audio hobbyist (I have scratch-built 25- 30 tube audio pieces in the past few years). I recently built my first tube phono pre.
I used the standard RCA receiving-tube manual circuit married to the standard RCA low impedance 'input amp' as an output stage. The unit sounds GREAT, i mean, really really great, but it has pretty bad RF in the first stage. WEBE 108 in stamford CT. This station, BTW, often gets into tube studio microphones in the area.
I use a Grado green series (AKA the $60 one) cartridge. 56k imp. I use a 56k resistor from input grid to ground in the circuit.
I do NOT use a grid stopper resistor (the circuit did not call for one).
Any help would be greatly appreciated. I have several solid-state cheapo phone pre amps that work totally fine with this same deck and cart., so i assume this problem is correctable?
here are links to the original circuit if you care to check it out:
as i said before, i am using a 56k input load resistor instead of the 47k indicated here.
Add a grid stopper.
If the first stage plate resistor isn't straight on the pin - try a 10R plate stopper. I had intermodulation distortion from RF instability on mine and a plate stopper helped.
I also recomend using a grid stopper.....
Normally the RIAA filter would filter the RF, but in your case the RF is entering the grid and being "detected"...demodulated to audio band...then your phono stage is amplifying the unwanted audio signal....
I also suggest using Hi-End resistors in that location.....first you want to hit the loading resistor of say 47K or 56K in your case....Then you want the grid stopper resistor in series close to the tube socket....
The tough part of this is choosing the correct grid stopper value...
Here is what the trade-off is.... The grid stop resistor makes a Low-Pass filter working into the Miller capacitance of your input valve.. Assuming your using a 12AX7A valve your typicall stage gain of roughly 60 , then you effective Miller Capacitance will be about 152pF......
Now keep in mind that the Pole frequency you choose will be at -3dB and a 45 degress phase shift .....
You want to place the Pole high enough that you don't intoduce any objectionable phase shift in the upper audio band....this you WILL hear and will make the Hi-End system sound cheap...
You don't want to place the POLE to high, then you will not filter out the RF...
So for example if you use a 2.2K grid stopper, your POLE will be at 478kHz and you will have a 1.2 degree phase shift at 10kHz and 2.4 degree phase shift at 20kHz..... this is negligible...
Your 108MHz nuisance of a radio station will have -47dB of attenuation, hopefully this will keep it from being "detected" in your 1st stage of your Phono stage....
You can alway work the value of the grid stopper up till you solve the problem, hopefull it will not alter the sound of your phono stage to were you can hear it...
Also, make sure you have the proper amount of shunt capacitance for that cartridge.
Grado recommends a cartridge load of 47K for all of their induced magnet (MI) cartridges including yours - so I would change this as well as add a grid stopper.
In phono pre-amps I have found 220 ohms - 1K to be more than adequate in all cases so far.. And Don't use a grid stopper larger than necessary to do this job because it is another noise source in series with the cartridge. The internal generator impedance is typically something over 600 ohms in series with the coil inductance - so at very low frequencies the grid stopper may generate appreciably more noise than the cartridge if the resistance is much more than the cartridge resistance. (Note the 47K load resistor appears in || with the cartridge and consequently doesn't contribute much LF noise.)
The grid stopper to be effective should be mounted as close as possible to the socket pin.
Small plate stopper resistors (like 10 - 22 ohms) can be helpful, but usually not required with the 12AX7A. Again mount close to the socket.
Take a careful look at your input grounding scheme, if you have star grounded the inputs add a small ceramic cap 0.01uF max from the RCA jack ground connection right to the chassis (one on each channel) - your internal ground wiring has appreciable inductance and isn't a good ground at 100MHz.
The phono stage chassis should be metal, if wood shield the inside with copper tape and connect to the star grounding point. Internal signal wiring should be fully enclosed. (Chassis box with bottom.)
Tube shields can help a lot with tubes having high RP like the 12AX7A.
Bypass filament wiring to ground at the socket with 0.01uF ceramic caps.
And yeah I have designed and built a lot of phono stages over the years. (See my site for some early designs.)
A common fix to using a smaller grid resistor is to put additional Capacitance at the input of the tube...
For example, a popular Hi-End pre-amp uses a grid stopper of 365 ohms and also puts a 47pF from grid to cathode.....
This is like -34dB attenuation at 108MHz ......
As the last poster mentions... NOISE is a issue ....
But also be aware that the signal current from the cartridge does not pass through the grid resistor ...it passes through the load resistor, 47K ohms.....
Kevin is correct about the low gm 'X7 not needing much resistance in the stopper position. 100 Ohm Carbon composition parts, with their bodies right at the socket lugs WILL get the job done. Noise is not an issue with control grid stoppers, as the current present is vanishingly small. Being both non-inductive and non-metallic makes Carbon comp. resistors SUPERIOR in the control grid stopper role.
Take additional steps against RFI. Use shields around the 'X7 bottles. Also, use clamp on ferrites on the cables right next to the I/P RCA females.
BTW, changing the 22 KOhm resistor in the EQ network to 24 KOhms improves RIAA accuracy.
Everything mentioned so far is good advice. So one more thing to try is small ferrite bead(s) on the tube side of the grid stopper resistor lead right at the socket.
Johnson (thermal) noise is independent of the current passing through the resistor, effectively it is a voltage noise source in series with the input. The only time this noise goes away is at absolute 0.:D
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