Hello,
Having some RFI issues with my 300B amps, that is to say I am "receiving" a particular radio station pretty clearly. It is not super loud, but noticeable in quiet passages for sure.
I moved about a year ago to a place where I am now surrounded by TV and radio towers. This hasn't caused too much trouble except with these 300B amps. (see schematic)
The radio signal is loudest when turning the amps off, or to a lesser extent when turning the amps on. When the voltages and temperatures stabilize, the RFI is at its quietest, though still audible.
I tried cobbling together an RF filter for the signal input of the amps from what I had on hand. I used a 680R series resistor and a 560pF parallel capacitor. This helped a bit, but the radio station is still there.
Due to the fact that the signal gets louder when the rail voltages sink, I was wondering if there might be a better place to put the RF filter than on the input.
Any thoughts on this would be much appreciated.
Regards,
John
Having some RFI issues with my 300B amps, that is to say I am "receiving" a particular radio station pretty clearly. It is not super loud, but noticeable in quiet passages for sure.
I moved about a year ago to a place where I am now surrounded by TV and radio towers. This hasn't caused too much trouble except with these 300B amps. (see schematic)
The radio signal is loudest when turning the amps off, or to a lesser extent when turning the amps on. When the voltages and temperatures stabilize, the RFI is at its quietest, though still audible.
I tried cobbling together an RF filter for the signal input of the amps from what I had on hand. I used a 680R series resistor and a 560pF parallel capacitor. This helped a bit, but the radio station is still there.
Due to the fact that the signal gets louder when the rail voltages sink, I was wondering if there might be a better place to put the RF filter than on the input.
Any thoughts on this would be much appreciated.
Regards,
John
Attachments
It depends where the interference is getting in.
If you disconnect the cap between pin 6 and pin 2 of the 12BH7, does it stop. If it does, reconnect it and disconnect the one between pin 6 of the EF86 and pin 7 of the 12BH7. Work all the way back towards the input to find out where it is coming from.
If it doesn't add a grid stopper to pin 3 of the output valve, (5k6 should be fine). If that doesn't stop it, there is an issue with mains borne interference and an RFI filter, close to the mains input should help with that. https://www.mouser.co.uk/c/passive-...e=Panel Mount&product type=Power Line Filters
If you disconnect the cap between pin 6 and pin 2 of the 12BH7, does it stop. If it does, reconnect it and disconnect the one between pin 6 of the EF86 and pin 7 of the 12BH7. Work all the way back towards the input to find out where it is coming from.
If it doesn't add a grid stopper to pin 3 of the output valve, (5k6 should be fine). If that doesn't stop it, there is an issue with mains borne interference and an RFI filter, close to the mains input should help with that. https://www.mouser.co.uk/c/passive-...e=Panel Mount&product type=Power Line Filters
Also try a dummy 8 Ohm output load with very short leads, and see if the RF pickup is reduced.
The RFI could be entering from the speaker wires, on the feedback loop through the 500pF lead capacitor.
The RFI could be entering from the speaker wires, on the feedback loop through the 500pF lead capacitor.
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Might try about a .05 mH air core choke at the output along with a snubber and see if that does it. Might look at some solid state amplifier threads on the output filtering.
If you done all, what JonSnell wrote, and interference still exist, possible EF86 demodulating radio broadcast.
The wire (even it's shielded) from volume potentiometer to EF86 G1 may carriage RF.
Try to put small (10..100pF) capacitor between grid and cathode.
Sometimes high Gm tube act as antenna. In this case shielding (even grounded aluminium folie around the tube) may helps.
The wire (even it's shielded) from volume potentiometer to EF86 G1 may carriage RF.
Try to put small (10..100pF) capacitor between grid and cathode.
Sometimes high Gm tube act as antenna. In this case shielding (even grounded aluminium folie around the tube) may helps.
This is the most likely entry point. Remove the 500pF capacitor. You may need to adjust the frequency compensation.
Ed
Oh man! Thanks everyone for your your suggestions!
All are very logical.
I will try these suggestions and report back.
Regards,
John
All are very logical.
I will try these suggestions and report back.
Regards,
John
If I understand your initial statements correctly, the station is loudest when you have the power switch Open.
That sounds like there is RF coming in at one of the two lines of your power mains outlet.
Are you using a 3-wire power mains, 3 wire IEC power cord and IEC socket?
Is the chassis grounded to the IEC socket?
I can not tell what country you are in, or the status of the power mains.
Are the RCA inputs floating from the chassis (insulating washers on the inputs, with the input connector 'ground terminal' going directly to the cathode bias circuit of the input tube? And the volume control ground end going there to.
Bringing those 'grounds' directly to the chassis, or to later circuits may produce ground loops that include the RF signal.
That 100 Ohm resistor at the input is the "ground' for the cathode circuit. The other end of that 100 Ohm resistor is the 3000 Ohm feedback resistor and 500pF from the output transformer secondary.
. . . As was mentioned by someone else above, the RF may be coming in from the speaker and speaker cable.
Grounds are Commonly Misunderstood.
All wires have DC resistance, and even more importantly, those wires have Inductance that has a high enough impedance at RF, which can cause a voltage drop/ground loop problem.
It seems you have no grid stoppers on any of the tubes. What about the input tube?
The volume control wiper is not a grid stopper.
Can you identify the 'offending' station?
Is it AM or FM?
Simulcast, so no way to determine that?
In the US, I use 3-wire power mains, cords, and sockets. My tube amp chassis are grounded.
Things like my tuners, CD players, and phono preamps are not grounded, they use 2 wire plugs.
But with my RCA input jacks on the tube amp not connected directly to the chassis, but instead to the ground of the input cathode circuit, i do not get ground loops (at least not than I can hear).
Lots of questions, but finding the cause(s) of RF interference can be difficult.
That sounds like there is RF coming in at one of the two lines of your power mains outlet.
Are you using a 3-wire power mains, 3 wire IEC power cord and IEC socket?
Is the chassis grounded to the IEC socket?
I can not tell what country you are in, or the status of the power mains.
Are the RCA inputs floating from the chassis (insulating washers on the inputs, with the input connector 'ground terminal' going directly to the cathode bias circuit of the input tube? And the volume control ground end going there to.
Bringing those 'grounds' directly to the chassis, or to later circuits may produce ground loops that include the RF signal.
That 100 Ohm resistor at the input is the "ground' for the cathode circuit. The other end of that 100 Ohm resistor is the 3000 Ohm feedback resistor and 500pF from the output transformer secondary.
. . . As was mentioned by someone else above, the RF may be coming in from the speaker and speaker cable.
Grounds are Commonly Misunderstood.
All wires have DC resistance, and even more importantly, those wires have Inductance that has a high enough impedance at RF, which can cause a voltage drop/ground loop problem.
It seems you have no grid stoppers on any of the tubes. What about the input tube?
The volume control wiper is not a grid stopper.
Can you identify the 'offending' station?
Is it AM or FM?
Simulcast, so no way to determine that?
In the US, I use 3-wire power mains, cords, and sockets. My tube amp chassis are grounded.
Things like my tuners, CD players, and phono preamps are not grounded, they use 2 wire plugs.
But with my RCA input jacks on the tube amp not connected directly to the chassis, but instead to the ground of the input cathode circuit, i do not get ground loops (at least not than I can hear).
Lots of questions, but finding the cause(s) of RF interference can be difficult.
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Agree that speaker wires are the likely antenna - bypassing them (.01 uF) at the entry point may help. Clamp-on Ferrite cores won't have much impedance at medium-wave frequencies, but may help.
I once lived in he shadow of a 50 KW MW tower - my record player would play it loudly when it shut off at the end of a record. My theory was that the amplifier was linear when powered, but as voltage dropped, it became non-linear and "detected" (rectified) the RF modulation.
I once lived in he shadow of a 50 KW MW tower - my record player would play it loudly when it shut off at the end of a record. My theory was that the amplifier was linear when powered, but as voltage dropped, it became non-linear and "detected" (rectified) the RF modulation.
It is also worthy to check your house grounding. My house was previously owned by contractors who did some dodgy things as contractors are known to do.
I had nagging RF inteference problem that went away when I did a house addition and replaced the meter and circuit breaker panel.
While pulling the old panel out I had to realize there was no ground stake at the panel outside. The wire I thought clamped onto a water pipe did not really go anywhere useful.
Also the ground wire had been harvested from a nearby pole.
I had nagging RF inteference problem that went away when I did a house addition and replaced the meter and circuit breaker panel.
While pulling the old panel out I had to realize there was no ground stake at the panel outside. The wire I thought clamped onto a water pipe did not really go anywhere useful.
Also the ground wire had been harvested from a nearby pole.
So, just a little more info:
-The station appears to be 1580 AM.
-My input filter (680R, 560pF) has a knee at ~420 kHZ. (Should I go lower?)
-The station does not get louder when the power switch is off (open), but when voltage sinks..
-I built these as mono amps. No volume potentiometers for the signal input.
-I used a star grounding scheme for the amps. The star terminates at a point in the chassis. So the case is grounded at the "star" point.
-The input connectors are isolated from the chassis
-The EF86 and 300B have grid stoppers, albeit small ones. 100-330R.
-I'm in the USA
-All the cabling in the system acts as an antenna. I can make the RFI much worse by moving, connecting or disconnecting interconnects, but I can"t seem to make it better this way.
What I have tried so far:
- 8 ohm dummy load on the output = no difference
-.1uF capacitor in parallel with the output = no difference
-33pF cap between G1 and Kathode of EF86 = no difference
-Tried connecting EF86 internal shield to ground = no difference
Then I:
- Disconnected the 500pF cap across the feedback resistor = THIS HELPED A LITTLE!
Now, with the feedback cap disconnected and the input filter attached, the radio is gone. But I think the amp sounds a tiny bit less good, but his is probably just my imagination/audionervosa.
However, in the interest of thoroughness, I think I should probably spend a bit more time and do what Jon Snell suggests and trouble-shoot each stage to isolate the problem.
I will report back if I find anything new or relevant.
Thanks again for all your suggestions!! All were super helpful!
Regards,
John
-The station appears to be 1580 AM.
-My input filter (680R, 560pF) has a knee at ~420 kHZ. (Should I go lower?)
-The station does not get louder when the power switch is off (open), but when voltage sinks..
-I built these as mono amps. No volume potentiometers for the signal input.
-I used a star grounding scheme for the amps. The star terminates at a point in the chassis. So the case is grounded at the "star" point.
-The input connectors are isolated from the chassis
-The EF86 and 300B have grid stoppers, albeit small ones. 100-330R.
-I'm in the USA
-All the cabling in the system acts as an antenna. I can make the RFI much worse by moving, connecting or disconnecting interconnects, but I can"t seem to make it better this way.
What I have tried so far:
- 8 ohm dummy load on the output = no difference
-.1uF capacitor in parallel with the output = no difference
-33pF cap between G1 and Kathode of EF86 = no difference
-Tried connecting EF86 internal shield to ground = no difference
Then I:
- Disconnected the 500pF cap across the feedback resistor = THIS HELPED A LITTLE!
Now, with the feedback cap disconnected and the input filter attached, the radio is gone. But I think the amp sounds a tiny bit less good, but his is probably just my imagination/audionervosa.
However, in the interest of thoroughness, I think I should probably spend a bit more time and do what Jon Snell suggests and trouble-shoot each stage to isolate the problem.
I will report back if I find anything new or relevant.
Thanks again for all your suggestions!! All were super helpful!
Regards,
John
generally . . .
When the amplifier is warm, and then it is turned off, as the B+ starts to collapse, any self biased stages will have less current, and that reduces the cathode voltage.
That reduced cathode voltage, causes the grid to cathode to be a fairly efficient rectifier of very low voltages. It demodulates the AM radio signal.
Common Mode:
Some RF that gets into the amplifier does so with, both the conductors having RF voltage on them.
That could be the Hot and the Neutral power cord wires (and could also include the 3rd wire (ground).
The two wires of a shielded cable from a signal source (CD player, Turntable cartridge, Preamp, Tuner, etc.) can also have RF on both signal cable conductors.
Differential Mode:
Some RF gets on just one conductor. The 2nd conductor acts as the return wire.
Power Mains: just one of the following: Hot, Neutral, Ground. The 2nd (and 3rd) wires act as the return wire (differential)
Even the shield of a signal cable can have RF present, and no signal on the center conductor (differential)
It is possible to have both common mode, and differential mode; One on one cable or power cord, and the other on the power cord or cable.
Power, Input signal, speaker (3 different potentials for either common mode, or differential mode).
When it comes to using filters to reduce RF ingress into the amplifier, it must be noted that they are of two different kinds:
Common mode interference reduction
Differential mode interference reduction
Fixing this kind of problem is not simple.
When the amplifier is warm, and then it is turned off, as the B+ starts to collapse, any self biased stages will have less current, and that reduces the cathode voltage.
That reduced cathode voltage, causes the grid to cathode to be a fairly efficient rectifier of very low voltages. It demodulates the AM radio signal.
Common Mode:
Some RF that gets into the amplifier does so with, both the conductors having RF voltage on them.
That could be the Hot and the Neutral power cord wires (and could also include the 3rd wire (ground).
The two wires of a shielded cable from a signal source (CD player, Turntable cartridge, Preamp, Tuner, etc.) can also have RF on both signal cable conductors.
Differential Mode:
Some RF gets on just one conductor. The 2nd conductor acts as the return wire.
Power Mains: just one of the following: Hot, Neutral, Ground. The 2nd (and 3rd) wires act as the return wire (differential)
Even the shield of a signal cable can have RF present, and no signal on the center conductor (differential)
It is possible to have both common mode, and differential mode; One on one cable or power cord, and the other on the power cord or cable.
Power, Input signal, speaker (3 different potentials for either common mode, or differential mode).
When it comes to using filters to reduce RF ingress into the amplifier, it must be noted that they are of two different kinds:
Common mode interference reduction
Differential mode interference reduction
Fixing this kind of problem is not simple.
I notice that the 10uF cap bypassing adjustable bias voltage is drawn with backwards voltage. Only matters if electrolytic, but.
Lots of great info already, can only add that a Zobel across the input, terminating the interconnecting cable, can sometimes help, and can't hurt. Something like 220pF in series with 75R is about right.
All good fortune,
Chris
Lots of great info already, can only add that a Zobel across the input, terminating the interconnecting cable, can sometimes help, and can't hurt. Something like 220pF in series with 75R is about right.
All good fortune,
Chris