I've closed mine up and am calling it good enough, the only change will be to reduce the heater voltage a little more (Line voltage around here tends to run high, and I'm using 900mA heater power tubes rather than 1.5A. My guitarist has it now, the sound of the regular channel is so clean it took him a little while to get used to it. I put that down to the hum fix and the grid stoppers. I'm going to assume 3-4mA bias mismatch between tubes is good enough...?
The next thing I would want to try (if I was so inclined) would be making the raw supplies quieter, the first stage filter caps are pretty big for both the high and low voltage. That would require changing the initial C's to CRC, where the first C is smallish, but that would be some major board surgery.
The next thing I would want to try (if I was so inclined) would be making the raw supplies quieter, the first stage filter caps are pretty big for both the high and low voltage. That would require changing the initial C's to CRC, where the first C is smallish, but that would be some major board surgery.
If I short out the Receive input it is dead silent. So I don't think there is any noise coming from the power tubes. 3-4mA sounds like a lot to me. If they are mismatched it will increase hum. Perhaps you need better matched tubes, or at least to give it a try.
I am thinking my residual hum is coming from the channel switching blanking circuit. Running pspice shows that if there is ripple on the 12V supplies it can get injected by that circuit. I will try lifting the Source on both Q1 and Q2.
I also figured out I can just jumper from IC1 pin 3 to pin 5 and lift R23. That will remove the first opamp from the Send circuit and just send the signal directly to the second opamp.
How are you controlling the heater voltage?
I am thinking my residual hum is coming from the channel switching blanking circuit. Running pspice shows that if there is ripple on the 12V supplies it can get injected by that circuit. I will try lifting the Source on both Q1 and Q2.
I also figured out I can just jumper from IC1 pin 3 to pin 5 and lift R23. That will remove the first opamp from the Send circuit and just send the signal directly to the second opamp.
How are you controlling the heater voltage?
I put a resistor in each leg of the heater supply. I like to get it within 2 or 3% of spec. I added a terminal strip on the back of the chassis to the left of the power transformer.
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I suspect this problem is before the amp, inside the guitar.
Original single coil pickups are very sensitive to magnetic fields emanating from the mains transformer. Move the guitar into different positions and the hum will change.
In that case I recommend throwing away the single coils and replace them by some stacked humbuckers.
EMG select work fine in my guitars and are available at 35Euros/piece.
Thanks for the advice, but it's not the guitar. I get the same result with a 1/4" phone plug just shorted out at the plug. Besides, how would noise from the guitar explain the fact that the noise goes away when the guitar is plugged into the effects return with the guitar volume set to zero?
I did a pSpice sim of the blanking circuit, and it does couple any ripple voltage on the +/-15V supplies into the first opamp that drives the effect Send output. So once I get a chance to try it I will further isolate it down.
I did a pSpice sim of the blanking circuit, and it does couple any ripple voltage on the +/-15V supplies into the first opamp that drives the effect Send output. So once I get a chance to try it I will further isolate it down.
I assumed the guitar volume knob is the one at the guitar, at the input of the amp.
It is. No big deal, I just thought you misunderstood what I was saying.
It is definitely in the amp, and I have isolated it to this one opamp circuit.
The real root problem is the solid state rectifiers used in the power supply. They generate a small glitch that just travels everywhere, and get's coupled to both the low voltage and heater voltages, which all come from the power transformer. All the work I have done is to keep that signal from getting into the actual signal voltage. If that glitch wasn't there then I wouldn't have it coupling in at all the various places it has.
It is definitely in the amp, and I have isolated it to this one opamp circuit.
The real root problem is the solid state rectifiers used in the power supply. They generate a small glitch that just travels everywhere, and get's coupled to both the low voltage and heater voltages, which all come from the power transformer. All the work I have done is to keep that signal from getting into the actual signal voltage. If that glitch wasn't there then I wouldn't have it coupling in at all the various places it has.
I wonder if the source of buzz is just the big glitch of the rectified DC in the first cap section (used by some of the circuits)?
Yes, it is. But it is getting coupled into the signal path. It also is couple to the filament and the +/- 15Vdc supplies, since they all come from the same transformer.
I fixed a major source of this noise by rewiring the filament circuit near the second tube stage, as shown above. It is a lot better than it was, but now there is another trouble spot.
I fixed a major source of this noise by rewiring the filament circuit near the second tube stage, as shown above. It is a lot better than it was, but now there is another trouble spot.
I finally figured out tha last source of hum in this amp. I determined that if I grounded the effects loop input the hum went away. When in gain mode the is a Master volume control that if set to zero completely kills the signal to an op amp the drive the Send output of the effects loop, and also the rest of the amp. With that set to zero the hum is still there. So I knew the hum was between the master volume and the effects return input.
There are only two things between those two points, an op amp, and a connection to a channel switching blanking circuit. I assume the blanking is there to prevent a pop when switching modes. It does that by monitoring the voltage across the channel switching relay. It sense a change in the relay state and turns on a JFET for a fraction of a second, which in turn kills the signal until the switching has been done and is stable. If you look on the schematic published above it is transistors Q1 and Q2.
I simulated this circuit in Pspice and determined if the + and - 15.8 volt supplies had noise or AC ripple on them that that signal would leak through the circuit and be transferred through Q1 and Q2 into the signal path, even though no switching was going on.
Well yesterday I removed a jumper wire that disconnected Q1 and Q2 from the signal path, and guess what? The last remaining vestage of AC hum is completely gone.
Phew!!! Let me say that again, Phew! That was a tough one.
So now I need to figure out how to eliminate the hum from the circuit or maybe just use the amp without the switching blanking circuit. I should be able to filter the noise from the +/-15.8v supplies. If I do that then I can still use the blanking circuit.
There are only two things between those two points, an op amp, and a connection to a channel switching blanking circuit. I assume the blanking is there to prevent a pop when switching modes. It does that by monitoring the voltage across the channel switching relay. It sense a change in the relay state and turns on a JFET for a fraction of a second, which in turn kills the signal until the switching has been done and is stable. If you look on the schematic published above it is transistors Q1 and Q2.
I simulated this circuit in Pspice and determined if the + and - 15.8 volt supplies had noise or AC ripple on them that that signal would leak through the circuit and be transferred through Q1 and Q2 into the signal path, even though no switching was going on.
Well yesterday I removed a jumper wire that disconnected Q1 and Q2 from the signal path, and guess what? The last remaining vestage of AC hum is completely gone.
Phew!!! Let me say that again, Phew! That was a tough one.
So now I need to figure out how to eliminate the hum from the circuit or maybe just use the amp without the switching blanking circuit. I should be able to filter the noise from the +/-15.8v supplies. If I do that then I can still use the blanking circuit.
That's great that you figured it out. Several ways to clean up the +/- 15.8v supply come to mind, but the trick will be how to do it within the confines of the circuit board. Another possibility would be to use the +/- 15v supply, but I don't remember what else is on there and if you would have some unwanted interaction.
Yep
Me too!
I'm not sure if it's the +/- 12 or the +/- 15.8 just yet. I will try adding caps first to see if I can just filter it.
There is also a lot of hissing going on. I couldn't hear it before , because the hum was louder. I saw on the scope plots the signal was fuzzy on the screen, and it varied a lot based on where I had my hands around the wires and probes. I took it to be noise picked up by the probes, but now I think I have a high frequency oscillation, or just a high frequency noise issue.
Me too!
I'm not sure if it's the +/- 12 or the +/- 15.8 just yet. I will try adding caps first to see if I can just filter it.
There is also a lot of hissing going on. I couldn't hear it before , because the hum was louder. I saw on the scope plots the signal was fuzzy on the screen, and it varied a lot based on where I had my hands around the wires and probes. I took it to be noise picked up by the probes, but now I think I have a high frequency oscillation, or just a high frequency noise issue.
I was getting a lot of RFI in my amp, some of which was sort of a 'hiss'. The grid stoppers I put on the 12AX7s cleaned that up quite a bit.
Grid stoppers?
It appears this amp already has grid stoppers.
There is a cap on the PCB C44 that isn't in the schematic. It is in parallel with R15 which forms a voltage divider in the anode/plate of the last 12ax7 before the Send output. I'm thinking it may be an oscillation stopping cap.
It appears this amp already has grid stoppers.
There is a cap on the PCB C44 that isn't in the schematic. It is in parallel with R15 which forms a voltage divider in the anode/plate of the last 12ax7 before the Send output. I'm thinking it may be an oscillation stopping cap.
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Mine doesn't have the extra cap you mention, sounds like yours may be a slightly different version from mine.
Hi CapnDenny1
I also have a Ampeg Reverbrocket R212R reissue from the 90's (EL34 version)
I also have the same issue
I want to cancel the hum I read all your messages, I looked at all your pictures but I don't understand everything (I am French :-/ )
Could you sum up what you did exactly to remove the hum? with details and pictures, it would be vrey great!
I would really appreciate
Youw wrote :
"Moving that heater wire next to the signal at the R14-R13 junction got rid of about 90% of my problem. I can hear the noise, but it's so low I can live with it."
"The biggest helps were the moving the heater wire, and the humdinger pot. The pot had to be a little off center for minimum hum. So the grounded center tap would not have been the minimum hum."
What/where is the heater wire? How to move it?
Where is the humdinger pot? How to modify it?
Thank you very very much
All the best,
Max
I also have a Ampeg Reverbrocket R212R reissue from the 90's (EL34 version)
I also have the same issue
I want to cancel the hum I read all your messages, I looked at all your pictures but I don't understand everything (I am French :-/ )
Could you sum up what you did exactly to remove the hum? with details and pictures, it would be vrey great!
I would really appreciate
Youw wrote :
"Moving that heater wire next to the signal at the R14-R13 junction got rid of about 90% of my problem. I can hear the noise, but it's so low I can live with it."
"The biggest helps were the moving the heater wire, and the humdinger pot. The pot had to be a little off center for minimum hum. So the grounded center tap would not have been the minimum hum."
What/where is the heater wire? How to move it?
Where is the humdinger pot? How to modify it?
Thank you very very much
All the best,
Max
