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Squealing/Buzzing noise and weird 2nd stage voltages

5/27/2005 11:29 AM
Squealing/Buzzing noise and weird 2nd stage voltages
This is a problem that I had originally started discussing as part of another thread on the Design & Construction page, but now I moved it to this forum since it seems more appropriate.  
I have an Electar Tube 10 amp (1-12AX7, 1-6L6). I used to modify the circuit on the printed circuit board by removing components and/or changing components. All my changes always worked and never caused any problems. But I changed it so many times, the circuit traces started coming off in places. So I decided to redo the circuit with a turret board. But now I am having problems. I have worked through most of them with the help of others in my other thread, but I still have one problem I haven't solved.  
You can find an Electar Tube 10 schematic here:  
Here are some of the changes I have made to the circuit(both when using PCB and turret board) to make it more "Champ"-like for the most part:  
- changed R3 to 1.5K  
- changed R7 to 1.5K  
- changed C2 to .01uF  
- removed R5 and R6  
- changed C5 to .02  
- changed C6 to 500pF on turret board only  
- removed R14 (as a way to disconnect negative feedback loop)  
- removed C11 on turret board only  
- power transformer replaced with a Weber W022772  
- output transformer changed to something with more iron  
- disconnected preamp out jack  
When I turn the gain control way up, it starts to make a buzzing, squealing noise. You can hear it even without plugging the amp into a speaker.  
I took some voltage measurements on the 12AX7 tube at zero gain and max gain.  
Pin 1 (plate stage 1) min gain = 271V max gain = 278V  
Pin 2 (grid stage 1) min gain = .003V max gain = .004V  
Pin 3 (cathode stage 1) min gain = 2.08V max gain = 2.14V  
Pin 6 (plate stage 2) min gain = 272V max gain = 411V  
Pin 7 (grid stage 2) min gain = 0V max gain = -5.12V  
Pin 8 (cathode stage 2) min gain = 2.05V max gain = .13V  
Stage 1 looks OK to me, but Stage 2 looks funny at max gain. At max gain, there is very little voltage dropped across the plate resistor (the plate voltage is very close to the supply voltage) which indicates there is very little current being drawn by the tube (I learned that from Carl Gigun and Mike Setzer). It appears the bias is way too negative. How did it get so negative?  
Carl Gigun suggested I should try a different 12AX7. I did and got the same results. He also suggested checking the tube socket, I haven't done that yet.  
I wonder if it's because I'm using the gain control as the grid resistor? But that's the way a 5F1 champ does it. That's the way I did it with the Electar PCB and that worked.  
I figure that the gain control is suppose to limit the signal. But won't it also screw up the bias of the tube too? As I understand it, in cathode biasing, the grid is negative with respect to the cathode because a resistor is added between the cathode and ground. Now with the gain control at zero, the grid is connected to ground, so it seems like you would have a base-level bias voltage. In my case, -2.05V. But if you turn the gain control up, then the grid is no longer directly connected to ground, so wouldn't the bias be less negative? But then again, if it is less negative, more current flows, which means the voltage drop across the cathode resistor is greater than it was before. So I suppose that would counteract the effect of the grid being above ground. But in my case, turning the gain way up actually caused less current to flow!  
I'm confused.  
- Clint
5/27/2005 2:54 PM

I just read RG Keen's tube amp debugging guide. He mentioned that squealing could be due to a bad coupling cap leaking DC from the prior stage into the grid of the next stage. The leaking DC voltage would screwup the bias. He mentioned that the grid voltage should be at zero for cathode-biased amps. I'm assuming he means DC grid voltage. My measurements showed it was zero with the gain control at zero and around -5 volts at maximum. Its not a positive value, but its not zero either. If I did have leaking DC voltage coming into the grid, having the gain control at zero would've blocked it, while turning the gain up would've let it through. I'll check it out tonight.  
- Clint
5/27/2005 6:57 PM

Changing the capacitor didn't fix the problem.  
Actually, I made a mistake when I said RG Keen's debugging guide listed a bad coupling cap as a cause for squealing. That was listed as a cause for "sound cuts out or squawks on loud notes".  
- Clint
5/28/2005 4:28 AM

Clint, you didn't mention, how much your supply voltage is to your preamp tube - but it must be around 420V, if I've calculated right. That's btw. pretty strong!  
Anyway, I can't see anything on the schematic, from where a negative voltage couldt arise, what's found on your second grid.  
So the only idea, which arises in my head is, that maybe an arcing in your preamp tube socket has created an creeping current path (don't know, how to describe it better in English :)) with an unfortunately reversed diode effect, so that maybe from one of your AC heater pins couldt be creeping a negative voltage to your grid pin.  
When your gain is set to zero, then this negative voltage is shunted to ground, but when you're cranking up the gain pot (and you're increasing the grid's resistance seen to ground), then this negative voltage finds its way to your grid pin again and decreases the current through your tube finally to nearly zero.  
Although in this case it wouldn't be a nice negative DC, but with huge ripple. In this case you'd hear a strong hum, as more, as more you crank up the gain pot - but maybe that's the buzz, you're hearing.  
It's really stranged and I never had a problem like yours, so all I've written above is only theoretical - but it's also the only idea I got, when you're telling about a negative voltage on your grid. Maybe the "right" idea?  
BTW: The C5 into your tone stack isn't really needed and you can remove it too.
5/28/2005 10:14 AM

Thanks for the response novosibir.  
Today I measured the power supply voltage and it was 416 volts and the plate voltage was 400 with the gain control at max. So it is different today, but it is still pretty high.  
I guess I'll try changing the tube socket. Carl Gigun (I think) had mentioned that in the thread I had started under the Design & Construction forum.  
The noise is definitely not a hum. It is a buzz, a squeal. Its frequencey changes as you change the gain knob. When it is plugged into a speaker, it sounds like really bad distortion. With the speaker unplugged it sounds like a buzz, squeal.  
I noticed if I turn the master volume down, you can't hear it.  
- Clint
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