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In Reply to: RE: Help with Beveridge direct-drive amplifier oscillation posted by fredtr on March 22, 2021 at 08:45:17
The amplifier oscillates somewhere in the circuit that feeds the stators. The oscillation can be detected via an oscilloscope probe held within an inch or two of the 7.5M resistor (R1), seen in the schematic attached. (Can't make contact with the output circuit due to the very high voltages and lack of an HV probe.) The oscillation can also be detected, in phase with the field detected around the 7.5M resistor, with a probe touching the 1K resistor (R2) where it exits the (low voltage) input board. The wave form resembles a saw tooth. Frequency = 200kHz. Reasons to believe the oscillation arises in the output circuit: (1) I swapped input boards between the good and bad amplifier, and the problem remains, and (2) Using a dual trace oscilloscope, with one probe placed near the 7.5M (R1), and the other probe placed on the 1K resistor (R2), I started up the amplifier from dead cold. There is a delay before the onset of oscillation, and both channels detected oscillation at the exact same time. Wave forms are in phase.
The other amp of this stereo pair is configured exactly the same and does not oscillate, plays music nicely. I cannot find any difference between the two amplifiers in parts or circuit tracings. (I have actually replaced nearly every part on the oscillating amplifier output board, to no avail.)
There are two differences between both of my amplifiers and the circuit shown in the schematic. My amplifiers do not have the LED shown in the circuit that feeds g2 on the upper side of the schematic. And instead of a 4.7K resistor as shown, both of my amplifiers have 10k resistors going straight to ground. Also, R201 in the schematic is 22K, not 10K, in both the good and the oscillating amplifier. Schematic dated 1978. Inside my amplifiers, someone wrote "1979".
Thank-you for posting the schematic, I like unusual problems. My suggestion is poke it with a stick, I better explain that. When I try to figure out a problem where I don't seem to have enough information, but do have some guesses, I try some change that will tell me if I am on the right track or not, or just provide more info about the nature of the problem.
Looking at the schematic, it is possible that the oscillation is caused by the cathode circuit V201 and V203, specifically the varistor, 470pF cap, resistors and diode. The changes you could make, to see if it impacts the problem is to try a lower noise diode (which may be triggering the oscillation), or change the RC time constant, by changing the value of the 470pF cap or varistor which has a capacitance of 500pF. If you try a change, and the oscillation is still there, check to see if the frequency of the oscillation has changed.
Thank you for your excellent ideas. It is interesting to me, because I have an older schematic, dated 1977, in which the value of that ceramic capacitor is 290 pF, not 470 pF. Both of my amplifiers in both positions have a 470 pF capacitor in place. In addition, that is the one part that I have not double checked in recent months. And I need to take another look at that. Also, I had planned to just try substituting R 201 with a 10 K resistor as shown on the 1978 schematic I provided. Right now I have 22K, as I mentioned. As to a lower noise diode, I am using UF4007's, instead of 1N4007's. What would you use after having tried that?
You could try a soft recovery diode, but I would be inclined to focus on the RC time constant, trying different values for C202 and C204 or the varistor RV201 and RV202. Regarding R201/R203, I had missed that. That is a good question why one is 10K the other 22K. I agree with you, I would experiment with changing those. I would guess that it was simply a mistake on the schematic, someone meant to change them both to the same value, and only one was changed. But geez I have seen stranger things.
As you say, I have been assuming that the value shown for R201 (10K) was in error on the schematic. Especially since on both of my actual amplifiers, R201 = R203 = 22K, with the attendant C202 = C204 = 470pF. On the 1977 schematic, C202 and C204 are 290pF. So they were clearly tinkering with that part of the circuit.
1977: uses 47K and 290pF, both sides
1978: uses 22K and 470pF, both sides (assuming 1978 schematic is in error)
It looks like the other thing they changed were resistors 206 - 210, which will affect amplifier loading and the feedback path. Are these resistors 1.0M or 2.2M in your amplifiers? Have you experimented with different values for these resistors?
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