I have a problem with an Ampex 601 preamp, when I turn up the gain past about half way it starts oscillating (MIC. REC. LEVEL on the schematic, just after the first tube, which is a 5879).
I tried changing the 5879 tube, and cleaning the socket. I have already changed the two proceeding 10uF power supply filter caps. Using a cheap oscilloscope, after the input transformer it looks fine, but anywhere after the tube I get the bad signal. I plan to also change the 20uF filter capacitors as well.
I then changed the potentiometer itself and the .1uF capacitor before it, to no avail.
As it happens when I turn the gain pot up, what does this indicate? Does this necessarily mean it must be a problem before the potentiometer? I am quite a novice, but since the resistance increases, does this mean that more current is drawn, so it could be a power issue? The only thing I haven't changed now around that tube is the cathode bypass cap, and the resistors, but before I proceed what do you think is most likely to cause such a problem? I guess the second tube I tried could be wrong too, but it seems unlikely, and changing it produced no effect.
Thanks
I tried changing the 5879 tube, and cleaning the socket. I have already changed the two proceeding 10uF power supply filter caps. Using a cheap oscilloscope, after the input transformer it looks fine, but anywhere after the tube I get the bad signal. I plan to also change the 20uF filter capacitors as well.
I then changed the potentiometer itself and the .1uF capacitor before it, to no avail.
As it happens when I turn the gain pot up, what does this indicate? Does this necessarily mean it must be a problem before the potentiometer? I am quite a novice, but since the resistance increases, does this mean that more current is drawn, so it could be a power issue? The only thing I haven't changed now around that tube is the cathode bypass cap, and the resistors, but before I proceed what do you think is most likely to cause such a problem? I guess the second tube I tried could be wrong too, but it seems unlikely, and changing it produced no effect.
Thanks
Do you have a schematic? Pictures? Are the values of the new parts the same as the old ones? Did you verify/measure this? Did the preamp oscillate because of the mods or after the mods?
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I have attached the schematic.
It's been like this since I got it (I just used it with the gain down low but it's not useful for quiet sources), and as far as I can tell it's mostly original. It did this before I changed anything. The values of the new parts are the same. Since it's from the 50s/60s I guess it's caused by an old part... but which one?
I don't know if I have the equipment to measure it properly. I call it an oscillation but I don't know for sure - all I can say is that with the gain past halfway the VU needle goes up to the max and fluctuates up and down a bit, it makes a brr sound like this: https://ufile.io/fytp6m14 (it starts as soon as I turn the pot up)
It's been like this since I got it (I just used it with the gain down low but it's not useful for quiet sources), and as far as I can tell it's mostly original. It did this before I changed anything. The values of the new parts are the same. Since it's from the 50s/60s I guess it's caused by an old part... but which one?
I don't know if I have the equipment to measure it properly. I call it an oscillation but I don't know for sure - all I can say is that with the gain past halfway the VU needle goes up to the max and fluctuates up and down a bit, it makes a brr sound like this: https://ufile.io/fytp6m14 (it starts as soon as I turn the pot up)
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Seems like the 10uF supply decoupling capacitor is dried out. If you already changed that one,
try decoupling the pentode screen to ground with 0.1uF.
try decoupling the pentode screen to ground with 0.1uF.
Thanks, yes I changed the final two 10uF filter capacitors, but it did not fix the problem. I can try decoupling the screen, although I am a little reluctant as I suppose there must be a defective component without changing the circuit from the original design?
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Does the oscillation really not happen at all until above a certain position of the gain control?
That sure seems like oscillation through the power supply. Is this with the inputs shorted?
Will the position of the monitor selector switch change the behavior?
Does directly grounding the pentode's input grid change the behavior?
That sure seems like oscillation through the power supply. Is this with the inputs shorted?
Will the position of the monitor selector switch change the behavior?
Does directly grounding the pentode's input grid change the behavior?
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Yes, after position 5 on the input it will reliably produce the oscillation.
I tried a 60uF electrolytic to see if the PSU filter is a problem (+'ve to +'ve side of last 10uf power filter cap, and negative to ground). I also tried the 60uF from pin 3 of the pentode to ground (in case 40uF is bad). In both cases the oscillation still occurs after about halfway on the pot. I also clipped in 0.33uf from pin 7 (pentode screen?) to ground, and there was no change in behaviour.
I tested with multimeter to ensure continuity with component leads after clipped in, so it seems odd none of these solved the issue, as I suppose that leaves only the tube itself (and the second tube I tried), unless the resistors can be a cause?
I grounded the pentode input grid (pin 1) as suggested, and no change. How do I short the inputs? I have nothing plugged in (do you mean connect 2 and 3 of XLR together? - when I do this it still oscillates after position 5)
Switching the monitor switch from input to tape position there is no oscillation. And removing the pentode tube completely there is also no oscillation. I am assuming this means the issue is early in the input preamp, but I'm a bit baffled, I don't know if there is much left to try.
I tried a 60uF electrolytic to see if the PSU filter is a problem (+'ve to +'ve side of last 10uf power filter cap, and negative to ground). I also tried the 60uF from pin 3 of the pentode to ground (in case 40uF is bad). In both cases the oscillation still occurs after about halfway on the pot. I also clipped in 0.33uf from pin 7 (pentode screen?) to ground, and there was no change in behaviour.
I tested with multimeter to ensure continuity with component leads after clipped in, so it seems odd none of these solved the issue, as I suppose that leaves only the tube itself (and the second tube I tried), unless the resistors can be a cause?
I grounded the pentode input grid (pin 1) as suggested, and no change. How do I short the inputs? I have nothing plugged in (do you mean connect 2 and 3 of XLR together? - when I do this it still oscillates after position 5)
Switching the monitor switch from input to tape position there is no oscillation. And removing the pentode tube completely there is also no oscillation. I am assuming this means the issue is early in the input preamp, but I'm a bit baffled, I don't know if there is much left to try.
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Try a bypass capacitor across R118, in case the 40uF @ 25V capacitor across it is bad.
Around 10uF should still be ok.
Is C103A one of the capacitors that were replaced or jumpered with another capacitor?
Around 10uF should still be ok.
Is C103A one of the capacitors that were replaced or jumpered with another capacitor?
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I think in my case I should clarify that the unit is only being used as a microphone preamp, so I don't think tube V103A is in use that R118 relates to, as the preamp is disconnected from the tape machine. As I understand, when used as a mic preamp, only V101, V102 and V103 are in use when monitor is set to input. I suppose V103 and R118 cannot be causing the issue (actually V103 socket is currently empty, though I have a second 601 with V103 in place and having it in or out doesn't seem to change anything)
Scrap that, sorry - R118 and C101B are used for V102A as well aren't they? I'll try it
Scrap that, sorry - R118 and C101B are used for V102A as well aren't they? I'll try it
Yes, there could be unwanted feedback between stages if that bypass capacitor is bad.
But not if V103 is gone.
But not if V103 is gone.
If that doesn't work, connect the scope to pin #1 of the monitor selector switch.
Is the oscillation still visible on the scope when the switch is in the tape position?
If so, that would rule out the circuits after the switch.
Is the oscillation still visible on the scope when the switch is in the tape position?
If so, that would rule out the circuits after the switch.
Thanks, yes I still detect the noise on the scope on pin #1 with selector on tape.
cap on R118 to ground didn't change anything I am afraid, but yes V103 is gone.
cap on R118 to ground didn't change anything I am afraid, but yes V103 is gone.
Ok, then the problem must be in the first three stages of the mic amplifier.
Have you tried adjusting or cleaning the line record level pot? The wiper is
the only thing that gives the third tube's grid a ground return.
Have you tried adjusting or cleaning the line record level pot? The wiper is
the only thing that gives the third tube's grid a ground return.
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Hi, I thought you were on to something then, the line record level pot is on the back on this unit and needed a good clean, unfortunately it hasn't solved the problem though. I also tried directly grounding after the 270k resistor there, and that did not change anything either.
I have cleaned the other pots too. I clipped in a capacitor over R111 and also tested to make sure rec. cal. and mic. rec. level. pots are grounded. I swapped out V102 as well, and I cleaned the socket. Still no change. I can't seem to find anything prodding about either.
I have cleaned the other pots too. I clipped in a capacitor over R111 and also tested to make sure rec. cal. and mic. rec. level. pots are grounded. I swapped out V102 as well, and I cleaned the socket. Still no change. I can't seem to find anything prodding about either.
Make sure C102 isn't leaky. There should be zero volts after it. I've been tripped up by leaky coupling caps because they shift the tube bias. A cap can pass every test, but the amount of leakage to create a significant voltage on a high impedance can be quite small. Usually it just causes excessive THD. Also, check the wire routing to be sure signal isn't being coupled back to the input. Oscillation with increasing gain suggests some feedback path.
Yes, sorry for delayed reply, I had already tried swapping C103, but no luck.
I finally got around to taking some voltages of the coupling caps. I wanted to wait until I could borrow a better meter as mine is not the best. C102 I have already tried swapping as well, but I have taken a VDC reading after it anyway (the new cap is still in there). I took the VDC readings at a gain level before oscillation occurs, and at a gain level where oscillation is present.
C102 before oscillation 0 VDC, and at maximum gain (oscillation occurs) fluctuates from 3.2 to 4.2 VDC
C104 before oscillation 0.35 VDC, and at maximum gain (oscillation occurs) fluctuates from 13 to 19 VDC
C106 before oscillation 0.15 VDC, after oscillation this one is weird, I guess due to the oscillation - it seems to fluctuate (comparatively slowly) between -0.6 to +0.6 VDC at a gain setting just when the oscillation starts, but as the gain pot is increased the fluctuation is faster until after say 3 or 4 VDC (where it fluctuates between -3 to +3 VDC) and beyond that the voltmeter can't lock onto it as presumably it is fluctuating too fast or too widely.
I am not sure at all what these values indicate - how much above zero do you think suggests leakage? I did clip in a capacitor over C104 and then also (separately) over C106, to no avail, but I am not sure if that would actually work in preventing the oscillation if one of those capacitors is the cause?