Hello,
I am having a small but annoying problem. I have a Conrad Johnson PV-10A that has developed a high pitched squeal that is independent of volume and intermittent. It will usually go away for a while if I cycle the power on the preamp.
The problem has persisted through several pairs of tubes. I finally paid some serious(ish) cash for some very carefully screened, ultra quiet, new production 12AU7's. Squeal is still there. Changed amps. Still there.
Any thoughts?
Thanks for your help!
Cheers,
John
I am having a small but annoying problem. I have a Conrad Johnson PV-10A that has developed a high pitched squeal that is independent of volume and intermittent. It will usually go away for a while if I cycle the power on the preamp.
The problem has persisted through several pairs of tubes. I finally paid some serious(ish) cash for some very carefully screened, ultra quiet, new production 12AU7's. Squeal is still there. Changed amps. Still there.
Any thoughts?
Thanks for your help!
Cheers,
John
Last edited by a moderator:
Other thoughs...
I don't THINK it is interference from other electronic devices. If I try another tube pre in the same physical location, hooked up to the same equipment, even with the top off, I get no squeal.
It sounds very similar to when I installed some old NOS Allen-Bradley resistors as grid stoppers on my SE amp. THis situaltion cause a high-pitched squeal that was very similar. That problem went away when I swapped the original metal-film resistors back in.
Could it be bad grid stoppers? Could loose/dirty tube sockets cause such a thing? Power supply? I have read that the CJ power supplies can be finicky.
For convenience I have attached the schematic:
I don't THINK it is interference from other electronic devices. If I try another tube pre in the same physical location, hooked up to the same equipment, even with the top off, I get no squeal.
It sounds very similar to when I installed some old NOS Allen-Bradley resistors as grid stoppers on my SE amp. THis situaltion cause a high-pitched squeal that was very similar. That problem went away when I swapped the original metal-film resistors back in.
Could it be bad grid stoppers? Could loose/dirty tube sockets cause such a thing? Power supply? I have read that the CJ power supplies can be finicky.
For convenience I have attached the schematic:
Are the grid stoppers soldered directly on the tube socket pins?
I would check for a bad connection/component around the RIAA network.
I would check for a bad connection/component around the RIAA network.
Thanks for your reply!
All components are soldered to PCB. The squeal is independent of volume and independent of source selection. I would think it might have to do with the last stage rather than the RIAA.
All components are soldered to PCB. The squeal is independent of volume and independent of source selection. I would think it might have to do with the last stage rather than the RIAA.
Obviously, you are experiencing some oscillation behavior, but have eliminated the gain stage tubes by having already replaced them. Perform a careful visual check for anything that appears not right, such as a discolored component or odd looking soldered joints. Also, the tube sockets are likely suspects. Clean their contacts and make sure they still tightly grip the tube pins.
Generally speaking, the first active circuitry to check when troubleshooting electronics is the power supply. The fact that the circuit sometimes works normally after cycling power further suggests starting there. Since the supply is solid state you won't be able to conviently replace tubes to see if the problem is corrected. This will be much easier if you have access to an oscilloscope.
Generally speaking, the first active circuitry to check when troubleshooting electronics is the power supply. The fact that the circuit sometimes works normally after cycling power further suggests starting there. Since the supply is solid state you won't be able to conviently replace tubes to see if the problem is corrected. This will be much easier if you have access to an oscilloscope.
I would suspect if this is an older unit that it may be time to replace the supply electrolytics. I assume this is present on both channels in which case this would point to something common to both channels.
If you have a scope take a look at both the HV supply rail and filament supply. I have had more than one experience with an oscillating monolithic filament regulator and an audible squeal as a result. (Coupled through filament to cathode capacitance)
If you have a scope take a look at both the HV supply rail and filament supply. I have had more than one experience with an oscillating monolithic filament regulator and an audible squeal as a result. (Coupled through filament to cathode capacitance)
Hey! Thanks everyone!
Unfortunately, no scope. That would make things too easy!
I may replace all the SS PS components anyway. Not too expensive.
I'll have another look-over at everything. Nothing obvious, for sure. The squeal is low in volume, but enough to be distracting in quiet passages of music. I will look for a cracked solder trace near the tube sockets.
The weird part is that it is not usually in both channels at once, even when it has been, it wasn't to the same level. That's why I first suspected tubes. Hmmm...
Unfortunately, no scope. That would make things too easy!
I may replace all the SS PS components anyway. Not too expensive.
I'll have another look-over at everything. Nothing obvious, for sure. The squeal is low in volume, but enough to be distracting in quiet passages of music. I will look for a cracked solder trace near the tube sockets.
The weird part is that it is not usually in both channels at once, even when it has been, it wasn't to the same level. That's why I first suspected tubes. Hmmm...
Any possibility of borrowing a scope from someone? Random parts replacement is not necessarily the most effective approach to this problem and if it ultimately turns out to be a latent design problem in the pre-amp you'll never fix it. (I designed a pre-amp with a similar problem, I fixed it before the design got into the wild, however without a scope I would not have ever understood the origin of the problem which was not an obvious malfunction.)
A couple of threads over at the Audio-asylum about this.Think they rectified it by increasing the grid stopper(to 1k ohm if I recall).Stock stopper is only 45 ohm.
Thanks Brik!
I searched Audio Asylum. Strange that I didn't find anything where the problem was resolved. This is the direction I was thinking of going anyway, only I was planning on trying 100R carbon-comp. Might try some 1k's. Sure I have some in the parts box.
I searched Audio Asylum. Strange that I didn't find anything where the problem was resolved. This is the direction I was thinking of going anyway, only I was planning on trying 100R carbon-comp. Might try some 1k's. Sure I have some in the parts box.
Hi Brik!
I just noticed your last response! That is very generous! I need to make a Mouser order soon, anyway. So I will just tack on a couple carbon comps to that.
I just noticed your last response! That is very generous! I need to make a Mouser order soon, anyway. So I will just tack on a couple carbon comps to that.
Don't ignore the tube sockets. The worst "squeal" I have ever encountered was from a Fender 5E3 clone that I had made, It looked beautiful but it was an unruly beast. The squeal I finally figured out was from one of the preamp tube sockets.