Help with sp14 left channel 60hz hum issue

Hey all, I haven't been around in a while and was wondering if anyone could offer some advice regarding this problem. I have a SP14(Purchased pre assembled) and two M125 Monoblocks which I built myself. The Preamp and monoblocks are about 6-7 years old. The monoblocks are still working great, and I recently got them a new set of tubes as the originals were getting up there in hours. My problem is with the SP14, lately it's developed a moderately loud 60hz hum in the left channel. The hum isn't related to the monoblocks as they work fine with my schitt passive preamp. It's purely related to the left channel on the SP14, and it's a pretty loud hum. It's easily audible throughout the room with 85db speakers. Sounds almost like the filter capacitor for that channel may have failed, but IDK how to test for that and don't want to just assume so and replace the filter cap. I've checked all the wiring and solder joints on the preamp and it all seems fine. If anyone can offer some advice it would be great, thanks.

Corona wrote:Hey all, I haven't been around in a while and was wondering if anyone could offer some advice regarding this problem.  I have a SP14(Purchased pre assembled) and two M125 Monoblocks which I built myself.  The Preamp and monoblocks are about 6-7 years old.  The monoblocks are still working great, and I recently got them a new set of tubes as the originals were getting up there in hours.  My problem is with the SP14, lately it's developed a moderately loud 60hz hum in the left channel.  The hum isn't related to the monoblocks as they work fine with my schitt passive preamp.  It's purely related to the left channel on the SP14, and it's a pretty loud hum.  It's easily audible throughout the room with 85db speakers.  Sounds almost like the filter capacitor for that channel may have failed, but IDK how to test for that and don't want to just assume so and replace the filter cap.  I've checked all the wiring and solder joints on the preamp and it all seems fine.  If anyone can offer some advice it would be great, thanks.

Good afternoon Corona.

The first thing I would do is check your RCA cables to make sure one of the cables isn't bad or that it is making good contact on the negative side of the RCA Jack.

What generally happens is when you plug the RCA cable into the Jack if the negative ground is not seated correctly and just the positive side of the lead makes good contact it will create a signal but there will also be a hum created from the lack of grounding.

If you have already done this then yes one of the filter capacitors may be starting to leak and short creating a hum in the signal path.

I would also check the driver tubes to make sure all of them are good.

Also I would add if you do have a electrolytic capacitor going bad and starting to leak and short if you don't have a lot of equipment but you do have a simple analog multimeter I would set it on 10k ohms to see if one of the capacitors is testing like a resistor. What happens is when electrolytic capacitors start going bad they start to leak and short and create a short circuit. If you use an analog multimeter and measure the capacitor and it starts to read like a resistor it generally means the capacitor is bad.

Usually using an analog multimeter the resistance will go up and then back down. In other words the capacitor is charging and discharging. This is normal.
If the capacitor doesn't discharge completely or doesn't slowly go all the way back down that means it could have a short starting to form and the capacitor will not filter properly.

Found this on YouTube to help if you have a electrolytic capacitor going bad.

Analog Meter Electrolytic Capacitor Testing: https://youtu.be/SajwOxjVcuE

Tom Pickett wrote:

Corona wrote:Hey all, I haven't been around in a while and was wondering if anyone could offer some advice regarding this problem.  I have a SP14(Purchased pre assembled) and two M125 Monoblocks which I built myself.  The Preamp and monoblocks are about 6-7 years old.  The monoblocks are still working great, and I recently got them a new set of tubes as the originals were getting up there in hours.  My problem is with the SP14, lately it's developed a moderately loud 60hz hum in the left channel.  The hum isn't related to the monoblocks as they work fine with my schitt passive preamp.  It's purely related to the left channel on the SP14, and it's a pretty loud hum.  It's easily audible throughout the room with 85db speakers.  Sounds almost like the filter capacitor for that channel may have failed, but IDK how to test for that and don't want to just assume so and replace the filter cap.  I've checked all the wiring and solder joints on the preamp and it all seems fine.  If anyone can offer some advice it would be great, thanks.

Good afternoon Corona.

The first thing I would do is check your RCA cables to make sure one of the cables isn't bad or that it is making good contact on the negative side of the RCA Jack.

What generally happens is when you plug the RCA cable into the Jack if the negative ground is not seated correctly and just the positive side of the lead makes good contact it will create a signal but there will also be a hum created from the lack of grounding.

If you have already done this then yes one of the filter capacitors may be starting to leak and short creating a hum in the signal path.

I would also check the driver tubes to make sure all of them are good.

Thanks, I'll check the RCAs when I get home although I'd think that would effect the passive preamp as well.  I recently put in new tubes in the SP14 (NOS RCA 6sn7's) and a new 6ax5 rectifier.  Just to check I'll go back to the old set of 4 Electroharmonix 6sn7s and 6x5 rectifier.  If all this fails I'll have to assume a bad filter and replace it for the problem channel. I only have a digital multimeter btw, will that method you described still work?

Yes a digital Multimeter will work but there are some small differences. Let me see if I can find a video on YouTube that you can watch that will be much better than I can explain it.

Yes it may not be the RCA's just make sure that the negative post on the jacks are nice and clean if so then I would look in a different area.

Changing the tubes will definitely give you an answer whether if it's the tubes or not.

How to Check a Capacitor and Measure Capacitance with a Digital Multimeter:

https://youtu.be/qDABYKoVO4Q

How to Test a Capacitor with a Digital Multimeter and LCR Meter

https://youtu.be/wrJXdJbX5zg

I run into the same problem when I rebuilt a SS amp, and I could not remember whether or not the RCA inputs were grounded, so I did not ground them, the result was a very loud hum, sending the amp into a thermal shutdown. So it would not hurt to check for that, as TOM recommends.

Found the problem, apparently the amp doesn't like running 6ax5 tubes instead of 6x5GT tubes. Seems like it should be compatible, but switching one of the 6ax5s into the rectifier slot in place of the 6x5GT causes the hum issue. Very weird, the amp works otherwise with the better 6ax5 tubes. Idk why they would make it hum in one channel only.

Corona wrote:Found the problem, apparently the amp doesn't like running 6ax5 tubes instead of 6x5GT tubes.  Seems like it should be compatible, but switching one of the 6ax5s into the rectifier slot in place of the 6x5GT causes the hum issue.  Very weird, the amp works otherwise with the better 6ax5 tubes.  Idk why they would make it hum in one channel only.

Awwwww....... The old switching the tubes trick.... In most cases.... Never fails lol

I don't think it's the tube configuration Corona. I think you might have a bad tube.

Here is the tube datasheet. Pin 3 Plate Diode 2 Could be weak or bad or Pin 5 Plate Diode 1 Could be weak or bad. ether one could cause one or both channels to be weak or hum.

6AX5 or 6AX5-GT Datasheet pdf https://frank.pocnet.net/sheets/049/6/6AX5GT.pdf

A video proving you may have a bad tube Corona.

Two Types of Tube Amplifier Hum and How to Determine the Source

https://youtu.be/GrVtX0QGNls

Tom Pickett wrote:I don't think it's the tube configuration Corona. I think you might have a bad tube.

Here is the tube datasheet. Pin 3 Plate Diode 2 Could be weak or bad or Pin 5 Plate Diode 1 Could be weak or bad. ether one could cause one or both channels to be weak or hum.

6AX5 or 6AX5-GT Datasheet pdf https://frank.pocnet.net/sheets/049/6/6AX5GT.pdf

Interesting, I actually have a pack of 4 NOS 6ax5gts and I've tried two of them with the exact same issue. What's weirder is how the issue occurs. As the preamp warms up it briefly starts humming, then the hum goes away and about 10 seconds later the left channel hum comes back really loud. With the 6x5GT there are no issues.

I think you resolved the problem by using a 6X5 instead of a 6AX5 rectifier tube.

The 6AX5 rectifier is not completely interchangeable with a 6X5.
It has a somewhat greater forward voltage drop than the 6X5, but there is a lot of overlap in the tube specs, and the 6AX5 will work in a lot of cases if you have one that is on the low side for forward voltage drop.

The SP14 creates two somewhat separate power supplies by wiring the full wave rectifier to create two half wave rectifiers - one for each channel.
It also floats the regulated 6.3AC volt  filament heaters at  20 to 40 volts DC above B+ to minimize HUM.
If your rectifier forward voltage drop is too large, your filament voltages will not be floated high enough to eliminate the Hum.

My SP14 was kit built and the 6X5 is specified as the rectifier tube.
During the initial  checkout procedure there is a requirement that B+ voltage be 265-272 Volts DC.
I checked with Roy Mottram about the use of the 6AX5 rectifier and he saw no problem if the B+ voltage requirement was met.

In my case the B+ was met and I heard no difference between the two, but I see no reason not use the 6X5 since they're cheap reliable and plentiful.

Pablo wrote:I think you resolved the problem by using a 6X5 instead of a 6AX5 rectifier tube.

The 6AX5 rectifier is not completely interchangeable with a 6X5.
It has a somewhat greater forward voltage drop than the 6X5, but there is a lot of overlap in the tube specs, and the 6AX5 will work in a lot of cases if you have one that is on the low side for forward voltage drop.

The SP14 creates two somewhat separate power supplies by wiring the full wave rectifier to create two half wave rectifiers - one for each channel.
It also floats the regulated 6.3AC volt  filament heaters at  20 to 40 volts DC above B+ to minimize HUM.
If your rectifier forward voltage drop is too large, your filament voltages will not be floated high enough to eliminate the Hum.

My SP14 was kit built and the 6X5 is specified as the rectifier tube.
During the initial  checkout procedure there is a requirement that B+ voltage be 265-272 Volts DC.
I checked with Roy Mottram about the use of the 6AX5 rectifier and he saw no problem if the B+ voltage requirement was met.

In my case the B+ was met and I heard no difference between the two, but I see no reason not use the 6X5 since they're cheap reliable and plentiful.

Thanks for the technical explanation, it makes sense now what might be happening. As for why I changed over to a 6AX5, it's because on various forums I heard a bunch of horror stories about the 6x5 being prone to shorting and destroying your power transformer. They suggested that the 6AX5 is generally interchangeable with the caveat that it takes double the filament current(which the SP14 should have no problem with). Out of curiosity, what brand of 6AX5 rectifier are you using? I assume that some brands or makes of the tube have lower forward voltage drops than others. I got the following type: 1967 Black Plate RCA 6AX5GT Rectifier Tubes TV-7 Tested NEW
https://www.ebay.com/itm/324434820552