Help with sherwood S-5000 hum

[jera]

Recently picked up and S-5000 of unknown working condition. Upon first inspection measured some of the electrolytic caps and noticed mostly all where high. So I ordered an new set from Hayseed ham fest and installed them. There was only one other electrolytic cap a 50uf 6v cap and I replaced that as well. I cleaned all the switches and tube sockets and brought it up on a variac. I got music out of both channel but I also got a hum from both channels? The hum is present in all selection phono, aux,tape etc. The hum also increases with the volume ? I rechecked all of my connections and they seem fine. I have a parts unit although slightly new version it appears everything has been put back the way it was . Any ideas of what I should be looking for?

 

[gadget73]

Does it hum with the volume full down or no?

Also, is it 60 or 120 hz? There is probably a phone app that will tell you if you're not sure. That will help narrow down what sort of issue it might be.

also, which S-5000 ? The 7189 one or one of the half dozen others?

 

[jera]

This is that early S- 5000 with the 7189 output Tubes. The hum appears to be there even with the volume down but Definitely increases with volume. Sounds like a 60Hz tone but I could be wrong . What is the app you are referring to ? I would like to check it against that .

 

[cademan]

Try removing the preamp tubes and see if it still hums.

 

[Fairlane]

There is a hum balance adjustment on this early version. Move the switch behind the left output transformer to test and adjust the shaft below the switch.
It is a split shaft and the top does one channel and the bottom of the shaft adjusts the other channel. Turn them to the quietest you can hear then put the switch back to normal.

Report back. Good luck.

 

[jera]

Ok I removed the preamp tubes and it still hums.
I adjusted the bal/ hum pot and it may have gotten quieter but very marginally but the hum is still quite loud.

 

[gadget73]

I don't have a specific phone app in mind, but there are free spectrum analyzers that will ID stuff.

Does it continue to hum with the phase inverter tubes pulled? If it does, you may be dealing with really badly matched output tubes. I'd expect that to be 120 hz hum. Same if it was power supply noise. If you have heater-cathode leakage in the inverters, it will stop when those are pulled and it would be 60hz.

 

[jera]

Well I checked it with the 7199 phase inverter removed and the hum was very low . I also checked it with a another set of unknown quality 7189 and a set of el84 and the hum with all tubes in was still loud. I Also noticed that as I turn it up the volume the hum increases more in the right channel then left but still Present in both

 

[gadget73]

Try some different 7199's if you have any.

Also maybe worth trying is grounding the pentode grid on the 7199 (pin 7) to see if it stops. If the hum is the same, its a problem in that tube, probably heater-cathode leakage.

Might also try balancing the tubes with the 7199's out to see if things get quieter. if you can get it quiet with the hum pot while the 7199's are out and grounding the grid of the 7199 doesn't silence it, that pretty much puts a nail in the 7199. They're prone to H-K problems just because of the voltages all that stuff operates. I'll spare you the long drawn out discussions about it, but basically if you can minimize that difference it makes the tube last longer.

 

[jera]

Ok so I tried another pair of 7199 and it still hummed.
I did try and balance out the hum with the 7199 out and it did get quieter but was still there.
Not exactly sure if I did this correctly but I just ran a wire from pin 7 on those 7199 to a ground lug . With that I was able to adjust the hum very low .

 

[gadget73]

ok, so it sounds like the power amp behaves if grounding the 7199 grid makes it quiet. That would indicate the source of the noise is upstream of the 7199. See if there is a ground off between the volume control and the 7199. Not sure if that has shielded leads, a ground wrap, or just nothing. Shields or wraps I'd expect grounded only at one end. Also make sure the ground connections to the volume pot are good, and if it has a resistor to ground off pin 7, that needs to not be open. I don't have the schematic in front of me to see what exactly it has going on.

 

[jera]

I checked and replaced the resistor to ground on pin 7 on both of the 7199. It was at the high side of the tolerance and just barley out of spec. After I checked around the amp to see if I could find any ground issues with the volume pot . Nothing appears loose. I powered it up and after a few minutes the hum returned ?

 

[gadget73]

so it starts quiet and then gets worse? That could be output tubes going out of balance.

Did you figure out if its 60 or 120 hz ?

 

[jera]

It is 60hz . I have noticed that I can get the hum to be very low with bal/hum pot but still there . It also seems that it doesn't have as much punch as it initially did. So I will try and order some new output tubes and see if I can find some strong tested 7199 as I can not say if any of mine are any good without a tube tester. It looks like I should replace those 7189 with a set of 6P14P. Are all variation of those are plug and Play or should I be looking for a certain type ?

 

[Patrice B]

You need the 6P14P-EV, 7189 specs. Cyrillic name is 6Π14Π-EB.

 

[gadget73]

Or the EL84M, same thing.

 

[GuyNoir]

6P14P-ER are only a little more money than the EV iteration, and are rated for 10,000 hrs.