Broken kathode ground damaged amp?

[Jon Snell]

The EL36 or 6P31S was designed as a line output valve for monochrome television receivers, back in 1940, with a maximum 7.5kv peak anode voltage so no issues there. (There were two types of EL36 in production; an Audio version with no top cap and the type with a top cap. Like the 807 which is a 6L6 with a top cap, allowing for a much higher anode voltage with no fear of arking on the valve base).
I would suggest the hum ... is it 50 or 100 HZ I wonder ... .
50 HZ is mains hum, maybe the phase splitter valve has an unbalanced heater/cathode issue or the 100R heater balance pot requires some adjustment or is faulty.
100 HZ points to smoothing issues. Check the ripple on the smoothing capacitors and compare it with the hum ripple.

 

[Little Jay]

The EL36 or 6P31S was designed as a line output valve for monochrome television receivers, back in 1940, with a maximum 7.5kv peak anode voltage so no issues there. (There were two types of EL36 in production; an Audio version with no top cap and the type with a top cap. Like the 807 which is a 6L6 with a top cap, allowing for a much higher anode voltage with no fear of arking on the valve base).
I would suggest the hum ... is it 50 or 100 HZ I wonder ... .
50 HZ is mains hum, maybe the phase splitter valve has an unbalanced heater/cathode issue or the 100R heater balance pot requires some adjustment or is faulty.
100 HZ points to smoothing issues. Check the ripple on the smoothing capacitors and compare it with the hum ripple.

This is the EL36 with the top cap.

The hum is definitely 100Hz, when I play a 100Hz tone with my iPhone's tone generator app, it sounds identical. Also my FreqenSee app shows a peak at 100Hz.

Could the expensive F+T caps already have gone bad?

The problems occured after I played a gig outside and temperatures went down quickly when the sun set. All equipment was moist with dew.. I let the amp dry but at the next gig a week later it stopped playing after some intermittent on/off playing. Back home I discovered the broken connection at the cathode trim pot. After resoldering that I don't recall the hum was there, I think it was there suddenly some days ago after just playing the amp at home... but now it's pretty loud.

 

[Little Jay]

I do consider a faulty trim pot also....

 

[Jon Snell]

I would replace the trim pot first, (anything will do to test it), before replacing the EL36s but it could be an EL36 that is going soft.
Just a thought ... you have tried bridging the first smoothing capacitor with another to test I presume. Just as a temporary measure, to check the main smoothing is adequate?
Hopefully your F&T supplier wasn't fleabay!

 

[Little Jay]

Maybe it's worth mentioning that the amp has always started up with this pretty loud hum, that went away like after 5 seconds, as if the smooting caps kicked in with a delay or something. But now it just doesn't go away anymore....

 

[Jon Snell]

I would try a new set of EL36s as your description has the classic sign of a soft output valve.
I know it goes against the grain but the Russian 6P31S is an excellent replacement. Available from Ukraine suppliers.

 

[Jon Snell]

Electron tube 6P31S (6П31С) = EL36

beam tetrode 6P31S - (NOS)

retrostore.eu

If you haven't got a good supplier and these are very inexpensive!

 

[Little Jay]

I would replace the trim pot first, (anything will do to test it), before replacing the EL36s but it could be an EL36 that is going soft.
Just a thought ... you have tried bridging the first smoothing capacitor with another to test I presume. Just as a temporary measure, to check the main smoothing is adequate?
Hopefully your F&T supplier wasn't fleabay!

Hahaha! No, the supplier was a reputable one ;-)

I did bridge with a 50uF cap by hooking the positive side to the first step down resistor (point A in the schematic) and negative to ground. Is that the right way? It did not decrease the hum....

 

[Little Jay]

Oh and I have a spare set of EL36s and I did some swapping but to no prevail.... so I don't think the tubes are faulty. That leaves the trim pot I guess...

 

[Little Jay]

And I have nothing against those Russian tubes, I use the 6P3S in my 6L6 amps (A Twin Reverb and a Blues Deluxe)

 

[Little Jay]

One more thing: one of the tube feet of the EL36 is giving me a hard time, very hard to get the tube in and out and one the pin holders sits very loose in the tube base, it actually comes out when I pull the tube. I wonder if that can cause the hum? 'Chop-sticking' the tube feet did not reveal (or solve) any problems.

 

[Little Jay]

Electron tube 6P31S (6П31С) = EL36

beam tetrode 6P31S - (NOS)

retrostore.eu

If you haven't got a good supplier and these are very inexpensive!

Oh wow, that is indeed very inexpensive! It pays off to have an amp with uncommon tubes I guess......

 

[Little Jay]

Hallelujah! I seem to have made a breakthrough!

Indeed it is a smoothing cap problem... the F&T caps are radial and of the type with a plastic nut to hold them:

They are only grounded by contact with the chassis. On a hunch I loosened their fastening nuts and rubbed/rocked them a bit hoping to clear any oxydation between them and the chassis away and this did the trick!! The hum is now way less loud and now reduced to a more normal background hum! (*edit: with the trim pot I was able to dial it out almost completely!!)

Which reminds me what a sh*tty type of fastening system this is for smoothening caps....

Thank you all and especially Jon Snell for thinking with me! You sent me in the right direction!

 

[dan40]

They are only grounded by contact with the chassis.

I'm not sure I understand what you mean. The body of the capacitor should not be providing the ground. The ground connection should be coming from the solder lug with the - symbol. Is it possible that the solder joint is bad and moving the cap a bit helped to restore the electrical connection to the ground wire?

 

[Little Jay]

I'm not sure I understand what you mean. The body of the capacitor should not be providing the ground. The ground connection should be coming from the solder lug with the - symbol. Is it possible that the solder joint is bad and moving the cap a bit helped to restore the electrical connection to the ground wire?

No, these are weird, they only have possitive terminals and the ground connection is solely provided by contact with the metal of the chassis. And aluminium and steel don’t react great to eachother…. It’s not the best solution, I agree….

I did consider replacing them with the type with the solderable negative terminal:

But then I would also need to install brackets to hold them and drill holes for that. It would be a better solution, but now I know that if the noise level increases over time I have to clean the contact of the rims with the chassis…