Big Hum on "treble" pot on my 5e8-A

[pampeo]

Hello
i just finish my 5e8-a and i have a big hum on the treeble pot !
anyone have a solution ?
also i have a sound like a cruch ,not always but it apears after 5-10 min !
thanks a lot !

[pampeo]

a have a VVR but i don't think it's the cause
for the cruch ,perharps it's the brand new tube ?
i will check tonight with the vvr full and ground sheme on the treeble pot !

[pampeo]

so
i make a full reviews of my amp
the vol bright pot is not linear .....
the treeble pot buzzz
and a did hear any difference when a turn the presence pot !!!

[pampeo]

nobody have an issue ?
thanks in advance

[martin manning]

Are you using 12AY7's as in the original circuit? If so what brand? The cathode follower stage just in front of the treble pot has a high filament-cathode voltage that modern tubes may not tolerate. The original schematic shows 112V, and that is over the 90-100V maximum listed on the old data sheets. Failure of the insulation between the filament and cathode will produce 60Hz hum. What voltage do you have on the cathode there? Is the hum still present if you remove the first two pre-amp tubes? If so that would isolate the problem at the third tube.

[pampeo]

hello
thanks for your answer
i i have installed 3 12ay7 and 1 12ax7 as seen on original layout
it's a brand new TAD tube
I d'ont have multimetre yet to check the cathode voltage but a will check maybe tomorow
do you think it's a grounding problem ?
because i don't have so hum when the pot is all way down and the hum goes up in the course of the pot !
i maybe rewinding the filament section !

Another little problem : on the volume bright ,the level is not linear !
at 1 the level is approx 50% and at 12 the is 100%
maybe a bad pot !

Do you think a 470K grid stopper can help me ?

thanks so much for help me !

[martin manning]

If it's only the treble pot that affects the hum then yes maybe there is a ground problem there.

You can also try removing the 12AY7 pre-amp tubes one at a time beginning at the input and see if and where the hum stops to isolate the problem.

On the bright volume, are you sure you have an audio taper pot there? Sounds like maybe it's a linear taper.

If you can post a picture or two that might make it easier to find the problem.

[pampeo]

Oky
so i see a little proximity between the cathode and the filament on V3 tube
see pics
ia had another if you want
[img:912:608]https://lh3.googleusercontent.com/_WkVl ... G_0312.JPG[/img]
[img:912:608]https://lh5.googleusercontent.com/_WkVl ... G_0315.JPG[/img]
[img:912:608]https://lh6.googleusercontent.com/_WkVl ... G_0310.JPG[/img]

[Jana]

Are you using the backs of the pots as grounding points?

[pampeo]

yes i saw this technique on TAD pics

[martin manning]

It's possible that you have a poor ground connection at the one of the solder connections to the pot backs, or from the pot to the chassis.

I would also look at the placement of the wiring. I don't like that loop of yellow wire from the control panel hanging down close to the filament wires.

Often these "lead dress" issues can be discovered by pushing the wires around with an insulated tool (a chopstick) while the amp is on. In general I think you could clean up the wiring a bit more, using shorter lengths, and keeping the leads up agianst the chassis surface (except of course for the filament wiring).

[pampeo]

oky thanks
i will check the ground connection and replace the yelow wires loop far from the filament wires

[pampeo]

hello
i just cheked the frequencies of the hum and it's 50 hz (i'm in europe > paris france)>>>>edit ,it's normal is the france frequencies voltage
So I found a another problem (a big one ) when a push the standby , i have half time a big hum and the half time a little hum !!
i think it' a VVR problem .......or a big grounding problem
tomorow a check all the tension and a scan the result !

[pampeo]

hello
i just checked all the voltage and all is on the tolerence ....good news
I pushing the yellow wires around with an insulated tool and it's don't remove the hum
i a see a mistake on my wiring ....see pics
[img:912:608]https://lh6.googleusercontent.com/_WkVl ... G_0030.JPG[/img]

I REwiring the ground of the switchcrat 12A or the sleeve is already connected with the nut and screw !!!!!
maybe it's create a ground loop ????

for martin manning >>> the measured voltage is +106V in place of the +112V on the layout !

[pampeo]

Hello
replace the yellow wire who make loop in front of the filament wire and the hum was decrease good news
But is still here ...and I don't have an idea to solve !
nobody have an idea ?
thx