I have no wire between the case of the pot and the earth of the preamp. The wires to the pot are:
1 - input signal from the audio source (CDP)
2 - output to the grid of the tube
3 - earth pin of the pot to the earth of the preamp.
The pot is mounted on the chassis so the case of the pot is at the chassis earth. No wire to the preamp earth.
Miguel
1 - input signal from the audio source (CDP)
2 - output to the grid of the tube
3 - earth pin of the pot to the earth of the preamp.
The pot is mounted on the chassis so the case of the pot is at the chassis earth. No wire to the preamp earth.
Miguel
Buuuf!!!!!
100K...the maxi output resistance is +- 25k
10% - 25K= 2.5k
The resistor that you have need to put many time ago betwen wiper and grid is
+- 2,5k
The value that you have in hand
2,2k ....2,7....3.3k....is not so critical!!!
i hope that this time you put the resistor...or i give up!
100K...the maxi output resistance is +- 25k
10% - 25K= 2.5k
The resistor that you have need to put many time ago betwen wiper and grid is
+- 2,5k
The value that you have in hand
2,2k ....2,7....3.3k....is not so critical!!!
i hope that this time you put the resistor...or i give up!
This weeend I put a 2.2K resistor on the grid of the EF86's. Turn on the system and put the volume all way down. No hum 
. And again and again. No more loud hum at all . Jorge, you were right!
While I was there I decided to put the 1M resistor a bit more close to the grid tube pin. Well, I had not much else to do so I ended up remaking all the preamp section. All paths were shortened and it looks quite better now. I also shielded all signal cables with aluminium foil, attached in one of the ends to the chassis.
But now I have this medium/low hum all the time
. I saw that sometimes it disapears and when I tap the box it disapears too and comes again. Maybe I left something with a bad solder. It was late . Time to recheck.
Miguel
. And again and again. No more loud hum at all . Jorge, you were right!While I was there I decided to put the 1M resistor a bit more close to the grid tube pin. Well, I had not much else to do so I ended up remaking all the preamp section. All paths were shortened and it looks quite better now. I also shielded all signal cables with aluminium foil, attached in one of the ends to the chassis.
But now I have this medium/low hum all the time
. I saw that sometimes it disapears and when I tap the box it disapears too and comes again. Maybe I left something with a bad solder. It was late . Time to recheck.Miguel
The preamp Miguel!!!
Well!!!...It's done!!
Well Miguel the grunding arrangement in a stereo preamp is not a piece of cake...i suspect that the aluminium foil is doing more harm than good as it couples capacitivly to the various parts of the circuit...and the point of conection of this foil to the ground is critical...especialy with high impedance circuits...
So make a try without foil!....many times...less is more!!
Um abraço!
Well!!!...It's done!!
Well Miguel the grunding arrangement in a stereo preamp is not a piece of cake...i suspect that the aluminium foil is doing more harm than good as it couples capacitivly to the various parts of the circuit...and the point of conection of this foil to the ground is critical...especialy with high impedance circuits...
So make a try without foil!....many times...less is more!!
Um abraço!
Hello Miguel,
EF86 can be uninviting when it comes to layout. I know the problem you’re talking about and I can believe you don’t have ground loops and still have a hum. There are some tricky things there I did not figure. If you had a solution it is maybe best to go back to that. If you’re ready for experiments, try to wire shield (pins 2 and 7) in different ways. And take note at the changes when you move resistors physically. Anyway, this tube can work in given configuration without any hum, and without series resistor at the input. As I said already, I did not figure this story, but have one (PCB) solution that surely works. Send me an e-mail if you are interested for more info about it.
Pedja
EF86 can be uninviting when it comes to layout. I know the problem you’re talking about and I can believe you don’t have ground loops and still have a hum. There are some tricky things there I did not figure. If you had a solution it is maybe best to go back to that. If you’re ready for experiments, try to wire shield (pins 2 and 7) in different ways. And take note at the changes when you move resistors physically. Anyway, this tube can work in given configuration without any hum, and without series resistor at the input. As I said already, I did not figure this story, but have one (PCB) solution that surely works. Send me an e-mail if you are interested for more info about it.
Pedja
Hi Pedja,
Thanks for your help but I don´t want to go into making pcb's
... I have everything p2p wired, with the tube bases bolted to chassis. The layout is quite nice now, as the circuit is simple. I will try to move things a bit inside to see what happens and do something with the shield pins
Miguel
Thanks for your help but I don´t want to go into making pcb's
... I have everything p2p wired, with the tube bases bolted to chassis. The layout is quite nice now, as the circuit is simple. I will try to move things a bit inside to see what happens and do something with the shield pins Miguel
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