Does this bias circuit look ok ?

[mat]

Biasing is still quite hazy to me

I have 2 6L6's on my nonHRM (V5/V6 - plate = 465/466, screen = 463/465)

[img:617:594]http://www.pichotel.com/pic/16028B31e/117499.jpg[/img]

When I turn either bias pot it affects to the bias of both tubes. I have 1ohm's on power tube cathodes to ground and test point outside the chasis.

Should the resistors on the pots be half of their value ? How is the 470ohm next to the diode ? Should I aim for 35mA or 40mA ?

Thanks,

[epis]

Hi,Mat
Check schem of October on AX84.With a little bit of tveaking you gonna get it right.I did the same with my SLOclone and it works fine.Best regards, Damir

[mat]

Hi,Mat
Check schem of October on AX84.With a little bit of tveaking you gonna get it right.I did the same with my SLOclone and it works fine.Best regards, Damir

Ok, so I should build My Dumble clone bias as in this picture (October):
[img:718:219]http://www.pichotel.com/pic/16028B31e/117708.jpg[/img] ?

[heisthl]

The schematic placements are correct, but for 6L6 with that plate voltage you are shooting for a range of 45 - 55 volts. You will probably have to have a tail resistor of larger than 10K and an input resistor smaller than 15K. I would use test leads and start with 3K3 input and 33K tail, or better yet measure the bias supply available with no circuit and do the voltage divider math.

[Ed de Jager]

I am using the October bias circuit too.Don`t know if it works though;still have to wire the heaters
Ed

[mat]

I am using the October bias circuit too.Don`t know if it works though;still have to wire the heaters
Ed

Thanks for the tip but I wouldn't like to rebuild the bias circuit if it can be corrected with a resistor tweak

Well, I had the measuring points wrong way around. Two ground points and one test point (with both grids ).

I corrected that and changed the 470R to 1k but all I get now is between -90mV (mA) and somewhere to -160mV...

Should I make the 1k bigger ?

Should I put bigger/smaller resistors on the pots ?

Should I change the pots bigger/smaller ?

I'm out of bias...

[heisthl]

Mat - SInce it's for a Bias circuit with almost no current demand you can use fairly high resistor values to keep resistor wattage low. To get the correct ratio between first and last resistors think voltage divider with the proportion of difference between the resistors values setting the voltage. The pot values depend on how big a range you want. Here's a simple example to get you thinking the right way:

[glasman]

I would also put a small (22uf or so) cap on each of the wipers to ground, this will help to keep the PI plates looking into a constant AC impedance.

May not be a big deal, but that point (220K junction) should be AC ground.

Gary

[mat]

Hi heisthl,

Thanks for the lesson.. I feel stupid.. I changed the 470R in again and put a 43k after the 15k's to ground. Now I'm getting as warm as -40mV. Hope this arragment is ok. I will bias between 45-55 as You suggested.

[mat]

I would also put a small (22uf or so) cap on each of the wipers to ground, this will help to keep the PI plates looking into a constant AC impedance.

Gary, are You referring to the picture on the first post ? Am I correct assuming they (22uf's) should be positive to ground ?

May not be a big deal, but that point (220K junction) should be AC ground.

On that I'm lost

[glasman]

I would also put a small (22uf or so) cap on each of the wipers to ground, this will help to keep the PI plates looking into a constant AC impedance.

Gary, are You referring to the picture on the first post ? Am I correct assuming they (22uf's) should be positive to ground ?

May not be a big deal, but that point (220K junction) should be AC ground.

On that I'm lost

Yes, the picture on the first post 22uf wiper to ground with the + end to ground.

Don't worry about the second part of my post.

Gary

[Tdale]

I wired it just like the first picture. Two pots, one resistor from each pot to ground.. and it works. I had to huse higher values than 15K, if I remember correctely.

Is there any reason that the circuit shouldn't work?

I thought about one pot influencing the other, but I'm able to adjust both tubes individually...

Tommy

[mat]

Yes, the picture on the first post 22uf wiper to ground with the + end to ground.

Don't worry about the second part of my post.

Thanks Gary ! Will put them in now

Tdale, it seems to work ok now