I just finished building the Weber 5F6-A kit, schematic he

https://weberspeakerscom.secure.poww...5f6a_schem.jpg

I mounted a 50K pot in one of the speaker jack holes and installed tip
jacks with 1 ohm resistors as shown he

http://waynereno.com/Bias_Test_Points.htm

The Weber schematic indicates to replace the 56K R19 with a 50K pot for
adjustable bias.

I am thinking it might be a good idea to put a resistor in series with
the bias pot in case somebody turns the pot all the way down, like
maybe a 10K 1/2 watt in series with the 50K pot.

Would that be the way to go?

wrote:
I just finished building the Weber 5F6-A kit, schematic he

https://weberspeakerscom.secure.poww...5f6a_schem.jpg

I mounted a 50K pot in one of the speaker jack holes and installed
tip
jacks with 1 ohm resistors as shown he

http://waynereno.com/Bias_Test_Points.htm

The Weber schematic indicates to replace the 56K R19 with a 50K pot
for
adjustable bias.

I am thinking it might be a good idea to put a resistor in series
with
the bias pot in case somebody turns the pot all the way down, like
maybe a 10K 1/2 watt in series with the 50K pot.

Would that be the way to go?

Good call. You should add it at the grounded end of the pot. I'd use
15K or 22K though, so you have more range on the high end of the scale.
Also you need to know what you're measuring. Look up tube data charts
and figure how much current you can put through the tubes without
frying 'em. When I put together my first amp kit with an adjustable
bias, the instructions said for 6V6 tubes set the bias between 10mv and
35mv; for 6L6, EL34, 6550 etc, set it between 35mv and 50mv. That's not
very specific, and I've biased them pretty much at the low end of those
scales.

O.K. thanks. I will go with 22K. I found this article:

http://www.duncanamps.com/technical/lvbias.html

They suggest to do exactly as you recommend: Divide the bias resistor
by two and go with closest standard value in series with a pot having a
value close to the original bias resistor:

"Many amps which use "fixed" (negative grid) bias have provisions for
adjusting the negative grid voltage upward or downward. Making the
grids LESS negative will cause MORE current to flow through the tubes.
Some amplifiers don't have a bias-adjusting control (pot) but instead
use a fixed resistor to set the voltage. If you encounter one with a
fixed resistor, the best thing to do is convert it to an adjustable
type. Most of the time, the fixed resistor will be in parallel with the
bias capacitor; the lower this resistor's value is, the lower the bias
voltage will be. If you can locate and identify this resistor, you can
replace it with a simple network consisting of a (lower value) resistor
in series with a potentiometer. What you'll be shooting for is a range
of adjustment that goes from LESS voltage to MORE voltage than is set
by the (existing) fixed resistor. Take the value of the fixed resistor
and divide by two; pick the closest standard value to your result, and
put it in series with a pot which is as close to the original
resistor's value as you can find. Example: the existing resistor is
33K; use a 15K resistor in series with a 25K pot to replace it. The
original resistor was 33K; you now have the ability to adjust the value
from 15K to 40K. This should provide you with sufficient adjustment
range to set any plate current you wish. If not, use a different value
pot or resistor."

wrote:

O.K. thanks. I will go with 22K. I found this article:

http://www.duncanamps.com/technical/lvbias.html

They suggest to do exactly as you recommend: Divide the bias resistor
by two and go with closest standard value in series with a pot having a
value close to the original bias resistor:

"Many amps which use "fixed" (negative grid) bias have provisions for
adjusting the negative grid voltage upward or downward. Making the
grids LESS negative will cause MORE current to flow through the tubes.
Some amplifiers don't have a bias-adjusting control (pot) but instead
use a fixed resistor to set the voltage. If you encounter one with a
fixed resistor, the best thing to do is convert it to an adjustable
type. Most of the time, the fixed resistor will be in parallel with the
bias capacitor; the lower this resistor's value is, the lower the bias
voltage will be. If you can locate and identify this resistor, you can
replace it with a simple network consisting of a (lower value) resistor
in series with a potentiometer. What you'll be shooting for is a range
of adjustment that goes from LESS voltage to MORE voltage than is set
by the (existing) fixed resistor. Take the value of the fixed resistor
and divide by two; pick the closest standard value to your result, and
put it in series with a pot which is as close to the original
resistor's value as you can find. Example: the existing resistor is
33K; use a 15K resistor in series with a 25K pot to replace it. The
original resistor was 33K; you now have the ability to adjust the value
from 15K to 40K. This should provide you with sufficient adjustment
range to set any plate current you wish. If not, use a different value
pot or resistor."


Gosh, what a good technical writer. ;-)

Lord Valve
Expert