Page 1 of 1
Help me do bias mod on my Bravo 112
Posted: Tue Nov 18, 2014 3:02 pm
by budubum92
Hi.
Just a quick one.
Iv been drooling to test the shunt method biasing.
i understand that you need a good meter to do this.
Is this DVM that i have any good?
how do i test the meter or know if the meter is sturdy enough to take 400 plate volts and about thesame voltage at the center tap?
Which socket of the DVM should my test leads go to?
And at what setting should i be at to read current for the shunt?
I do know how to read the plate voltage and i understand they are not thesame when doing B+ to ground when checking for current.
Re: Help me do bias mod on my Bravo 112
Posted: Tue Nov 18, 2014 3:15 pm
by budubum92
im gonna be adding a bias pot on this bravo and it has a fixed 33k resistor R60.
ill change that resistor to 15k and add a 25k pot in series. does it matter if its linear pot or logarithmic A pot?
looking at the back of the pot from left to right 1-2 (wiper)-3, ill connect the 15k from board bias splitter resistors to log 1 of pot. then i bridge log 2 and 3 together then connect that to ground.
are all this correct?
why id like a confirmation about the shunt bias is because the bravo112 preamp board is vvery difficult to cut the traces and will take a lot of effort to put 1-ohm resistors on cathode to ground on this circuit.
Re: Help me do bias mod on my Bravo 112
Posted: Tue Nov 18, 2014 8:35 pm
by CoffeeTones
Use a linear pot only.
Turn the pot completely clockwise. The two lugs that have the highest reading when jumped through the meter, will be the two to use. That will insure that clockwise rotation increases bias current. You may connect the remaining lug to the center lug.
You may want to buy or build one of these cheap bias tools. No need for 1 ohm resistors to be installed.
http://hoffmanamps.com/MyStore/perlshop ... =!ORDERID!
http://el34world.com/charts/BiasChecker3.htm
Re: Help me do bias mod on my Bravo 112
Posted: Wed Nov 19, 2014 5:11 am
by budubum92
thank you coffeetones.
so when testing the two lugs, you mean the lug 1 and 3? that id connect to wiper lug 2 and check lug 1 and 3 which of them gives the highest reading?
Re: Help me do bias mod on my Bravo 112
Posted: Fri Nov 21, 2014 4:48 am
by budubum92
id like to bumP this thread furthur with a choke mod since the bias resistor is already close to, what i think is, the choke resistor.
id like to get a confirmation though if R62 400ohms 5W FPP is the choke resistor.
what choke can i go with an amp with 2x el84 running at 400 VDC plates?
