Correct bias circuit

[Funkalicousgroove]

So, I see alot of wrong bias circuits on the schematics out there, so here is the correct bias circuit for my chassis, HRM's and BM's.

[tele_player]

Why the extra cap?

[ayan]

Why the extra cap?

The schematic supplied follows the actual Dumble bias supply, for starters. Secondly, it is a good idea to have a well-filtered bias supply while keeping the time constant of the circuit under control. Thus, using a cascaded filter is a good idea; one might be tempted to use a larger single cap instead, for exmaple, but that could be a problem because the supply network might start loading down the 220K feeding resistors that carry the bias voltage to the power tubes' grids.

Cheers,

Gil

[mdroberts1243]

Hi Funk,
What is the correct way to use your rectifier board in the assembly?
I found an old posting of yours that seems to have the rectifier board pretty much the same as the current pics you shared...
I couldn't see the 27k resistor on the board, but there appears to be a 'tail' resistor on the bias pot itself... I assume this is so you only have to route two wires from the bias/rectifier board.
Please see the attached pictures... two excerpts from your previous assembly drawings and the 'layout' proposed in the visio drawing that I've been posting... let me know if I've got it right!

Thanks,

[skyboltone]

This is for a transformer with a separate bias winding. Normster has published a couple of schemes for tapping off the HV winding.

[glasman]

Hi Funk,
What is the correct way to use your rectifier board in the assembly?
I found an old posting of yours that seems to have the rectifier board pretty much the same as the current pics you shared...
I couldn't see the 27k resistor on the board, but there appears to be a 'tail' resistor on the bias pot itself... I assume this is so you only have to route two wires from the bias/rectifier board.
Please see the attached pictures... two excerpts from your previous assembly drawings and the 'layout' proposed in the visio drawing that I've been posting... let me know if I've got it right!

Thanks,

As I remember, the 27K is grounded to the body of the pot. Gil or Brandon can correct me.

BTW, A nice little add is to put a small poly cap (.1uf or so) across the second cap to get rid of the HF stuff that can appear on some amps and to ensure that the bottom of the 220K's stay at AC ground.

Gary

[mdroberts1243]

This is for a transformer with a separate bias winding. Normster has published a couple of schemes for tapping off the HV winding.

The Twinman replacement tranny (I'm using the new Hammond 290FX) has a 60V bias tap on it.

[Funkalicousgroove]

MD,

You have it almost correct, look at the schematic.

The 2 positive ends do indeed go to ground, but as your board is wired you have no voltage. Try bringing the negative ends of each of your caps to an eyelet, and run your ground wire directly from the positive of one of the caps.

Both the 27K (I usually use 24K but the big guy used 27) and the bias circuit ground at the same place near the bias pot.

[mdroberts1243]

MD,

You have it almost correct, look at the schematic.

The 2 positive ends do indeed go to ground, but as your board is wired you have no voltage. Try bringing the negative ends of each of your caps to an eyelet, and run your ground wire directly from the positive of one of the caps.

Both the 27K (I usually use 24K but the big guy used 27) and the bias circuit ground at the same place near the bias pot.

Thanks Funk!
Does the attached look right?

[Funkalicousgroove]

MD,

I wire mine up a bit differently, but what you have works just fine.

Not that it makes a difference, but I generally wire the caps down to the 2 eyelets in the corner, and take the ground directly off the eyelet that the caps are connected to, My reasoning is that the more stuff you shove in with a diode the longer you have to heat it to get the solder to melt, and the greater the risk of damaging the diode.

Perhaps I'm just being anal....

[dguidry]

I had a hum problem on a BF Deluxe I built and adding the extra cap on the Bias output cleaned it right up. Very cheap hum insurance.

I never thought about adding the small poly cap, but it also makes good sense. Thanks Gary!

Dave

[glasman]

I had a hum problem on a BF Deluxe I built and adding the extra cap on the Bias output cleaned it right up. Very cheap hum insurance.

I never thought about adding the small poly cap, but it also makes good sense. Thanks Gary!

Dave

No Problem, the small poly cap makes for a much better AC ground than the electolytic by itself.

Gary

[Funkalicousgroove]

how big should the poly cap be?

[glasman]

how big should the poly cap be?

.1uf is big enough in most cases.

[Guitarman18]

glasman wrote:

BTW, A nice little add is to put a small poly cap (.1uf or so) across the second cap to get rid of the HF stuff that can appear on some amps and to ensure that the bottom of the 220K's stay at AC ground.

Just to clarify, do you mean the cap that is connected to the bias pot wiper?

Thanks, Paul.