How To get Fixed Bias when you Don't Have a Bias Tap

[Gregg]

If you need quick and easy bias supply from a non center tapped transformer check out Marshall's JCM900 series amps. I've used it several times without any problems.

[leadfootdriver]

The value of 220K comes directly from the data sheet.. If anyone objected, it would be the power tube itself..

-g

RG1 is for the pulldown resistor right?

http://tubedata.milbert.com/sheets/163/e/EL84.pdf

[leadfootdriver]

If you need quick and easy bias supply from a non center tapped transformer check out Marshall's JCM900 series amps. I've used it several times without any problems.

Thanks, but my amp doesn't have a bias tap, only a secondary with no center tap.

I needed to do another FWB.

[Tom Phillips]

Thanks, but my amp doesn't have a bias tap, only a secondary with no center tap.

I needed to do another FWB.

Respectfully, you missed the point that Gregg made. Check out the Marshall schematics and you will see how they generate the bias supply without a bias tap and without an additional FWB.
Cheers,
Tom

[mooreamps]

RG1 is for the pulldown resistor right?

http://tubedata.milbert.com/sheets/163/e/EL84.pdf

Yes.

-g

[leadfootdriver]

I brushed over the JCM900 to quickly at first. But a closer look revealed (to me) a no center tap secondary. So I gave it a shot, and actually got it to work! I don't know how it works, but I'm getting a ground reference and a variable negative voltage now. This amp (I like it) loves fixed bias over CB.

I threw in some different resistor values by chance, and am getting a sweep from -4.48 to -14.52. That's fine for a single ended JJ EL84. The FWB setup was way easier to mock up tho given my recycled radial caps with nubby leads!

[leadfootdriver]

Yes.

-g

K then.

[leadfootdriver]

BTW, Thanks!

[agadis]

What if you have a 0-50V-320V non-center tapped PT and using FWBR?

Hi Gregg,

I wondered if you managed to work that 50V tap as a bias supply effectively.

I have the same situation here and would be happy to use 50V tap instead of full B+ tap.

Thanks in advance for any help!

[Gregg]

Hi Gregg,

I wondered if you managed to work that 50V tap as a bias supply effectively.

I have the same situation here and would be happy to use 50V tap instead of full B+ tap.

Thanks in advance for any help!

No. As I mentioned earlier I used the JCM900 way to get the bias voltage.

[catalin gramada]

Hallo.
Can someone do a sketch how to use the 50v tap from a hammond power tranny into a useful voltage doubler configuration for kt88 bias supply.please? My atempt to imagine any configuration put one diode in short. I cannot imagine how could be done. The tranny have 400-50-0-400 secondary.please. Thanks. Catalin

[The Dude]

How large of a bias supply are you looking for? Wouldn't a simple half wave rectifier be enough here?

Edit: If that can't get you where you need to be, you could run another rectifier off of the HV winding and use a divider to lower the voltage.

[Jazz P Bass]

Voltage Multipliers - Doubler, Tripler, Quadrupler

[pdf64]

Can someone do a sketch how to use the 50v tap from a hammond power tranny into a useful voltage doubler configuration for kt88 bias supply.please? My atempt to imagine any configuration put one diode in short. I cannot imagine how could be done. The tranny have 400-50-0-400 secondary

I think that a voltage doubler needs to have a separate winding; it can't utilise that 50V tap.

[loudthud]

There are two forms of Voltage Doubler. One does not require a floating winding.