need more (negative) bias voltage

[eddie25]

On a whim the other night I just completely re-built a p-t-p amp I had done a while back that sucked. It was cathode bias 6V6, and I decided to go with fixed bias this time.

I figured I would just take the bias off the HV secondary, but there is no center tap, and apparently this doesn't work when using a bridge rectifier.

Turns out there is a 24VAC secondary that I wasn't using so I wired that up as a standard D-style bias and there is -35VDC at most. This is just barely not enough for my 415VDC B+. I'm currently running some JJ's at about 12.5 watts each and the amp sounds promising, but I just need a little more bias voltage.

Can I run the 24VAC tap as a bridge as well, (with the + and - of the rectifier reversed) to get more voltage? Do I need to run some kind of voltage doubler? Anyone ever has this problem?

[tubeswell]

... I figured I would just take the bias off the HV secondary, but there is no center tap, and apparently this doesn't work when using a bridge rectifier...

Capacitor coupled bias supply will work

[pdf64]

With a FWB on the B+ (ie no ground referenced centre tap), the winding sits at a Vdc of about 1/2 the VB+.
A coupling cap could be used to the bias supply, as per some Marshalls etc http://schems.com/manu/marshall/jtm30_30w_jtm310.pdf
Or a voltage doubler on the 24V winding would be great but be careful with cap polarity.
Please don't read anything into that, certainly not that I may have got it wrong and filled a chassis with exploded cap innerds.
Pete

[sluckey]

-35 is about what you'll get with a bridge on that 24VAC winding. You could use a voltage doubler. Or look at page 5 of this pdf to see how Marshall does bias when the B+ is developed using a bridge.

http://home.comcast.net/~seluckey/amps/ ... apbook.pdf

[pdf64]

Please note the distinction between a balanced / bi-phase bridge rectifier and a full wave bridge rectifier.

[eddie25]

Who told you that? Just copy the BFPR bias supply (run it off the active side of the high-tension winding with a voltage divider and a reverse-bias diode and a reverse-polarity filter cap)

I told myself this after trying it. I got no voltage whatsoever. (There was voltage until the reversed diode). Racked my brain for a bit, then consulted google, which I may have read where this wouldn't work.

Plugged that 24V tap in and there it was.

I did only try one side of the winding, so I suppose that was my problem? Running downstairs to try...

Thanks for the input guys.

[eddie25]

...runs back upstairs

Still nothing using the other side of the HV rectifier... You sure this should work Tubeswell?

It shows high voltage right before the diode, then after the diode it specifically goes down to 0.00VDC.

On the PDF that sluckey referenced it shows a typical bias circuit, but with a .047uF cap off the winding and a 56k to ground, is that all I am missing here?

PDF, I believe I am using a FWB. I will check that schem soon, but I have trouble reading those modern Marshall schems and I gotta head off to work.

[sluckey]

Still nothing using the other side of the HV rectifier... You sure this should work Tubeswell?

That will not work with a FWB. You must use the circuit I linked.

On the PDF that sluckey referenced it shows a typical bias circuit, but with a .047uF cap off the winding and a 56k to ground, is that all I am missing here?

That typical bias circuit is an exact copy of the JCM-900 circuit. If you build that circuit exactly as shown you will have enough negative bias voltage.

[potatofarmer]

Yeah, you either have to use the capacitor-coupled bias supply or run a voltage doubler on your 24V winding. For simplicity's sake, I'd vote for the latter...

http://hyperphysics.phy-astr.gsu.edu/hb ... oldoub.gif

There can be issues with capacitvely-coupled bias supplies - if they're not loaded enough (or is it too much?) they can act as charge pumps and put double your B+ down your power supply, ruining all your filter caps. Ask me how I know.

[tubeswell]

Ah yes I forgot about the effect of the absence of a CT on the neutral voltage reference point.

As you were

[eddie25]

Alright, thanks guys that's exactly the info I needed.

The Marshall style off the HV with the cap input would actually be more convenient, but I might try the doubler. Any idea what cap size would be ideal for the circuit that potatofarmer linked?

[martin manning]

Don't do the doubler unless this 24V winding is isolated from all others, i.e. not a tap on the HT secondary.

[eddie25]

OK, thanks. Yeah it is, it's not like a Fender type it's a seperate winding. 24v at 1A.

[martin manning]

For the doubler I'd make the caps 47uF at 50V, and don't forget to reverse the polarity of the caps and the diodes re the posted schematic.

[eddie25]

I was thinking run it as is, into the reverse diode of a standard bias circuit, won't that get me a negative voltage?