Princeton Reverb build - FWB rectifier - 0V Bias voltage

[J3ona1]

Hi folks,

I'm building a Princeton Reverb (AA1164 circuit) with adjustable bias (10k pot) wired like the attachment from Rob Robinette's site and I'm getting 0 Bias Voltage at the anode of the bias diode, but I'm getting 440V B+  (unloaded). I'm using a full wave bridge rectifier (HV winding of PT has no center tap) and the bias diode along with the diodes in the bridge rectifier (1n5408 for all) test good. Did I connect the feed for the bias voltage in the right spot?

[Willabe]

You can't wire it up like that with a FWB rectifier set up. It's has to be capacitance coupled, so it needs a series cap.

From our friend sluckey's web site, go down to pages 9 and 10. Wire it up like that. It's what Marshall uses.

https://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf

More detail here from Merlin's web site, bottom of the page;

https://www.valvewizard.co.uk/bias.html

[stratomaster]

You're not building an AA1164 as that uses a pair of half wave rectifiers and a PT with a grounded center tap on the HT winding.  Without more detail we can't tell what you're building.

It appears you're attempting to use a bridge rectifier and may or may not have a CT on the HT winding.  So what PT are you using?  A datasheet of your transformer would be best. You may have a dedicated bias tap ready to use.

You can get away with a 2 phase rectifier and get something functionally similar to the AA1164 if you have a CT.  Otherwise check out the Valve Wizard page for a way to generate a bias supply from a bridge.  https://www.valvewizard.co.uk/bias.html

[Willabe]

It appears you're attempting to use a bridge rectifier and may or may not have a CT on the HT winding.

I'm using a full wave bridge rectifier (HV winding of PT has no center tap)

[Willabe]

So what PT are you using?  A datasheet of your transformer would be best.

Yes, this would help greatly.

[J3ona1]

Thank you for the links fellas! I wish the pt had a bias tap! This is the link to the spec sheet for my PT. https://www.antekinc.com/content/AS-1T300b.pdf

It leaves out the current handling capability of each HV winding which is .17A.

[J3ona1]

Hi folks,

I wired up the attached diagram (from sluckeys amp scrapbook) and I'm unfortunately still getting 0vDC at the bias point. I've put voltages in the diagram as well with the hope that you can help me trouble shoot. Thanks in advance for any help you can provide.

[Willabe]

How can you have 139.3 dcv at the junction of C1 and R1?

C1 will block any dcv from the FWB and it's before the backwards/reversed biased diode.

Do you have at least 1 filter cap wired up to the FWB's output? 

[J3ona1]

I have one 47uF @500v cap attached to b+ on the fwb. I'm confused for real. Maybe it's my meter.... Here are pics of the wiring and the measurements. The .047 uf cap measures okay at least in terms of capacitance.

[Willabe]

Thank you for posting those pics, but I can't tell what's what with that build style. 

It's fine to build like that, just can't see the circuits flow.

You must have something wired wrong.

Try a brand new set of batteries in the meter. We had a problem not long ago, turned out all it was was the meters batteries were weak. We, the builder and us, spent a lot of time on that post.   

[Willabe]

This is a great way to double check a new build.

https://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630

[Beezerboy]

 nevermind...