bias circuit frustration

[firemedic]

So I'm powering up my bass amp build slowly on a variac & lamp limiter. As I slowly bring up the voltage, the B+ looks good, no problem there. I can't bring it up all the way because without tubes in, I will exceed my max voltage on the main caps. So we're up to 660VDC, @ 85vac primary. OK fine.
 
Meanwhile, I'm trying to get a good ballpark estimate on the bias circuit resistors. My problem is, I was getting a good 50vAC on the AC side of the diode, but no DC voltage at all on the other side. Negative or positive.

I double- & triple-checked the bias pots & cap and it all looks fine.

The PT is the Twin Reverb job from Weber. It's not center tapped, but the bias winding is a separate 50vac pair of secondary wires. I had one of them feeding the bias circuit & the other one capped off.

So in my always cautious wisdom I figured maybe I needed to make the capped-off bias wire, a ground reference by connecting it to the chassis.
Which was maybe not the best idea.

So now, I believe I have accidentally shorted the PT bias winding to the HT secondary, since I now have MUCH higher VAC at the bias circuit connection, and also a comparable +DC voltage. Just like the HT bridge rectifier connection.

But there is still no voltage at all on the other side of the diode: negative, positive, AC or DC. I have tried changing diodes to no avail.  
Thankfully, the B+ is unaffected & still nominal. But I can not proceed until this bias circuit makes some -vdc. I am stumped. I am humbly requesting assistance.  
 

[gldtp99]

Info for Weber Twin Reverb replacement PT : https://taweber.powweb.com/store/022756sch.jpg

This PT has a center tapped HT winding (red/yellow is HT center tap)------ it doesn't have a seperate bias winding, but it does have a 50V Bias Tap (blue/red).
I suspect that you've incorrectly identified the PT leads and have possibly damaged the new PT.

[sluckey]

The PT is the Twin Reverb job from Weber. It's not center tapped, but the bias winding is a separate 50vac pair of secondary wires. I had one of them feeding the bias circuit & the other one capped off.

Are you using the Weber W022756 PT?

If not, then show us your schematic and a PT wiring diagram.

If you are using the W022756 PT, then look at the diagram and fix your wiring. This PT does have a CT and it does not have a separate bias winding.

EDIT...     Haha... duplicate replies. I'm getting slow!  

[firemedic]

That's the same PT.
If it does not have a bias winding, what is the 50v secondary for?

If you look carefully you will see the Red/yellow wire is not a CT. When I connected it to ground & powered up, the lamp lit very brightly even at 10vac primary. That can't be right. Before I did that, my 50v tap had a normal voltage. Now, it has the same AC/DC voltage as the HT secondary. So much for the CT theory. With all due respect.

The question remains, why am I not getting -VDC on the other end of the diode? No matter which diode I use? Even before I shorted the bias tap, no DC. Even with nothing else connected, there is no DC at all. On the line side, there is now AC and +DC. Will that cause a diode to not work? You see how I might be frustrated.
 

[Willabe]

Just like the HT bridge rectifier connection.

If your using a full wave full bridge, you can't ground the CT. If the CT is not grounded, I don't think the 50v bias tap will work.

660vdc with only 85vac on the primary? Going to be more at full 120vac. That scares me.    


                                  Brad      

[Willabe]

Red/Yellow is the CT, then you have a choice of 600vac or 640vac HT. Which is 300-0-300 or 320-0-320.

Red/Yellow to Red/Blue is shown as 50vac.

Ct to 50v tap to 300v taps to 320v taps.


                        Brad     

[sluckey]

If you look carefully you will see the Red/yellow wire is not a CT. When I connected it to ground & powered up, the lamp lit very brightly even at 10vac primary. That can't be right. I turned it off pretty quick.

You're reading that wrong.The red/yel IS the CT. The red/blue IS the bias tap. The reason your lamp lit up so bright is because you have a FWB tied across the 660VAC HT winding, which BTW explains your insanely high B+. When you connected the CT to ground you effectively put two diodes directly across the HT, shorting out the HT windings.

I assume we're talking about your B25 project? If so, the quick solution is to disconnect the two diodes that are connected to ground and connect the red/yel wire to chassis ground. This makes a conventional rectifier. Doing this will fix your bias problem too, that is, if nothing else has blowed up and the bias circuit is wired correctly.

[Willabe]

If it does not have a bias winding, what is the 50v secondary for?

It's not a _ separate _ bias winding. It's a bias _ tap _ taken from/off of the HT winding.


                    
                        Brad

[firemedic]

Okay, I'm gradually figuring this out.
So I do not ground the CT.

Since the B+ is so High I can use the lower voltage HT windings for the B+, which should have been obvious to me but I was busy w/ the bias circuit.

And use a LARGE dropping resistor to supply the bias, hopefully the PT is not too badly demented.

Sound about right?

Oops, I just read yer post sluckey.
Dumb question #3: on a conventional bridge, where does the HT secondary connect? Same place?

@Willabe: you sound like my wife

 

[Willabe]

If so, the quick solution is to disconnect the two diodes that are connected to ground and connect the red/yel wire to chassis ground. This makes a conventional rectifier. Doing this will fix your bias problem too, that is, if nothing else has blowed up and the bias circuit is wired correctly.

Listen to sluckey. Don't use a bridge. You don't need it and the HT voltage will be too high.

@Willabe: you sound like my wife

Is that good or bad?   


                Brad      

[sluckey]

on a conventional bridge, where does the HT secondary connect? Same place?

Same place. But... Now you need to use two diodes in series to get the PIV rating higher. So, the real fix is to totally remove the two bridge diodes that are connected to ground. Now use the extra diodes in series with your other diodes.

[Willabe]

Maybe this will help?


                    Brad    

[firemedic]

Good or bad depends on the day.

I'm too busy tonite w/ the fam to work on it anymore, but I'll let you know!

[firemedic]

I think I have learned an expensive lesson. I made the changes. The HT secondary reads 65vac.....
Time for a new PT. oops.

[sluckey]

The HT secondary reads 65vac

Exactly how are you measuring that voltage? Be specific.

[sluckey]

This is a Weber 6A80 that uses your PT. Your power supply needs to look very similar. This PT is not meant for a FWB.

[firemedic]

I rewired it just like that. The rect. is giving me @100vdc, after checking & rechecking connections. The 300-0-300 secondaries are also giving me a low reading. Bias tap is 10VAC. I am sad. I checked the primary voltage to make sure & it's 119VAC from the wall. Another one bites the dust.....

 

[firemedic]

Update: I installed the new PT. At first I had the same problem, i.e. low B+ voltage. So I bypassed the lamp limiter.....B+ is great, right where it needs to be. No tubes yet. The good news is I've figured out my bias resistors' values

Then the smoke escaped from my aux. filament tranny. Having visually inspected the whole strand, I can't for the life of me find any kind of short. But I should have kept that limiter connected.
 
I disconnected the primary wires and CT from the aux transformer, and found both secondaries shorted to ground. But then, the heater wires on the main PT also short to ground.

I am going to take a nap. Maybe when I wake up there will be some sort of revelation.

[firemedic]

Pilot lamp holder.

To install it in the BF Twin style chassis, turns out I had bent the bracket up, shorting the whole thing out.

There is no such thing as a free education.