Tubes up #102 build

[pompeiisneaks]

the one I linked shows a separate 60V tap and I'm not sure I get why it wouldn't work, seems right to me no?

bias.png

~Phil

[sluckey]

That's the one I'm talking about and it will not work as drawn.

[pompeiisneaks]

That's the question I'm asking, why wouldn't it work? it has the anode drawn the right way, the cap the right way, it should produce negative voltage, would it be too low? Just trying to understand what's wrong with it.

~Phil

[talbany]

That's the one I'm talking about and it will not work as drawn.

That's the standard Fender AB series bias supply on 102 and the one Phil posted used on thousands of amps and it does work as drawn
What's the AC your getting off your bias tap?

If you are using a parasitic supply that's a different circuit.

Tony

[sluckey]

Tapping into a FWB HT winding for the bias requires a cap between the winding and the bias circuit. The AC bias source must be provided from a completely separate winding to use that bias circuit. This is what it would take to make THAT bias supply work with a FWB...

[martin manning]

In a FWB rectifier arrangement there is almost no negative voltage swing in the AC, so you can’t use the typical circuit. Unless there is a separate bias winding, you have to place a capacitor in line to block the DC, provide a ground reference for the the AC on the other side, and THEN rectify it to get DC below ground.

[pompeiisneaks]

Again, maybe I'm missing something obvious, but doesn't that transformer SHOW a separate 60V winding?

I'm feeling like I'm being thick here and missing something obvious.

bias.png

~Phil

[pompeiisneaks]

Oh I think I just figured it out myself. The drawing you're showing, sluckey, is a completely separate winding with ground, not a single tapped point on an existing winding?

The problem being that without a ground reference via the other end of the winding, the FWB rectifier steals that away and is blocking it behind diodes that won't allow the right circuit closure?

Is that what I'm missing?

~Phil

[talbany]

So since you are using the same winding (Tapped) as the FWB you need to add a parasitic supply
DC blocker w/Gnd Ref

Tony

[sluckey]

Finally found a 102 schematic that shows a proper PT and bias supply. I'll just attach the file because I already lost the link. It's way down a rabbit hole!

[talbany]

Export transformer primary color codes are on the schematic...

https://el34world.com/charts/Schematics ... _schem.pdf

I am totally confused?.. The Bassman Schematic showed you basically how to do it.Why not just go with the one you posted earlier
BTW. The schematic of 102 you found is a guess of what's in the original probably taken from the same Bassman 100 PS.

Tony

[martin manning]

Andre is using a Bassman 100 PT, but he originally followed the 102 layout which shows a FWB. That was rectified (heh) a while back, but the M.D.Roberts ODS 102 schematic that was linked along the way has an error (as pointed out by Steve), showing a standard FW bias supply tapped off the HT on a FWB rectifier. The recent fuss is about why that won't work. The solution as pointed out above is to use a capacitively-coupled rectifier. At the same time there is a search going on for a schematic matching the 102 layout.

[sluckey]

...The Bassman Schematic showed you basically how to do it.Why not just go with the one you posted earlier
BTW. The schematic of 102 you found is a guess of what's in the original probably taken from the same Bassman 100 PS.

Tony

I don't have any problems wiring it IAW the Bassman 100 PS schematic. The only reason I'm stuck to this tar baby is because I saw Andre struggling with a B+ of 800V! Turns out that he used a Bassman 100 PT but he connected it to a FWB. Wonder where he got that idea? Once he rewired for a conventional rectifier, such as the Bassman 100 used, and connected the HT CT to ground, all problems were resolved.

So, please tell. Where did you get the idea to use a FWB in that nice layout you did? Your other drawings correctly show a conventional rectifier.

Tony, it's not my intention to poke a finger in your eye. I admire the drawings you've shared. And I don't want to argue about this. I'm ignorant about Dumble, but not so about electronics. All I ever wanted to do was help Andre solve his problem and offer an explanation. Peace... Steve

[martin manning]

Tony's layout specifies PT 026554, which is an export unit designed for use with a FWB. It does have a separate bias winding. Assuming the PT part number is correct, the errors I see in the layout are that the bias winding leads on that PT are brown, the red/yellow CT lead should not be used, and the dual primary connections are incorrect or incomplete (they may be correct, but hidden inside the bell).

[talbany]

Steve
No problem this a tough thread to follow?. So if he used the CT and things worked out all he needed to do was to add a parasitic supply like the one I posted right on the HT and problem solved.He would have had to tweak some values depending on the current draw on the tubes but works like a charm. This was why I was confused.
If you look up the part numbers on the PT of 102 you will find this is a power transformer from a Red Knob Twin!..The 554 is the 100V Export multi-tap primary version. It uses a full wave bridge rectifier. The bias winding (Brn-Brn) yields +- 75V. 102 uses 1/2 of the winding to get the supply voltages. If memory serves the red/yellow are the CT of the primary but in the red knob was used for a high low switch. Of course, if you go with a FWB you don't want to use the CT of the winding.This is why the RD/Y is not grounded on the layout
BTW.If anyone wants to use the same power transformer on the 102 amp I did then they need to do the research and pull up a schematic of the number posted and find the amp it was in.If they are using a different transformer then they need to know or figure out what voltages their transformer yields and how to properly design the PS before they build the amp..Not jumping on anyone's case just using some common sense!

Tony