Bias voltage source - HV or 50vac tap

[pdf64]

I don't think that the regular Fender BF arrangement that's fed from the ~50V tap off the HT winding could be classified as a low impedance source; granted its impedance is a bit lower than the regular 50W Marshall / BF Princeton arrangement, but it's not like it's in a different ball park.
And given the typical 220k power tube grid leak resistors, any difference between the arrangements would seem be swamped by that.

As I see, the 'feed from full HT' arrangement has a useful advantage in that the HT winding's CT return to 0V can be fused, thereby making it simple to implement a good level of protection for the winding.
Yes the CT 0V return could still be fused with the 'feed from tap' arrangement but high voltage caps would be needed for the bias supply, as regular 70-100V caps would be stressed if the CT fuse blew.

[pompeiisneaks]

It can be made to work either way but, as frankdrebin hints, the large dropping resistance required for HT-derived bias makes it a high impedance source. That means it needs a couple more parts (another RC filter) to make a bias supply that has both low ripple and reaches the required negative voltage quickly. If the main rectifier is a FWB, then yet another capacitor is required. Some people like to add a couple more parts, and draw from both legs of the HT.

Are you saying that it requires the extra cap and resistor that the dedicated winding doesn't? I'd think the cost of those parts is significantly less than paying someone to add a winding to a transformer no?

Just making sure I get the point.

Maybe I didn't.

~Phil

[wpaulvogel]

Pjd3, I was just going off topic and suggesting the output transformer not the TW power transformer. The DR power transformer will do everything you want with the 6v6 tube.

[martin manning]

Are you saying that it requires the extra cap and resistor that the dedicated winding doesn't? I'd think the cost of those parts is significantly less than paying someone to add a winding to a transformer no?

At its simplest, Fender's bias circuit is a diode, a resistor and a cap, plus the extra secondary tap, which is really an extra coil. The other circuit needs more parts, more eyelets or turrets, more board space, and more in-house assembly labor. I don't know how that nets out exactly, but Leo was notoriously miserly.

[martin manning]

I don't think that the regular Fender BF arrangement that's fed from the ~50V tap off the HT winding could be classified as a low impedance source; granted its impedance is a bit lower than the regular 50W Marshall / BF Princeton arrangement, but it's not like it's in a different ball park.
And given the typical 220k power tube grid leak resistors, any difference between the arrangements would seem be swamped by that.

The current limiting resistor in the Fender BF bias is ~1k ohms, whereas the resistor in the Marshall circuit is ~100k ohms, or two orders of magnitude higher. Re the 220k grid leak resistors, I'd classify them as being part of the load, not the source.

[pompeiisneaks]

Are you saying that it requires the extra cap and resistor that the dedicated winding doesn't? I'd think the cost of those parts is significantly less than paying someone to add a winding to a transformer no?

At its simplest, Fender's bias circuit is a diode, a resistor and a cap, plus the extra secondary tap, which is really an extra coil. The other circuit needs more parts, more eyelets or turrets, more board space, and more in-house assembly labor. I don't know how that nets out exactly, but Leo was notoriously miserly.

I wouldn't disagree with the statement that he was cheap

I would bet that it's much more likely he could find a source for off the shelf transformers in bulk that already had the winding so he could save the cost of a few more parts, but I guess coming from the custom builder, it seems time is the least worried about item and the total cost of the BOM is... buying transformers in smaller quantities and being able to choose a more common one w/o a winding vs one with the winding is going to be impacting that cost.

The same basic transformer with extra taps, always has some extra cost to it I'd assume, not sure though what that cost may be. Custom wound is even more of course. If you have a known PT that has the winding, use it! If you can buy an easily accessible one with that same winding, use it!

I guess really I'm often using the transformer I was taking from a donor amp. Maybe that's part of sluckey's point. He had one, it had no bias winding, it didn't stop him and it works perfectly.

~Phil