P/T bias winding?

[MitchK]

Maybe I'm over looking something here but I see alot of PTs for sale that having ratings like 360-0-360 at 200ma and then the other secondaries of say 5V and 6.3V. here are the couple things that are really confusing me if anyone can shed some light on this for me.

first, I know the 6.3v is for the heaters, but what about the 5v? I'm trying to build a jcm 800 2203 clone and the big thing that gets me is the bias secondary? I see the P/T that metro amps sells for example and they have a secondary in the range of 100v, that makes sense to me. What do people do with these PT that don't have that winding?

the other thing thats making my brain hurt right now is the how the centertap works. I see ratings like 360-0-360 and I know that will give me 720v, would that be noted as 700 VAC CT? when should the center tap be connected to ground and when not? I'm just unsure about it in general.

oh and the last thing is mA rating, most PT for guitar amps I see have up to 500mA ratings for the main secondary, and then like way higher like 8A for the heaters. is there a way to figure out how much current I'll need? I'd assume look at the max draws on all the tubes and add it up? is there anything else?

does anyone know where I can buy transformer laminations online?

[Enzo]

360-0-360 is the same thing as 720CT. You use it as is appropriate to your needs. Grounding the center tap and running the two 360s to rectifiers will yield about 500 volts DC once filtered. If you apply the 720VAC to a full bridge, then you get about 1000 volts of DC. I doubt you want that. SOme amps use a full bridge, and some use the grounded center tap. The voltage of the winding will reflect which style is in use. In other words ig a maker wants a full bridge, then he uses a 360V transformer winding. If he wants the grounded center tap circuit, then he specs the 360-0-360 type. SO you are going to either use the single winding and a full bridge or four diodes, or you will have two diodes and a center tapped winding. The ful bridge will require a winding of half the voltage that the other wants for a given DC voltage at the first filter.

6.3VAC is for the tube heaters, yes. The 5VAC winding is there for tube rectifiers specifically. It is a separate winding because in use heating a 5Y3 or 5U4, the heater of the tube will be at 500 volts above ground. SO it is separate to isolate it from the others. Many rectifier tubes have 5 volt heaters, though there are also some with 6v heaters. If you wanted to use a 6v heater rectifier tube, you might need a separate 6v winding just for. Many higher power power transformers had multiple 6v windings. That is one reason for it among others.

The tube manuals or the data sheets tell you what each tube heater draws. Yes, just add them up. Then add some margin. If you want 6 amps of heater, don't spec a 6 amp tranny, go higher. if you have a tube rectifier, then treat its 5v winding the same way.

The high voltage winding provides power to the tubes. All the output power comes from the power transformer ultimately. SO if you have a 50 watt amp and 500v on the plates, then you will be needing at least 100ma just for that. Of course the system is not 100% efficient, so add some margin there. The preamp tubes don't draw a lot, maybe a milliamp or two per triode. Add them up. And consider the current wasted in bleeders and other paths to ground besides the tubes.

If there is a bias winding, you can use it, otherwise, tap off the high voltage winding and add a suitable dropping resistor.

[Ray Ivers]

Enzo,

Just FWIW - your last post is now 'Spec-ing Power Transformers.doc' on my hard drive. It's actually one of many of yours... just wanted to let you know.

Ray

[Enzo]

Well gosh. One of these days I will post my secrest of making good chili, and your collection will be complete. I'll keep my cheesecake recipe secret for now.

[soundmasterg]

Thats fine cause I don't like cheesecake!

Actually Ray, you have quite a few entries on my hard drive too to keep the ones from Enzo company.