Does this PT have a center tap?

[dude]

Hi all,

I modded this crate VC2110 amp with help from the forum basically from Sluckey, thanks. But like many suggested why don't I build what I want so I decided to go that route. Below is the layout of an 18 watt super lite I'd like to put in the chassis of this Crate. I want to use the same PT as the tube layout is the same.

My question is in the layout there's a full wave rectifier and in the Cate schemtic a bridge R as there's no center tap.  So I planned on using the same bridge R as in the Crate. But I see a green wire on the top of the PT attached to the PT (a ground) what is this ground? I see no center tap on the schematic?

Also there is a 100uf e cap on the crate schematic, and the layout is a 32uf smoothing filter cap, which should I use for the first cap?

If I change the 18 watt super lite to 6V6's (with necessary changes) would this PT be able to handle the current draw?  I see the B+ on the Crate schematic is 320v and know no other specs.

Thanks in advance,
al

[kagliostro]

To me that green wire connected to ground may be the CT of the heather winding

or, at least, a shield

You can easily verify with your tester

K

[dude]

The only wires off the the PT are the black and white on one side and two red on the other side (AC). The three wires next to the reds are  the heaters, the yellow being the heater CT.

Since the schematic has a bridge rectifier with this PT and no CT for the red windings, I figured that greed ground wire is not a CT? How do I test it with a meter?   

I read somewhere that a bridge rectifier needs a large first e cap and noticed that the Crate has a large 100 uf cap. Can I use a 32uf like the layout instead? Can the PT handle two 6V6's?

thanks,al

[kagliostro]

Since the schematic has a bridge rectifier with this PT and no CT for the red windings, I figured that greed ground wire is not a CT? How do I test it with a meter? 

You must disconnect the green wire from ground, then use your tester and give a control for continuity between the green wire and the other wires that comes out of the transformer (may be you have to disconnect it from circuit)


If the response is that the green wire is not connected with any other winding present on the transformer, it is an internal shield

---

I read somewhere that a bridge rectifier needs a large first e cap and noticed that the Crate has a large 100 uf cap. Can I use a 32uf like the layout instead? Can the PT handle two 6V6's?

May be you have problems with a large capacitor ad Vacuum Rectification, using Solid State rectify you can use big capacitors, but this isn't strictly obligatory

K

[HotBluePlates]

My question is in the layout there's a full wave rectifier and in the Cate schemtic a bridge R as there's no center tap.  So I planned on using the same bridge R as in the Crate. But I see a green wire on the top of the PT attached to the PT (a ground) what is this ground? I see no center tap on the schematic?

Look at your schematic.

There are 2 secondaries on the right side of the transformer schematic symbol. The upper secondary has 3 wires, with one of them in the middle of the other two; this is a center-tap. Those wires run to the right to a pair of symbols marked "Header" with 2 wires showing a 6.3vac output, and the middle wire connected to a ground symbol. That is a grounded center-tap for the tube heaters.

On the layout you posted, the voltage is not indicated as "6.3vac" at the power transformer, but in volts relative to the center-tap. So it shows "3.15" for one side of the filament winding, "0" for the center-tap and "3.15" for the other end of the filament winding. 3.15+3.15 = 6.3, which is how we (and the tube) normally think of it.

The high voltage secondary in your schematic has no center-tap, and you are correct that you'll need to use a bridge rectifier just like the original amp did.

[dude]

yes, that's what I thought, CT for the heaters and none for the secondaries, hence the bridge in the schematic.

I was confused about that green ground wire on the outside of the PT, thinking that was the secondary CT. Without the schematic I would have thought that green wire was the CT for the ac voltage.

Thanks for the confirmation. Looking up heater draw for 6V6s seems it's less than for EL84's so 6V6's should work with that PT.

And since the Schematic has a 100uf at the power supply, I'll just keep it there and forget a cap can, saves me from drilling a hole too.

thanks all,
al 

[HotBluePlates]

I was confused about that green ground wire on the outside of the PT, thinking that was the secondary CT.

The filament winding is a secondary winding. But so is the high voltage secondary winding. So we always have to specify which transformer winding we're discussing to avoid confusion.

[dude]

I'm probably not using the correct terminology but to help me use the correct description tell me if I got it right:

The primary on a PT is where the wall voltage goes, the secondaries can be high voltage or filament wiring or a rectifier or a bias voltage?

If you look at the picture below of the "outside case of this PT", you see the two high voltage red wires (HV secondaries) also you see the filaments, two orange 3.15v each and a yellow center tap for the filaments.

But also is a green wire grounded to the tranny bolt. I thought this green wire was a center tap for the two HV secondaries (red wires). But couldn't figure out why the schematic showed a bridge rectifier. So then I thought it isn't a CT tap for the high voltage.

What is it? Hence the post with a few other simple questions.

"Kagliostro said if there is no continuity between that green wire and all the other secondaries the nit's not a center tap but an "internal shield".

I've never seen a wire grounded like that on any PT I ever worked on....?

Hope I'm not out to lunch here but I may be, ha

[mresistor]

Yes  Primaries connect to the input AC voltage, through a power on switch and a fuse. And you are absolutely correct on the usual secondary windings.

I've got a few PTs that have a shield wire that bolt to ground (PT mounting bolt). It's fairly common on some brands like Heyboer. The wire colors are different. Usually heater wiring is green and the center tap would be green with a stripe on it. So the color coding on this transformer is a little non-conventional.  If you make sure to do a continuity check from the yellow to both oranges and orange to orange to make sure there IS continuity and then from the yellow/orange to ground and to the other taps to make sure there is NO continuity between any others, you will know for sure. Plus you should be able to see where they were connected in the old amp. The orange wires would have connected to pins 4&5/9 on the 12ax7 tube sockets, 4/5 on the EL84 sockets, and the yellow to a chassis ground connection. 

Here is a link to Allen Transformers, he specifies that his transformers have a hum shield lead brought out to ground on the chassis ....
I know some of his transformers are made by Heyboer.     http://www.allenamps.com/parts.php#transformers

[eleventeen]

The pri on a PT is where the wall voltage goes, the secondaries can be high voltage or filament wiring or a rectifier or a bias voltage? <--right.

If you look at the picture below of the "outside case of this PT", you see the two high voltage red wires (HV secondaries) also you see the filaments, two orange 3.15v each and a yellow center tap for the filaments. <<--technically you don't really know the voltage of that filament or whether it even is a filament winding until you measure it or know its specs from a datasheet. Sure, absolutely, you suspect it to be a filament winding. My question would be "why isn't it green"?
 
Traditionally, by convention, yellow-stripe is the color for a CT on *any* winding, but of course, this becomes problematical if the tranny has a CT on the 5V winding because the expected color of a 5V winding is itself yellow. So we might see a yellow with a black or blue tracer thread on cloth wires or yellow with a contrasting stripe on thermoplastic insulation for the CT of a 5 volt winding. Fairly uncommon but they are out there.

Unless it was a really, really high voltage transformer for a tube-based radio transmitter or an X-ray machine or some strange piece of test gear.....I would expect any transformer that had a "tube B+" winding, eg, a winding capable of supplying 250+ volts with an ordinary rectifier or voltage doubler to have a 6.3 winding. Kind of comes w/the territory. You will definitely find 1200-0-1200 volt transformers for transmitting equipment that have no filament winding because by the time you are up in the thousands of volts zone, nobody knows if you are going to use SS rectifiers or mercury vapor tubes (many of which use 2.5 volt filaments) and frankly, you do NOT see 6.3-volt fired tubes in such rectifier circuits. What you want when you are making a 2500-volt linear power supply is for the filament winding NOT to be a part of the HV tranny because the insulation demands are so high. Plus, in trasmitters and the like you have things like interlocks that shut off tubes when cages have been opened (to prevent big-time sparking or electrocution) and you generally want independent control of DCHV and heaters. 

Most 6.3 volt winding wire colors would be green, again, by convention. Here you have no green "paired" wire(s) coming from the tranny. So if there's a heater winding, it must be non-green. Thus, if only a single green wire is present, we suspect "green" implies it's other typical meaning, eg; "ground".

It MAY OR MAY NOT be the case that a particular circuit GROUNDS the CT of a rectifier (or any other) winding. Depends on external factors.   

No transformer that has a center tap on any winding should "force" you to ground said center tap by internally (and thus invisibly) connecting it to the frame of the tranny = ground. That idea strikes me as....odd. While 95% of any sort of let's say 350-0-350 volt + 6.3 volt + 5 volt power transformers will ultimately ground the HV CT >>as part of the circuit-designer's choice of external circuitry<< the idea that the mfr would supply such a transformer with the CT "pre-grounded" is....just odd. Some designers, for example, lift that CT from ground for a standby function. Couldn't do that if it was permanently attached to the tranny frame. If you look at certain tube-based bench power supplies (Hewlett-Packard, EICO, Heathkit) you will see that many times, a CT of one HV Winding > Rectifier is referenced to some voltage level that is decidedly not ground. So, if I saw that green wire on a tranny like you show, it would strike me as "shield", that I would definitely want grounded, versus "center-tap" which I am free to ground or not ground depending upon ckt design.
Some transformer mfr's have a shield of some sort, internally (and thus invisibly) connected to the frame. Some bring that connection out with the other wires. It probably depends upon what the user of the transformer ordered. A maker of sensitive high-end instrumentation or radio gear might want that connection brought out so he could make a "better", more visible, more testable ground to a serious ground in his piece of equipment. 99.5% of all other users might be completely satisfied with a plain-Jane internal ground to the frame that's never seen or particularly thought of. 


"Kagliostro said if there is no continuity between that green wire and all the other secondaries the nit's not a center tap but an "internal shield". <<--Agree.