Transformer center tap and non

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Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
Hi

I got a question for you guys. I'm looking to build a AM864 and I was looking at the psu and started interogating myself about the heaters winding (both for the rectifier and the tubes).

When looking at, for exemple,  Hammond / Allied transformer, some have a 6.3V coil and 5V coil too. Some are center taped and some not. Sometime, the 6.3v coil is cente taped but not the 5v coil. Then I looked at the XPWR012 from Edcor.

This one has a 6.3V coil that is center taped, so basically 3.15V-0V-3.15V. Which means that betwen one of the lead and the center tap, we have 3.15V, same with the other end, so we have a total of 6.3V between both ends when center tap is connected to ground. Isn't it?. Does that mean each wire carries 3.15V? I mean, if I run them to a tube and connect to pin 4-5 and 9 so we're in paralelle and should read 6.3V between pin 4-5/9, meaning that each pins is fed by 3.15V? If I run them to pin 4 and 5, so I need to provide 12.6V, so we're better off with a 12V filament coil? What happened with a non center taped coil? we have 3.15v at both end, too, or only one wire run all 6.3V and the other lead is our common, like the primary side (for exemple 120V-common)? We need to make an artificial tap to ground at each end to help shunt leakage from the primary, isn't it?

Now, looking at the 5v coil of the XPWR012, it has no center tap and it indicate 5V at one end and common at the other end. Does that mean it is not balanced? I doupt it's what we need to feed the 5Y3 filament pin 2 and 8 isn't it?  This one struggle me a little loool.

Sorry if that seems to be a nooby question. I like to understand the maths behind everything I did, not simply painting by number :)

 
Also, if I use the 5V coil to feed the rectifier, Would I be better off to make an artificial ground connection on each lead?
 
Ok, after some research I found on valve wizard that "A valve rectifier requires no heater balancing since the HT has bigger noise problems to worry about" so if my 5V coil is 5V - Common (0V), that means it isn't balanced, but in our case, we don't care.
 
5V is for the rectifier and goes high, connected to the anode, so no ground referenece should be added or you will be shorting the B+.

6.3V center tap should be connected but not a deal breaker since using 2 matched 100Ω resistors works just as fine and have 2 advantages, first you don't need a center tap, if it is there doesn't hurt, but you don't need it, more options to chose. The second is if you accidentally shorted one side of the transformer to ground you will have 100Ω across 6.3V rather than 0Ω across 3.15V, I personally prefer the first one.

The center tap or the resistors are used to reduce hum in the signal. In the case of the 5V for the rectifier sometimes is there to get the output of the stage, but if it's not is not a big deal, you will have the resistance of the heater in parallel with the winding and a bit of DC in the winding. Either seems to be a good thing but the DC current is so much lower than the AC current on the winding and the impedance is so much higher than the load it's feeding that it really doesn't matter.

JS
 
transformers with a center tapped 6.3 wind will cost a little more, but it saves you two 100 ohm resistors plus installation, how much labor do you save by not having the girls solder two resistors? probably not much, but 100 ohms is not a commonly used value, like 1.5 k cathode resistors and 100 k plate resistors, so you have another bin in the stock room, another part to order, another part to pull when making a job kit, Fender mostly used the CT, Ampeg liked the resistor method, one advantage of having no CT is that you can arrange a hum balance control, that way you can be sloppy with your lead dress and layout and cancel the hum anyway, might save additional chassis soldered ground wires, which uses electricity for the big irons and a lot of solder, so you use a crappy grounding system like the  Ampeg daisy chained ground bus and get away with it,

 
Sorry to interrupt, but where exactly do the two matched 100Ohms resistors go?

I'm looking for a PT for my pultec project at the moment an I found a "Freed" one for only 20bucks right around the corner, but it doesn't have the 6.3V center tapped.  An it would be awesome if I could still use it since the other secondaries are at 535V CT / 85MA so it would almost be perfect!

Thanks!!
 
They go from each side of the 6.3V winding to ground, so they act as a center tap. As CJ said some use a pot to cancel the hum, if you have fixed resistors you have fixed hum cancelation, usually matching the resistors is enough and you don't need the added complexity, since the pot must have a few watts and low value usually WW pots were used for this task. I've used a 500Ω carbon pot without much trouble, the biggish ones, not 9 or 16 mm ones, but I don't know how reliable they are. It was in a stereo amp which had 3 WW pots, one for this and other two to balance the bias in each channel, one of them was broken and I needed the one at the hum cancellation to balance the bias cause I couldn't find the replacement.

JS
 
You will find when building tube gear that sometimes you will get some heater hum - whether you have a transformer with CT, whether you use matched 100 ohm resistors, or whether you get fancy and float them (like Pultec).  My point here  is - you can't really tell beforehand whether you  will have heater hum issues or not.  That being said I always allow myself the option of a balance pot as the last resort.  This is the equivalent of using resistors as far as what type of transformer you need. In other words you don't need a CT on the heater winding.  If you got it already, go ahead and try it.  Many times it works fine, but if you want to cover yourself more fully, the balance pot does it for all but the most stubborn cases.

Also, don't be afraid to do as they originally did and tie one side to ground if all else fails.  There are many pieces of commercial gear from the vintage era that did it this way without hum problems.    Do whatever works.
 
Many thanks folks. Let's take a transformer without filament C.T.  Is it always, for exemple, 6.3V - common (0V) or sometime, the coil is balanced, OR to balance the coil we need to make a center tap?
 

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