Center Tap Help

[cannikin]

Hey Guys;

I have a Power transformer with no Center Tap on the Filament (which is going to my heaters). On the gates Sta schematic there is a Center Tap from the heater wires going to the ground buss via a 0.5uf Cap, Since there is no Center Tap for this on my trans ... I need to fabricate one - I understand there is a way to do it?.

please Advise, or point me to any literature, would really appreciate it.
But if you Tell me how to do it... and I will love always :razz:

yes I'm still messing around with my Sta-Clone :?

 

[PRR]

This question is so last week.

 

[cannikin]

You rock my world thank you PRR...

 

[cannikin]

So after I take the Heaters 6.3V to each tube, I then take a the two heater Wires and connect a resistor to each wire going to the ground buss... Right?

now the question is 100 Ohm 1/2watt resistors are for 40mA for an La2A Power transf.

The Power transformer for My Gates is a 250-0-250 100mA..

I will need a different value of resistance.. Correct?

How do I calculate the resistance I need?

 

[adamasd]

You are dealing with the 6.3 volt heater taps not the HT taps, no difference from what you are doing to what freddy was doing. No need to calculate anything.

adam

 

[cannikin]

cool thanks a bunch

 

[CJ]

You can fine tune the values for least hum if you are in the "best in the world" mode, but stock resistors are fine. There might be a slight diff in the common mode aspect of the noise you are trying to balance out, this is because the heater winding in the transfomer is probably not perfetly balanced.

 

[cannikin]

excellent fellas

But for the Sta-Level there is a 0.5uF Cap from the Center Tap to Buss Ground.

:? how would I do that? :?

From Heater Wires - resistors - 0.5uF Cap - to Ground ??

 

[CJ]

That would work.

Have you tried grounding one end of the heater supply?
Was there bad hum, or something that made you want to create a virtual CT?

You might just try grounding one end of the 6.3 trans wire.

If it works, you get a couple of benifits, 2 less parts and assocoated wiring, plus, you create a poor man's electrostatic shield between the pri and sec, which might reduce nasty stuff getting from the line to your pwr supply. However, your pwr trans might already have a shield internally connected to the core/bracket assy.

cj

 

[cannikin]

Cj your knowledge blows me away... (along with NYD and PRR :green: )

The reason I'm creating a virtual center tap is because its in the schematic, Right now the unit is getting a low level buzz/hum and I'm also wondering if this is part of my microphonic tube issue...

I've been messing around and reworking the unit a little bit, just trying to learn and understand without shocking the hell out of myself. :grin:

 

[Rob Flinn]

Right now the unit is getting a low level buzz/hum and I'm also wondering if this is part of my microphonic tube issue...

In my experience if you have no connection between the heater supply & ground it is very common to get some hum. As CJ says it may be satisfactory to try the simple solution before going for more complex ones. Rather than use fixed resistors I sometimes use a 1watt wirewound 100 ohm pot. wiper to earth & either end to the 6.3v. you can then just tweak for least hum. However, sometimes tweaking it around makes no audible difference, which is why I suspect that the important thing may be just making the connction to earth.

 

[CJ]

And if you can get away with a grounded 6.3, you can save one wire coming back to the green wires of the transformer by gounding one end of the tube heater to the chassis, but this could cause more hum if your grounding scheme is such that the 6.3 current and the audio ground are sharing the same chassis. In other words, you would probably be less paranoid if you used a star ground for the audio, and the chassis for the heaters.

I never did like running one side of the heater to the chassis as if you ground changes resistance by 0.5 ohms, you might get a 1 volt drop in your heater supply if your dragging about 2 amps. unless you use a big soldeing iron to ground all the heater wires like Brother Leo did.

 

[cannikin]

This is what I did..

I measured the resistance of the Heater wires, which was around 10K.
I then Divided that by two, that became the value of the two resistors I need to make a center tap to a 0.5uF Cap to ground Buss. Hum has decreased dramatically, Voltages looking good, Here are some Pictures. But how does it sound?? Not Sure :razz:

The Center Tap

The Center Tap Overview

The Unit REWORKED - Different meter that tracks better :grin:

Approve???

(now I have to deal with the Input and Output PADS... :evil: )

 

[CJ]

Wow! Looks nice!

I like those standoffs. Really handy. I scored a whole bag of those off of evilbay.

6386?

 

[cannikin]

thanks Cj

Your a lucky man to score those on ebay... I looked everywhere and only found 20 of them.

Yup that is a 6386.... expensive little beast aren't they

 

[bcarso]

[quote author="cannikin"]This is what I did..

I measured the resistance of the Heater wires, which was around 10K.
I then Divided that by two, that became the value of the two resistors I need to make a center tap to a 0.5uF Cap to ground Buss. Hum has decreased dramatically, Voltages looking good, Here are some Pictures. But how does it sound?? Not Sure :razz:


[/quote]

I'm glad that you have gotten improvement---but where did you get this 10k again?? That's way too high to be either a cold tube filament or a transformer winding resistance. I would go lower on the two resistors.

The other trick, as discussed recently in a few threads, is to bias the filaments slightly positive with respect to the cathodes---this prevents the filament from acting like a little emitter and conducting any residual hum/noise to the cathode.