Dedicated PT Bias winding ..... how many mA ???

[kagliostro]

Assuming to be planning a PT for a fixed bias amp and wanting to have a dedicated winding for Bias on the PT

there is a rule to decide how many mA this winding must be able to supply ?

This question is because as far as I can know there is no consumption on Bias circuit ..........

Thanks

Franco

[shooter]

This question is because as far as I can know there is no consumption on Bias circuit ..........

I would agree with you, there shouldn't be.  If you're designing for failure though, do you make sure a resistor burns up before the tube or tranny? 

[sluckey]

There is practically no load imposed by the tubes. The main consumer is the voltage divider of the bias supply.

[jjasilli]

I had the same question some time ago.  I think it was PRR  who replied that the bias tap has the same mA  rating as the secondary winding , because it is a tap off that winding.  However the bias tap mA rating is not really important because the bias circuit draws near 0 current.

[Tony Bones]

Ohms Law?

I = supply voltage / sum of series resistance to ground

[PRR]

The tubes have a Maximum grid resistance, often 100K. This must include the bias supply, basically the bias trim resistors. So to truly meet specs we should probably have 50K on each grid and 25K in the bias supply.

We often let it go 100K per grid and 50K in the supply (which is really 200K each grid), but to design the bias let us use the lowest likely numbers.

70V raw DC with 25K total bias supply resistance leads to 3mA.

This is "never" a problem because a wire to melt at 3mA is too small to handle. Even the smallest stuff will take 30mA. As someone says, a bias TAP is often part of the 100+mA main winding, so really no-problem.

[kagliostro]

Thanks PRR

I was referring to a dedicated winding so your explanation is what I was looking for

Franco

[jjasilli]

Whoops, I somehow missed the point that you wanted a separate winding, not just a tap.  Is there a specific reason for a separate winding?

[kagliostro]

Ciao Jjasilli

Some time ago, I and a friend at DIYItalia, we were trying to apply to a Marshall amp a Fixed Bias VVR (with bias automatic regulation)

from Merlin's book and there were problems around the bias regulation, so, looking around, at Londonpower we have read that they

sell a separated small PT for Bias Circuits, planned to solve problems due to impedency of the bias windings

also, some time ago I was planning a mod for an Ibanez amp that had a separated winding for Bias Supply

I put together a bit of things and decided to collect info as to be able, when I'll plan a PT for a Fixed Bias amp, to try a Full Wave Bridge rectify

on the Bias PS circuit

and, this is all

Franco

[sluckey]

Some time ago PRR offered a good idea for a separate transformer for bias... Just use a small 120v to 12.6v filament transformer. Connect the secondary to your 6.3v filament winding and you have 60VAC available on the 120v winding.

[Tony Bones]

I like that. Cheap alarm clocks with LED readouts often have a tiny 12.6V CT transformer inside the size of a US quarter. Everybody has a broken or unused alarm clock, right? (I use my Android phone to wake me up these days.) Or buy a brand new one for $5 at your local big box. They're tiny.

[kagliostro]

Some time ago PRR ............

That is a good idea for sure

only I'm on the path (at least in the future) to plan and build my own transformers



Franco

[Lindsay]

That looks great Franco. Have you designed the bobbin mandrels yet. I've can photograph my method if you like. The bias is simple as mentioned above. I always wind a separate winding as it's easy to find space for the small amount of turns required.

Lindsay

[kagliostro]

Ciao Lindsay

Have you designed the bobbin mandrels yet

.... bobbin mandrels ......... ?

Do you mean the part that sorrect the sprochet of the transformer to be wind



or as bobbin do you mean the bobbin of copper that feeds the winding machine ?





Franco

[Tony Bones]

I think he means the first, and I'd like to see his method even if you're not interested. Making good mandrels has always seemed too difficult the way I do it.

[kagliostro]

Oh, but also I'm interested, there is always something to learn


Franco

[Lindsay]

Yes I meant the first. I will take photographs of my arrangement and post in a few days.

[kagliostro]

 

Franco

[drgonzonm]

1.  I take it that you are winding EI transformers transformers, and not C type? 
2.  From some of your early posts, I know you do a lot of experimenting with Power transformers, are you planning to do output, and if so, SE or PP? 
3.  From the bobbin picture, what is the temperature rating of the insulation on the wire? 

Thanks.   

[kagliostro]

1.  I take it that you are winding EI transformers transformers, and not C type?

At the moment the winding will de done with a friend (my winding machine still require some work before to be used) and an EI transformer  is on the way to be build

2.  From some of your early posts, I know you do a lot of experimenting with Power transformers, are you planning to do output, and if so, SE or PP?

I did some experimenting with existing transformers, still not building it, the transformer that is planned to be build is an SE OT (4200ohm / 2-4-8ohm)

3.  From the bobbin picture, what is the temperature rating of the insulation on the wire?

The picture is only one example I got ftom the Web, however I don't know also the exact temperature rating of the wire we will be using because the wire is supplied by my friend that knows someone in a factory where they build industrial transformers (they give him the wire)


Franco

[drgonzonm]

SE OT design.

As you are likely aware, RDH4 has design information for SE OT's.  RDH4 references Norman Crowhurst work. 
An additional source of information is the Norman Crowhurst' basic books "Basic Audio Vol 1, 2, and 3.  try "crowhurst_basic1.pdf" in your search engine.   

I am sure Crowhurst has addition material. 

Good Luck

[Lindsay]

Sorry about the delay, and the image quality is poor but I think you will get the idea Franco. It is a very simple system. I have mandrels for most sizes I have or can space out smaller ones with scrap material to make it work if required.

[kagliostro]

Nice realisation Lindsay

Many Thanks

I like the whole set of choices   

Franco