Fixed Bias PS Transformer question

[kagliostro]

I've always read that in the Bias path there isn't current flow

My question is

If I want to add a Fixed Bias somewere there are many methods to do that, but assuming I want to build a stand alone Bias PS, wich will be the smaller wattage transformer that can be used ?

In other worlds, will a 3VA or only 2VA transformer do the job ?

Thanks

Franco

[tubeswell]

2VA is fine

[kagliostro]

Thanks Tubeswell

Franco

[PRR]

I've always read that in the Bias path there isn't current flow...

There's always some current.

Suppose the bias voltage is 50V and the bias divider/bleeder is 50k resistance. That's 50V 1mA. So 0.050Watts. AC-to-DC conversion is hard work, pencil twice that or 0.1VA of AC.

[kagliostro]

Thanks PRR

My fault I wrote what is a diffuse opinion but in realty I know there is a very very very low current on the path, say so low that is negligible


Franco

[HotBluePlates]

I've always read that in the Bias path there isn't current flow...

There's always some current.

My fault I wrote what is a diffuse opinion but in realty I know there is a very very very low current on the path, say so low that is negligible

Notice that PRR is talking about the current from the source of AC Volts through the bias supply.

However, when you say "there isn't current flow" that is talking about from the tube grid to the bias supply's output.

Both are right; there is also some small current due to AC Volts passing through the coupling cap to the output tube grid(s).  That is presumably small enough to be sourced by the bias filter cap at the bias supply's output.

[kagliostro]

Many Thanks also to you HotBluePlates

Franco

[PRR]

...Notice that PRR is talking about the current from the source of AC Volts through the bias supply. ...

There has to be DC path. The power tube grid may leak 100uA or more. Much more when clipped. While we could use a PNP transistor to drain that only when needed, it is usually simple and cheap to have a 1mA bleed current.

[kagliostro]

While we could use a PNP transistor to drain that only when needed

May be I remember something like that (Geloso amp ??, don't remember well)


EDIT: Do you mean something like this ? (   those are NPN)




Franco

[PRR]

I worked the Fender 5F6s (before I saw your Geloso).

Although I figure you need 1.7mA of AC, actually you won't use wire that small: it would break or corrode too easily. While I figure 0.13VA of transformer needed, you really don't go below 3VA or 2VA because the price does not go down but up at very small sizes. (Unless you are building aircraft or balloons where lightness is really valuable.)

[HotBluePlates]

There has to be DC path. The power tube grid may leak 100uA or more. Much more when clipped. ... 1mA bleed current.

I see what you mean now.  Thanks!

[kagliostro]

Very good explanations

Franco

[pdf64]

… assuming I want to build a stand alone Bias PS …

I’m left wondering why you might want to do that 

[kagliostro]

I’m left wondering why you might want to do that

You are entering a restricted area

Only authorized personnel allowed

 


London Power do that why I don't ?

https://londonpower.com/bias-supply/





Franco

[kagliostro]

No, seriously, some years ago I and a friend at DIYItalia tried to put in a 50W (Fixed Bias) Marhall amp a VVR

the VVR is simple for the cathode biased amp, less for Fixed Bias amps

we tried Merlin's circuit



but my friend (he lives very far from me) wasn't able to have it working correctly on the amp

a search we did at that time pointed the attention to Bias PS impedance as reason for the missed positive result

Searcing on the web at that time I've find the London's Power Bias Kit that say LOW impedance PS

that thing was ended at that time

this days I'm planning the conversion of a Geloso G1 1020 A PA amp into guitar amp, the 1020 has a Fixed Bias and we decided to adopt Mixed Bias, from this the idea to plan a small board for a DIY Fixed Bias PS to be used in conversions (also from not so skilled guys) of cathode biased amps of various kind into guitar amps

That's all folks

Franco

[sluckey]

I’m left wondering why you might want to do that

London Power do that why I don't ?

https://londonpower.com/bias-supply/

That's your reasoning? That's crazy! BTW, I disagree with most everything stated on that londonpower link.

[kagliostro]

The reason Is to plan something that can be used easily by guy who ask for something like that at DIYItalia

No commercial purpose

And... my personal knowledge if we want to say which Is my personal interest

Franco