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Multiple primary impedances output transformer

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Elerion

Member
Joined 2017
Have anyone tried to use an OPT with multiple primary impedance taps?
Not for UL operation, but for regular anode connection.

Imagine we have a 4 sections OPT, like this
P:S:p:S:p:S:p
(Primary section, series wired)
(S:secondary section, series or parallel wired)

Imagine the primary impedance is 5000 ohm (for EL84 or 6V6)
If we make a primary tap at the beginning of the last primary section:
B+ --- P:S:p:S:p:S( here )P --- anode

We will get around 2.8 kohm impedance, if moving the anode there.

Suitable for paralleled EL84/6V6 or even for single EL84/6V6 triode mode.

The idea is using the full primary, OR using the shortened primary, leaving out the last section, to be able to use in other configurations such as those I just mentioned.

I suppose it will work perfectly fine, as winding sectionalization is still ok, and leakage inductance shouldn't suffer.

What do you think?
 
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E

Elerion

Member
Joined 2017
I was thinking that, usually first and last sections are half of a regular section. This is suggested to improve (reduce) leakage.

If we leave out the last section, then we have a non symmetrical transformer.
We will go from:

B+ --- p : S:p:S:p:S : p --- anode
(p: half turns primary section)

to:

B+ --- p:S:p:S:p:S --- anode

Not ideal, I suppose.
 
jazbo8

jazbo8

Ex-Moderator
Joined 2011
Here is an example of an asymmetrical SE OPT from Hashimoto:

Screenshot - 6_1_2018 , 12_19_01 PM.png
 
E

Elerion

Member
Joined 2017
haiqu said:
Menno van der Veen has a thing going called "The Project"
Click to expand...
Very interesting.

For example, he uses a push-pull OPT for SE too, by leaving another tube (as if they worked in push-pull) as current source (no signal input, just bias) to keep the OPT happy, so it does not saturate.

If it were me, I'd replace the second tube with some other solid state current source.

A slight digression on the starting topic.

Nice way to reuse PP OPT, or to wind a multi-functional OPT as Menno does. Or even, wind a PP from the begining to be used as SE, to avoid gapping, and achieve higher inductance.
 
E

Elerion

Member
Joined 2017
Yes jazbo8, as I said in my previous post, for SE operation, "leaving another tube (as if they worked in push-pull) as current source to keep the OPT happy, so it does not saturate."
I just find it much more efficient to use a solid state current source for these, rather than leaving a hot tube "idle".

Thanks for the link to the paper.


Going back to the matter
I designed a low power SE OPT like this:

(p: half turns primary section)
B+ --- p : S:p:S : p : p --- anode

It taking the output from a tap between those last primary sections:

B+ --- p : S:p:S : p --- anode

I get some less inductance, and I'm able to suit better UL-triode modes, than just add a switch (to connect either UL tap or anode via a 100 ohm resistor to the screen) and use the same impedance, as many people do (which is just unfair for comparisons).

I suppose the leakage inductance will not be much worse.

Any alternatives to this?
How would you do it?
 
E

Elerion

Member
Joined 2017
Sure.
In this particular case, the full primary is used for UL operation.
Leaving the last primary section out is for triode operation.
I thought that triodes strapped pentodes, due to their low output impedance, would requiere less primary inductance, and probably it will work just fine.
 
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