Simple trick to change UL from 40% to 60% ?!

[Gwaltinger]

Hello,

I have an EL34 SE Ultralinear amplifier with 40% screen tap.

I would love to be closer to Triode mode, but not 100% triode mode.

Is it possible, by just flipping both the two primary connections and the two secondary connections, to get 60% Ultralinear (closer to triode !) ?

Or does it have any bad side effects (has an audio transformer a polarity ?!)

Kind regards,

Günther

 

[Osvaldo de Banfield]

No, you need to rewind the transformer's primary, or simulate it with resistors, but they waste AC and DC power.

 

[Gwaltinger]

Thank you ! - Could you explain me why it does not work and what will happen when done so as I've described ?

Günther

 

[Osvaldo de Banfield]

The tap is an intermediate point in the winding, so to change that point, you will need to unwind the transformer.

An example, suppose you have a 220 to 110 autotransformer. If you want to get 150V from the same transformer, you must get the 150V in the winding. No other way, unfortunately.

An poor solution may be to add an inductor with taps, in parallel to the transformer's primary, but it will increase undesirable capacitance at the anode circuit, and lower the primary inductance, also augmenting the lower frequency cut off frequency, also undesirable.

 

[artosalo]

Is it possible, by just flipping both the two primary connections and the two secondary connections, to get 60% Ultralinear (closer to triode !) ?

Yes. This way it is possible to have 60% UL.
But if the secondary has multiple outputs, then the impedance steps do not stay as original.
But if you only need one output (8 ohms ) then no problem.

 

[Gwaltinger]

Hi,

The transormer has just one 6-8 Ohm output...

Is it a problem, that by crossing out the primaries, the outer and inner windings will not be connected at the potentials intended by the manufaturer ? - Has ever one really tried it and nothing was burnt down ? - I really don't want to burn my valuable output transformers...

(I must say, I would not even need to cross out the secondaries too, as the amp does not have any negative feedback. I once even connected the wrong secondary line to ground and once even let the secondaries float without grounding and it didn't have any negative effects one the sound)

Günther

 

[artosalo]

You can connect the primary both ways, reverse and original. It will not be damaged.
I have used my Hammonds (1628SEA) reversed to get 60% UL.

 

[kylej1050]

So I understand, you want to swap the screen and plate taps on the xfmr?

 

[smoking-amp]

For a Single Ended OT, just swap the B+ and plate taps. (and reverse secondary if NFBK used). That puts the 60% section between B+ and UL then. There could be some capacitance (and bandwidth) effect if one end of the primary is preferred for AC.

 

[Gwaltinger]

Exactly, that's the idea: I just swap B+ and plate tap !

Currently, the (single ended) output transformator falls off -1db at 20khZ (without overall feedback). This is just acceptable, but it shouldn't get worse after the change.

How do I find out if there are bandwidth problems ? - Only by measuring the amplitude of a sine wave at e.g. 16khz before and after the change ? - Unfortunately, the manufacturer of the output transformator does not respond to queries.

Günther

 

[BenY]

I don`t know if it`s possible with your transformers, but it could be possible to separate the primary windings into four sections and reconnect them again the way you want it to be.
The windings are usually connected together at the points where the five wires are going in, under the outer paper. if you can see the solder point through the gap, and if it`s the last/top layer, then it can be done.
I assume it`s a PP amp.

 

[artosalo]

The bandwidth at high side seems reasonable, but how does the trafo perform below 100 Hz, down to 20 Hz.
Take 1 kHz and some 90 % of full power as a reference and observe with an oscilloscope how the waveform remains and what happend to the available power at bass end.
This is a good way to analyze the quality of the output transformer.

 

[lcsaszar]

I don`t know if it`s possible with your transformers, but it could be possible to separate the primary windings into four sections and reconnect them again the way you want it to be.
The windings are usually connected together at the points where the five wires are going in, under the outer paper. if you can see the solder point through the gap, and if it`s the last/top layer, then it can be done.
I assume it`s a PP amp.

This is true for push-pull, but the question was regarding a SE transformer. No need a surgery there.