Single ended transformer as pushpull?

[jim]

Hi,

I have a Lundahl 1620/90ma SE transformer with 3k3 and 6k primary impedance.
Is it possible to use it in pushpull configuration with say 6L6, 6550 or EL34?
Any drawbacks?

Thanks in advance,
Jim

 

[DF96]

Does it have a centre tap? SE transformers rarely do.

 

[euro21]

No.

3k3:8 turns ratio 20.31:1
6k:8 turns ratio 27.39:1

For example primary turns is about 2031+739, if secondary is 100 turn.

 

[jim]

Does it have a centre tap? SE transformers rarely do.

Yes, it has the same winding configuration as the pushpull version of the LL1620.
See : Datasheet LL1620

 

[DF96]

The datasheet shows how to wire it for PP. See top of page 4. By buying the SE version you have simply lost some primary inductance and so will have poorer LF performance than you would have got from the PP version.

 

[jim]

Yes, I also figured it out how to wire it.
Poorer LF response? In what magnetude do you think?

I forgot to mention I already have these transformers and would like to know what the consequences would be by using it in PP so that I could achieve some higher power.
The alternative is selling these and buy some real PP-transformers.

 

[DF96]

They don't list a 90mA SE in that datashet, but the 80mA has 45H primary inductance. The PP has 300H. Quite a difference.

 

[jim]

Do you think it's better to buy a dedicated PP transformer then?
A Hammond 1650N for example?

 

[jazbo8]

If you are sure it is wound exactly the same as the PP version, you can remove the air-gap to improve the LF response, i.e., restore it back to the PP version.

 

[kmtang]

SE OPT shouldn't be used for PP. The crossover distortion should be very high, and the micro details will be lost - I believe.

 

[DF96]

Why? I would have thought that the air gap will lead to better linearity, not worse.

 

[kmtang]

Air gap prevents the magnetic core from saturation when there's DC current through it.

 

[jazbo8]

I think you have it backwards... properly adjusted push-pull output stage should have no net DC current flowing through the OPT, thus no air-gap is required.

 

[kmtang]

Yes, that's true. I asked this question to Kevin of K&K Audio. Kevin confirmed the Lundahl PP OPT has very small air-gap allowing small DC unbalance of the output tubes.

 

[DF96]

The datasheet mentions a small air gap in the PP version, to allow for some DC imbalance. The SE version will have more gap, therefore a more linear magnetic circuit.

 

[rongon]

...and a larger air gap means lower primary inductance (all other things being equal).

 

[nigel pearson]

I feel tempted to say send them to me and I will send you 2 x Danbury PP UL 6K6 for EL34. Better not as I have a use for them.

SE is very nice and perhaps you have already tried it? I have always suspected SE would do nicely for PP if the centre winding is available.

Funny thing is I was tempted to brake the Danbury and turn it into SE by rearranging the E and I lamination's. Someone said thin paper to set the gap. Mr Brooks of Danbury I think?

 

[316a]

Funny thing is I was tempted to brake the Danbury and turn it into SE by rearranging the E and I lamination's. Someone said thin paper to set the gap. Mr Brooks of Danbury I think?

Try getting the things apart if there is varnish involved . The only way you'll get a gap in there is with a hacksaw or a virtual gap using an added offset winding

316a

 

[nigel pearson]

Sometimes they fall apart, never when you want them to. SE to PP might just be lucky. The other way, no way. Clean everything if you do and try not to bend things. Don't use varnish to seal. Mostly the sounds of output transformer isn't helped much by it. For mains TF different. My Sowter TF buzz like crazy on 1 kHz tests. OK on music. Get it as hard clamped as you can. That might be a struggle. Wax is worth trying. It might melt. I have potted some and it worked OK (PE).

 

[indra1]

It's a messy job on Ludahls, but with the right tools you can adjust the gap. Use a mini grinder to grind the weld points, replace the spacer with a 25 micron or 1 mil plastic film and reweld the fastener. It's DIY after all.