Dear Mal, LF Group,

No – it is possible to design these transformers with a small air gap, but there does not appear to be a great advantage to doing so because the required number of turns increases, increasing the “copper” losses and offsetting any reduction in core loss. There is always a quite large gap in the original LOPT – often this is produced by inserting plastic film spacers between the core halves. These, and any glue, should be removed so that the core halves fit together perfectly. Sometimes the gap is produced by machining the core legs to unequal lengths, in which case you can’t really use the core in this application.

Cheers, Jim Moritz

73 de M0BMU

Tnx Jim for the info. My LOPT cores have equal leg length, and in the past I have removed the plastic spacers when present and they work well. This appears to be what u have done ie no gap.

Thanks to all the others as well for their info.

I am working on the project at present. I notice up to a few hundred watts there never seems to be a problem with heat, explosions etc but when aiming for an o/put of 1 kw things are more critical, 2 cores needed, more robust wire on the transformer. My present project uses 2 x 26A/500V Fets on each leg ie total 4. I am hoping to get up to about 100v and see what happens. So far I have managed 800w out at 40v/22amps but above this things get tricky, pa getting hot and the winding on the o/p transformer getting warm. The design is similar to YXM/MRF with a few refinements.

I happen to have these fets so I am trying them out, lets hope they survive !!!!!!!!

All suggestions welcome. In the past I have used low voltage high current up to 60Amps and all has been well but going to a higher voltage and lower current the PA performs differently even though this would appear a better approach, using the above Fets.

I should really use my Decca and save myself a lot of trouble with home built gear, but then there is not the same satisfaction when working the DX !!!!!!

73 De Mal/G3KEV

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of hamilton mal
Sent: 31 March 2005 13:36
To: rsgb_lf_group@blacksheep.org
Subject: LF: TX Cores

Hi All Jim M0BMU.

Jim ur mod to the G0MRF using a TV line output transformer. Did u leave a gap between the cores or not ?

73 de Mal/G3KEV