6L6G modulator. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 28th Oct 2013, 5:24 pm

I've used EF91s in VFOs plenty of times in the past - invariably a Colpitts electron-coupled setup with the screen-grid acting as the "anode" for the oscillator and the RF output taken from the genuine anode. If I were to run the EL91 in Class-A I could probably get enough output without needing the second EF91 as a buffer: again, class-A puts least loading on the preceeding stage so getting me the greatest grid-voltage-swing without drawing actual power from the VFO.

Given that I've already got the modulator chassis/transformer and all the valve-base cutouts are for octals, it's the Path of Least Astonishment to stick with these. The RF side will need a new chassis and my stash of Q-Max chassis-punches are only for B7G B8A and B9A bases so that's what I'll be using.

I m a bit concerned about the electrical health of the modulator mains-transformer though: it's distinctly frayed round the edges and so I'll be giving it a go on a Megger to check that the insulation's still healthy.

28th Oct 2013, 5:24 pm	#21
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: 6L6G modulator. I've used EF91s in VFOs plenty of times in the past - invariably a Colpitts electron-coupled setup with the screen-grid acting as the "anode" for the oscillator and the RF output taken from the genuine anode. If I were to run the EL91 in Class-A I could probably get enough output without needing the second EF91 as a buffer: again, class-A puts least loading on the preceeding stage so getting me the greatest grid-voltage-swing without drawing actual power from the VFO. Given that I've already got the modulator chassis/transformer and all the valve-base cutouts are for octals, it's the Path of Least Astonishment to stick with these. The RF side will need a new chassis and my stash of Q-Max chassis-punches are only for B7G B8A and B9A bases so that's what I'll be using. I m a bit concerned about the electrical health of the modulator mains-transformer though: it's distinctly frayed round the edges and so I'll be giving it a go on a Megger to check that the insulation's still healthy.