[Amps] G2DAF four 4CX250Bs G2DAF amplifier

[Peter Chadwick]

Hsu said:
>the non-linearity or 4CX250B's weak grid and screen?<
Both. There's been a lot of argument about the basic G2DAF ciruitry over the years. Right back when it was published (and I'm probably the only person on this reflector to have read the original article and comments in the RSGB Bulletin - shows my age!) there were several comments in the letters page, especially from George Jessop, G6JP. Now George was a cantankerous old wotsit at the best of times - he and I had many fights at RSGB Council - but we got on fairly well, and George told me a year or so before he died that he'd actually built one at work (he worked for the MO Valve Company, doing manufacture, applications and design of valves) and his opinion was that under certain conditions of adjustment with certain tubes, it might just be acceptable. But as a sound technical approach, no. Other people here have heard them all over the band......
The 4CX250B with its capability of negative screen grid current has the capability to really get screwed in such a circuit, and it may well try floating the screen up to the anode with the screen emission..even in AB1, it is according to my data book, a -30dB on PEP capability tube. I do not expect G2DAF operation to improve that. Ordinary AB1 operation with negative feedback should improve matters - few tetrode amplifiers at HF appear to use NFB, which is why they are comparatively poor. Incidentally, the PEP measured from one tone to the IMD product and then with 6 dB added to get relative PEP is a common approach, but not correct. But if you define what it is that you're measuring, I suppose it doesn't matter, and it helps marketing by giving them another 6dB...
And to me, a GU50 is an old British made mercury vapour rectifier tube, with performance similar to an 866A!
73
Peter G3RZP