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In Reply to: Re: Confessions of a plate choke convert & a few CCS questions (long) posted by Doug Flynn on June 3, 2002 at 14:28:45:
If you want to run a 6V6 as a driver, and drive a 300B, then you would need a much high HT than 350V.I recall that in my first 813 amplifier I was using my 6V6s at about your operating point with 4.7k load resistors - utterly useless - the distortion beggared belief - and this was push-pull! They couldn't knock the skin off a rice pudding.
I would want to run the valve with at least 250V on the anode. Also to get Ra down to somewhere sensible, I would want to run it at around 10W Pa; this would mean 40mA. Now at this operating point, Ra would perhaps be (if you are lucky) about 2k. This means that you should be running an anode load of at least 15k to get reasonably low distortion. 40mA across 15k = 600V, so you would need an HT of 850V.
The reality is that when using output valves as drivers, large anode loads, high currents and therefore high voltages are required.
I know that you are sold on chokes, but if you want to try RC coupling in the future, I have a couple of suggestions. One is the GEC A2134 (CV4062). This is a 9W 7-pin pentode. Triode strapped, its Ra at 20mA is about 1.2k. It can take Va of 330V and has mu of about 8. ALternatively, apart from the single triode 6AH4, what about 6BX7? mu is again about 8 and Ra a soothingly low 1k or thereabouts. This one can take 500V. I run one at about 275V and 15mA per section and it drives my 813s very nicely indeed. It would be better at 25mA of course. Remember that the makers' figures always refer to flat-out operation.
It is no use picking a valve and saying "Ah! this one has mu of 10 and Ra of 1k" if you are not running it at the the maker's operating point! Figures are very disappointing, quite frequently, at lower currents.
My objection to chokes is their frequency characteristics - I always try to get flat frequency response and wide bandwidth - and hopefully low distortion. Chokes offer a high AC anode load at most, but not all frequencies and indeed, they make the HT supply cheap - but they cannot provide the frequency linearity that I would like to see - and they are very expensive - especially when compared to a 25W resistor - silicon, or even a pentode.
7N7
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- Re: Confessions of a plate choke convert & a few CCS questions (long) - 7N7 06/3/0215:45:10 06/3/02 (0)
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