29-10-2012, 12:48 PM
(29-10-2012, 12:17 PM)Radio Fixer Wrote: HI Lawrence
I didnt put my input very well and was about to edit it when the server went away.
Yes! I agree the smoothing is in the -ve rail. The 100mf of C24 across the field coil is the reservoir cap. The 250 of R12 is a filtering resisitor and the other half of C24 (100mF) is the smoothing capacitor. The HT current flows via R12 and drops 7.8V. The non earthy end will be negative w.r.t earth and the 6Q7 and 25L6 cathodes. So if there grid returns, with filtering by R11, C19,R10, and C18, connect back to here they will have -ve bias. By doing it this way high value small electrolytics (not readily available back then) could, for cathode bias, be avoided.
I'm doubtful that the speaker with the 450 Ohm field with a padding resistor in series (definetly needed) will produce enough volume as it will have far less turns and so the ampere turns and flux density will be much reduced. It wont increase distortion in my experience but volume will drop off ... but it may be enough. And of course the 2.2K could be lowered, with increased rectifier load but these are good for a max of 150mA. 1.5K would maybe up the current through the field to 60mA.
If it is enough I would consider altering the configuration ... Load could be reduced on the rectifier and heat reduced from it and that in the wasteful 2.2K padder.
The alteration would be to simply put the -ve end of the reservoir cap (100mF C24) directly to the 25Z5 cathode. The 450 Ohm field would not need the 2.2K padding as it is now series fed. If the HT current stayed the same and scaling the voltage across the 250 (7.8V) it would drop 14V but this is likely to be compensated for by a rise in HT voltage. Just need to try it...
From the circuit diagram both field and HT current are given as 40mA so the replacement speaker should be no worse off doing it this way.
Gary
Hi Gary, my input wasn't very clear either (nothing new there) however I think we are on the same wavelength so to speak. Yes the rectifier could stand a bit more current draw if lowering any series resistance, as you say if the 450r field coil was designed for a pi filter then in the average small domestic radio the HT current drawn from the rest of the circuit it was intended to fit wouldn't be likely to exceed 35 to 50 ma so that can be used as a guide for working out a series resistor value.
Lawrence.
