My Second Radialva Groom Restore

[pwdrive]

Hi all, looking at the diagram that John posted it looks like the field coil is not in series with the HT but partly across it, it's the termination to the bottom end of the heater chain/bias resistor I am not too sure of so far as a circuit description goes. Looks interesting, any ideas anyone?

Lawrence.

[Radio Fixer]

To me it looks as if the field coil is across the un-smoothed HT but with 250 Ohms in series to earth, which is the the -Ve HT line. The field has C24 across it for some smoothing.

The 250 drops voltage (-7.5V) and so is negatibe w.r.t the -Ve HT line and after some smoothing is used as bias for the valves.

I have a US set where the field coil is across the HT, cant remember which one. Of course they are much more likely to be O/C as the wire used is much thinner and there is a lot more of it.

Gary

[pwdrive]

Hi Gary, yes that's more or less as I see it, it looks like the filtering is all done in the -ve rail as opposed to +ve rail with a combination of the 2 x 100uf and the 250r ? The PD across the 250r is as you say used as the bias potential.
If that reasoning is correct then the field coil would contribute little to the smoothing, I could be wrong on this?
I am thinking that the voltage drop across the field coil would be too much for a 110v set if the coil was connected in the conventional pi filter way. It looks a very efficient design.

Lawrence.

[Yorkie]

We still haven't answered John's original query - will it be detrimental to the field coil or operating conditions of the set if the 450 Ohm field coil is fitted as a substitute for the original 3000 Ohm one. True, it doesn't have to carry the full HT as would be the norm, though I can't help but think that it's less than ideal, and that to be on the safe side - initially at least - it should perhaps have a 2K2 resistor in series to bring up the total resistance to nearer 3K. (From the voltage drop across the coil, the current drawn through it can of course be determined).

Any thoughts anyone?

[pwdrive]

Hi David, yes, a field coil with a DC resistance of 450r wouldn't do the rectifier much good, if a series resistor is included then as I see it the the mag. flux generated is going to be somewhat less maybe a lot less?

Lawrence.

[pwdrive]

Forgot to add: I suppose it depends how the replacement speaker was configured originally, if designed for higher current then maybe a seperate rectified DC feed might be needed.
As you say, try it with a series resistor of 2.2 or 2.5k of a suitable rating and see what the results are, if lowering the value is required then the valve rectifiers operating parameters might have to be taken into consideration.

Lawrence

[Radio Fixer]

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.

Described this incorrectly so done it again lower down

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

[pwdrive]

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.

[Radio Fixer]

Got two inputs UGH!

G

[Radio Fixer]

Hi Lawrence

You didnt comment on that if the 450r speaker produces enough volume with a 2.2K padder, in shunt, it may as well be moved to be series fed saving 40mA off the rectifier and reducing heat in the cabinet. Seems sound to me, but I described the modification incorrectly (its OK on my diagram but I got in wrong in translation). Sorry to confuse.

"The alteration would be to put the 450r field, with no padding 2K2, before the 250r that produces the -ve bias (no change to this). The reservoir cap (100mF) -ve end would go to the 450r free end. If the HT current stayed the same and scaling the voltage across the 250 R (7.8V) the 450r field would drop 14V but this is likely to be compensated for by a rise in HT voltage. Just need to try it..."

Gary