My Second Radialva Groom Restore

[tiv48]

hi john would this be what you are looking for?
rob t

[Geaney]

hi john would this be what you are looking for?
rob t

Hi Rob, that looks exactly like the one I need - especially if the mounting holes are about 42mm apart. I'd be delighted to come to an arrangement as they say if you can spare it.
Cheers, John

[Geaney]

I may have had another stroke of luck finding replacement parts - I found a 4in 3.6 Ohm field coil speaker on eBay over the weekend so I figured I'd plash out the $20. The item doesn't indicate the resistance of the field coil but I figure it would give me a good head start versus going with a PM speaker.

Cheers, John

[tiv48]

hi john let me have you addess and its yours
rob t

[Geaney]

hi john let me have you addess and its yours
rob t

Major thanks Rob, details sent in a PM.
Cheers, John

[Geaney]

Some eBay goodies have arrived so I now have a full set of valves. The field coil speaker also arrived but that will require some metalwork to make it fit properly. The field coil resistance is lower than the original but I'm sure I'll be able to make it work.

I've ordered up some cloth covered hook up wire as well, just to keep with the spirit of the thing.

Cheers, John

[Yorkie]

It's looking good John.

Ironically, what stated off as a 'scrapper' - a donor for your first Radialva - will probably end up looking better than the first one!

(Any progress of the dial situation?)

[Geaney]

It's looking good John.

Ironically, what stated off as a 'scrapper' - a donor for your first Radialva - will probably end up looking better than the first one!

(Any progress of the dial situation?)

Hi David, yes I hope this one will look really nice when done. Ben said it will be a few weeks before he gets back to me re the dial - he has a guy who does the layout masks for him and this takes a while. I've got plenty to be going on with in the meantime.

Cheers, John

[Geaney]

At this stage I've got the new speaker properly mounted on the chassis. Next will be more metalwork to fabricate a bracket to mount the OPT to the top of the speaker. The long bolts are temporary - all will be cut to length before final assembly. The final octal socket is also now fitted - thanks again Rob! I ended up fitting the socket with 3mm bolts and locking nuts. I had intended using snap rivets but the force to snap the rivet shaft was too much for the paxolin and I didn't want to risk breaking the socket. The new one was ever so slightly larger than the original so wouldn't fit in the rectifier location. I removed the socket fitted in the centre of the chassis and used that for the rectifier. After some slight enlarging of the mounting holes in the socket I fitted the new one in the centre area without any difficulty.

I'd appreciate some guidance re the value of the field coil and it's effect on the power supply filtering. The resistance of the field coil measures 450 Ohms whereas the original measured 3000 Ohms. I assume the speaker will be OK so long as the field coil can handle the required 45mA or so but I'm not sure how to compensate for the lower inductance value in terms of power supply filtering. All help is as ever much appreciated.

Cheers, John

[Yorkie]

If I’ve done the sums right, with a 3,000 Ohm field coil, if 45 mA was to be drawn, then the voltage dropped across the coil would be .045 A x 3000 Ohms = 135 Volts. That Wattage dissipated in the coil would be .045 x 135 = 6.075 Watts.

With a 450 Ohm field coil, if 45 mA was being drawn, then the voltage dropped would be .045 x 450 = 20.25 Volts. The Wattage dissipated in the coil would be .045 x 20.25 = 0.91 Watts. (Call it one Watt).

Presumably a resistor in series with the 450 Ohm field coil to increase the total resistance to 3,000 Ohms would be in order? IE, 2550 Ohms. No such thing, so a 2K2 resistor would I assume be OK. The voltage dropped across that resistor would be 135V – 20.25V = 115 Volts, so the Wattage of that series resistor would be 135 x 0.045 mA = 5.15 Watts.

I appreciate that the field coil is of course an inductor – not a resistor, which has the dual role of energising the speaker and acting as a smoothing choke, but it terms of voltage dropped, 3000 Ohms is 3,000 Ohms. If for example - as often happens - someone substituted the speaker for a modern permanent magnet one, they'd dispense with the field coil (the usual reason being that it was OC anyway), and substitute a resistor of a similar value to the field coil across the smoothing/reservoir caps in the power supply.

Don’t know if that makes sense – any thoughts anyone?