Radford SPA.50 resistor values? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

GW4FRX · 14th Dec 2018, 8:19 am

Quote:

Originally Posted by Herald1360

OK, I've had another play. With my sim and the devices I used, I reckon you could try R9 3k3, shunt P2 with 120R across ends and make R10 1k5.

Chris, a quick question -- with those numbers, should R8 keep the original Radford schematic value of 100R or the as-found value of 2K2?

Herald1360 · 14th Dec 2018, 11:47 am

Go with the 100R- that's what was used in the sim.

GW4FRX · 15th Dec 2018, 7:05 pm

Just to tie the ribbons on this problem for the sake of anyone else who has cause to wrestle with an SPA.50, Chris’s values turned out to be almost spot-on except the resistor across P2 wasn’t needed. With it in circuit the control range was only a few millamps. Without it, P2 now gives anywhere between 4 and 300mV across the emitter resistors R25 and R26, so the standing current in the output pair is now settable at anything up to 0.5A or so. It will be interesting to find out what the optimum value for lowest THD turns out to be.

The cause of the difficulty in setting P2 initially turned out to be intermittent HF instability. I had been using an external pair of MJ15003/4 transistors for the output devices for ease of accessibility and the wiring to them was about 5in long. Cutting this back somewhat and connecting 22uF and 10nF capacitors between the emitters of the transistors immediately stopped them hooting at a few MHz and restored order. The Bailey/Radford design is inherently quite wideband and there seem to have been several different attempts to solve this problem in production.

Sincere thanks to those generous souls who provide information for all our benefit on this excellent forum.

14th Dec 2018, 11:47 am	#22
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Radford SPA.50 resistor values? Go with the 100R- that's what was used in the sim. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

15th Dec 2018, 7:05 pm	#23
GW4FRX Pentode Join Date: Aug 2005 Location: Shrewsbury, Shropshire, UK. Posts: 149	Re: Radford SPA.50 resistor values? Just to tie the ribbons on this problem for the sake of anyone else who has cause to wrestle with an SPA.50, Chris’s values turned out to be almost spot-on except the resistor across P2 wasn’t needed. With it in circuit the control range was only a few millamps. Without it, P2 now gives anywhere between 4 and 300mV across the emitter resistors R25 and R26, so the standing current in the output pair is now settable at anything up to 0.5A or so. It will be interesting to find out what the optimum value for lowest THD turns out to be. The cause of the difficulty in setting P2 initially turned out to be intermittent HF instability. I had been using an external pair of MJ15003/4 transistors for the output devices for ease of accessibility and the wiring to them was about 5in long. Cutting this back somewhat and connecting 22uF and 10nF capacitors between the emitters of the transistors immediately stopped them hooting at a few MHz and restored order. The Bailey/Radford design is inherently quite wideband and there seem to have been several different attempts to solve this problem in production. Sincere thanks to those generous souls who provide information for all our benefit on this excellent forum.