|
Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
|
Thread Tools |
14th Dec 2018, 8:19 am | #21 |
Pentode
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
Posts: 149
|
Re: Radford SPA.50 resistor values?
Chris, a quick question -- with those numbers, should R8 keep the original Radford schematic value of 100R or the as-found value of 2K2?
|
14th Dec 2018, 11:47 am | #22 |
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 |
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. |