PhonoII - rebuild with new pcb - R23/24 value question and silver mica caps

Hi all,

I have a new PhonoII pcb and I’m about to start building it up and planning to some higher tolerance components to the original WAD supply.

Firstly I'm planning on using some 0.1% 0.4W metal film resistors from Rapid Electronics. From the Rapid web spec sheet these look as though they are made by Vishay (but I may be wrong!) and are labelled Truohm 0207 series. Can anyone advise me if 0.4W will be OK as the WAD originals were 0.5W. Also can anyone advise on the criticality of the 316k value of R23/24 as this isn't available in the range? There are 300k and a 330k available - would either of these be OK? Or I could try to get closer to 316k by putting some resistors in parallel or series... Incidentally has anyone else had problems getting 316k resistors? I've looked for them at all the usual sources and find 316k in any type and shape to be virtually nonexistent except for very low wattage surface mount types.

Secondly I was planning on getting some 1% silver mica caps for C14/15 (8200pF) and C16/17 (220pF) from Rapid or RS. I not that there's been some discussion on these in other threads and any comments on experience of these would be welcome. I'm guessing that the original blue WAD caps were polystyrene but it's just a guess.

I plan to put in Auricaps at C10/11 (0.1uF) and C18/19 (2.2uF) - I used these in the mods of my original PhonoII pcb and the improvement was great. Note - I'll be putting pins in for these fellas this time as I ended up damaging some of the pcb foil last time trying to get the wires in!

I also plan to carry out the 3.18uS RIAA time constant correction by inserting paralleled 75k and 18k resistors on left and right channels between C16/17 and junction of R31/32 and V4b/a. I did this mod on my original pcb and found the result to be amazing!

Hope someone can help!

[NickG]

RS 2508908924 would give you 316k.

Or you could use 300k and 16k in series.

I wouldn't get too hung up on 0.1% resistors, the caps will be a limiting factor, as will the Ra of the valves.

None of the RIAA resistors are dissapating any power worth worrying about, so don't concern yourself with the power rating of them.

It is worth remembering though, that the anode load resistors play almost as much a part in the RIAA accuracy as the obviously RIAA parts.

Thanks Nick - I found the Vishay 316k 1/8W resistor at RS. I was going to order the the silver micas from RS as they have both values at 1% so will add this....

[The Shadow]

Hi Steve

For future reference:
if you need to calculate any critical resistor values there is this nice little calculator at Pete Millett's site here

Steve.

Thanks Steve - I was trying to get my wife to build a calculator like this using Excel - but this is great!

She's just commented that she feels redundant now!

Steve

[John Caswell]

Hi all,
I found that the 220pf cap C16/17 would consiiderably affect the square wave response depending on type so finished up with a 180pF SM cap // 50 pf trimmer on standoff pins plus the 3.18µS mod as well, and then adjusted the trimmer for best Sq wave response using Hagerman's inverse RIAA network.

John aka dr Dr John