Resistor value for Phono III with Pre III CF.

[bob orbell]

Maybe one for John, Richard or Matthew, or anyone that knows, I have just sold a Phono III to a member who is building a Pre III CF and has the PSU, question, what value or resistor ( I think it is R28-9 ) inside Phono before the 100uF capacitor is used with the CF, at present it is 1K 3watt. Thanks. Bob

[bikerhifinut]

I think the 1k should be OK Bob.
Given the 15k from the PSU3 HT out to Phono3.
It's not as critical as people would have you think, it's a simple circuit and capable of handling a spread of HT voltage with no appreciable ill effects.
Now then, my way would be the old "Adjust on test" method.
Plug the amps in to the PSU, measure the Volts at the HT in of each amp (Preamp and phono) and tweak the dropper in each unit to suit.
A working knowledge of good old Ohms law is all that is required here to get a ballpark value and choose the nearest preferred value. 2W is probably more than needed, again mr Ohm and the power law is your friend.
measure the voltage drop across the existing filter/dropper R and from that you will know the current passing through. Use that knowledge to calculate your resistor and select the nearest available value to it.
For the really OCD person, you could make series/parallel networks of resistors to get closer.......... Me I'd just get it inside the toleraince which for me would be 15V either way, you've got more variability on the UKmains so its not really all that critical, valve kit will work on a fairly large voltage range (within reason) around the nominal designed circuit values.

Indeed tweaking the Ht filter/dropper resistors is a valid way to compensate for your local Mains voltage AVERAGE values.

Andy.

[colin.hepburn]

Hi all
My Pre 4 CF schematic states HT from PSU 3 first resister Which is R9 as 20k 5 Watt.
Colin

[bob orbell]

Thanks both, Andy, I only have the Phono here, this I have sold to a member in Wales, he is building a Pre III CF and has the PSU, so, this is why I asked. Colin, that sounds too large to be honest, but if it is on the diagram it must be correct, I wonder, would this value (20K) be just to run the Pre IV CF on its own, the PSU open circuit will give well over 300 volts, the CF will consume a very small current as will a Phono, so I think this 20K must be just PSU and Pre IV CF. Bob I will stand corrected, said the man in orthopaedic shoes.

[Richard]

Hi Bob, I think you may be right about Col’s 20k being for Pre4CF and PSU3 alone but I don’t have the later build sheets or circuits to check.

Phono3S build instructions say,

1. If you are using WDPSU3 with WDPhono3S and WDPre3 then R35 is 1K 3W.
2. If you are using WDPSU3 with WDPhono3S alone then R35 is 10K 3W.

Pre3 draws quite a large 23ma
Pre3CF will not, probably under half the load of Pre3 looking at the CF in Tweaks.

So I’d use 6k8 in Phono3 with Pre3CF, check voltages later, but it’s not critical as Andy says.

[bob orbell]

Thank you all, I shall fit 6K8 and ask the buyer to give me some feed back when he completes his build. Bob