WAD Pre II circuit query

[buddam]

Hi all

with help from people here I've acquired most of the parts to change my PAS to Pre II cath foll. But, both Neal's CF instructions and the Pre II instructions have thrown me. According to both signal in passes through a resistor junction (?) with one leg going to a 1K res and the other through IM to grd. But the pcb I have shows the junction coming after the 1K res, where the signal 'splits' (?) to valve (pt 2 or 7) and grd. Does this matter?

I have a few more Qs on the CF mod:

Neal's original WAD BB notes say “Solder to the input PCB pins one end of a .047uF (47nF) quality polypropylene capacitor”. 2 questions: i) the other end is connected to the i/p end of the 1K resistor?; ii) what are the recommendations for a quality polyprop?

Neal then suggests a “1uF o/p capacitor....quality film&foil polypropylene”. Again, is there a good recommendation? And what is the voltage rating?

Finally, again from Neal, “if the impedance of the next stage is lower than this then increase this [100K] resister to 1M ohm”. How do I measure the impedance of the next stage (a KEL 84)?

Thanks

Adrian

[John Caswell]

Hi Adrian,
To measure input impedance you will need an oscillator or test CD, a 1meg linear pot and a meter to measure the amplifiers output plus an 8R 10 watt dummy load.
Place the dummy load across the LS terminals of one channel and the DMM set to 2 V AC across the dummy load. Feed the signal into the amp and set the input level to give 1V ac across the o/p. Then feed the signal from the source with the 1 meg pot, set to the maximum from one end to the wiper in, series with the signal and turn the pot until the output level drops to 500mV ie -6dB. disconnect and measure the pot. That is the input impedance of the amplifier.
If this is not possible or beyond you, simply change the input grid resistor to 1M which will be more than satisfactory, it is only there to hold the grid at 0v if the input is removed. (See the FAQ)

The output capacitor needs to be at least 250VDC rated of a type that you like, but PIOs or polypropylene are excellent here ditto for the input .047µF cap which should alkso be rated at 250VDC min.
Ideally the 1K input R should be wired directly to the valve base pin with the 1M at the remote end as in the cct diagand the .047µF cap wired directly to the junction of the 1M and 1K.

Regards

John aka Dr John

[buddam]

Dear John

thanks so much for the detailed response. If I haven't siad so before, your (and others') specialist knowledge is really appreciated, as is the time you take to explain things to novices like me.

In terms of the sequencing of the components, I'll stick to Neal's diagram, even though it still seems to me that the description in the prose (and that of the original instructions) differs.

Best wishes

Adrian

[NealG]

Well I'm sure we can change the FAQ if it's in error, others have built it OK from my description though. Never intended it to be a soldering by numbers exercise that's why the cct diagram is there.

The 1M doesn't go to ground in the CF, it takes the input and feeds it to the junction of RK(820) and RL(33K)

[buddam]

Neal

hope you didn't take offence. I have so little elec theory I need to double check everything. And if there's a slight anomaly I need reassurance. I was going to go with the cct diag in any case.

Best

adrian