Phono 3 with 5751 valves

[Richard]

Originally Posted by bearded fiddler

Hi Richard,

Thank you for your very prompt answer which makes a lot of sense in my opinion. There is a slight problem though: the Phono 3S parts list mentions a "5-50pF variable capacitor" for C9A/C10A. So the question is, what range have I got?

Jan

Hi Jan,

These trimmers are specified for Max Cmin (Maximum of Cminimum) and Min Cmax (Minimum of Cmaximum) if you follow! (So a nominally 5-50pF can be 2-70pF but not 10-40pF for example.)

Your 5-50 pF caps should look like those in the pic with the green colour denoting the range. In practice, I've checked 3 different supplies of those, and some plastic film ones, and they come out around 5-70 pF. (Being adjustable of course this is fine as the user just needs to know the range he has available and the exact value is then set with the screw.)

In our case of setting up without test gear it only affects the estimated mid point value a little so follow the previous guidelines and you'll be fine. Certainly you'll not be able to do better without test gear and nor will it make much difference

[bearded fiddler]

Thanks Richard. All crystal clear now.

Jan

[Shian7]

Originally Posted by Gerry

Wow! Sad to see that.
I was thinking of waiting for their promised KT88s but didn't in the end.
Great pity, but you never know what might emerge out of the ashes with thes "pre-Pack" deals when a firm goes insolvent.
Mike those valves might make some of the old NOS stuff look cheap in a few years time...don't burn them out!!!

Gerry,

Looks like you could be right!

http://cgi.ebay.co.uk/ws/eBayISAPI.d...=STRK:MEWAX:IT

It's 'a shame' (understament?) I've killed two...

Bah!

[Richard]

Originally Posted by Shian7

Gerry,

Looks like you could be right!

http://cgi.ebay.co.uk/ws/eBayISAPI.d...=STRK:MEWAX:IT

It's 'a shame' (understament?) I've killed two...

Bah!

Ah, but you have one which is low hours, boxed, tested, and looks as new, which might pay for 3 nos 5751s......

I think it must be a duff anode or cathode resistor/connection. There's nothing else to cause it to draw enough current to wear out one section in that time. Just change them all and re-check voltages and it should be fine. It may even be that those valves are fragile or have a short life as they've not been around long enough for us to know. Ah, one last thought, are your heater voltages raised to protect the last output valve cathode follower? That would perhaps not be likely though as only one section seems to be failing and that valve does both channels equally, so back to plan A.

[Shian7]

I think Ill just keep it for posterity...

While the phono stage is out of action I might as well see if I can find a few 'posh' resistors to pop in as well as look for a nice set of 5751's.

[Shian7]

Originally Posted by Richard

Ah, one last thought, are your heater voltages raised to protect the last output valve cathode follower?

Yes...

[Richard]

Originally Posted by Shian7

I think Ill just keep it for posterity...

While the phono stage is out of action I might as well see if I can find a few 'posh' resistors to pop in as well as look for a nice set of 5751's.

Low noise is important for high gain stages so I splashed out and used metalfilms for the anode loads

[Shian7]

Funnily enough, I was thinking of THESE or maybe THESE.

[Richard]

Originally Posted by Shian7

Funnily enough, I was thinking of THESE or maybe THESE.

Your choice, I wouldn't use the carbons, I'm a cheapskate and used these cost 2p each from RS or 17p each from Maplin

[Ali Tait]

Mike,I redid my Pre2 with Takmans.Very good IMO.