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Old 19th February 2010, 12:36 PM
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Shian7 Shian7 is offline
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Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by Gerry View Post
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!!!

Erm...

Something 'happened' but I'm not sure what!

I switched on one day to find all sorts of weird sounds and after a bit of valve swapping became convinced there was a problem with at least two of them. So I asked Rich if he'd take a look, and this is what he found...

Quote:
Hi Mike,

Sorry to be the bearer of bad news but 1 is good, 2 are knackered.

Test spec for the Avo is at 250V when they should draw 1.2mA current and show a slope of 1.6mA/V.
(A new lively one might have a higher slope for that current or meet that slope with less current. A worn one will draw less current and have less slope.)

Valve 1
anode 1 1mA / 1.7mA/V
andode 2 1 mA / 1.7mA/V

Valve 2
anode 1 0 / 0
anode 2 0.6 / 1

Valve 3
anode 1 0.7 / 1.4
anode 2 1 / 1.7

All test fine for heaters, gas, and shorts/insulation.

So,
Valve 1 is fine both sections well matched. It draws a little less current than spec and gives a slightly higher slope.
Valve 2 is all but dead on anode 1, there is a little movement showing it's not disconnected but it is useless. Anode 2 is low, will still make sound but you can see the way it's going.
Valve 3 is low on anode 1 but will still work. Anode 2 is fine.

Both bad ones have suffered in a similar way but anode 1 in partic. I'd first check V at the anode pins without valves fitted. If ok (a bit high due to no load) install other working ecc83 and check all voltages. Particularly the cathode voltages as either these valves simply have a short life or they've been run with high bias current on anode 1 in one or 2 valve positions in the amp. Is it possible you've a wrong cathode resistor value in one position for instance? Or could be just one bad position and they've been switched around only partially wearing out the other valve.

Rich
After poking and prodding around with a meter I can't find anything wrong, all the voltages are there or thereabout. So I don't know what exactly 'happened'!

It has to raise a big question mark over the reliability of these valves IMO... Has anyone else used them and had any problems?

Cheers,
Mike.
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