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Old 23rd Feb 2017, 5:50 pm   #190
Techman
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Join Date: Nov 2009
Location: Lincolnshire, UK.
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Default Re: Restore a Beau Decca

Quote:
Originally Posted by thejazzageuk View Post
So the dimmer valve does have a slightly lower resistance.
Are you sure about that? I would expect it to have a slightly higher resistance.

However, those resistance readings are not that far apart. One thing, though, those resistance readings are higher than they should be and I suspect the valves had just been run and were perhaps still warm when you took those readings. Ideally, the valves need to be stone cold when taking the readings as the resistance of the heaters increases considerably with temperature as they warm up. The other tests you've done do seem to now be showing that the fault is actually following the valve to the other position this time, so is a little more conclusive than it was before.

I saw your post earlier, so did some readings on a pair of valves myself, as I couldn't remember what they were from when I last did this test, pictures of these readings are below. The second valve in the picture is one that had a problem with a high resistance heater due to oxidised connections within the valve base pins, but has now been repaired. The third picture shows the meter with its test leads shorted together giving 0.3 ohms, a bit lower than yours. The forth picture shows the base of the previously faulty valve after I repaired it. I must stress the point that you have to make sure that the test leads make perfect connections to the valve base heater pins and this can only really be done with crock clips - you should invest in some good crock clip leads if you don't already have them. The three handed operation of trying to hold test probes onto valve pins is a disaster waiting to happen if the valve flips out of your hand and lands on the floor. Also, not being able to make a good contact, which doesn't matter with the 'high' resistance readings you've been making, but is very important with these 'low' resistance readings. To make a point, those 1.0 ohm readings both read 1.1 ohm until I 'jiggled' the crock clips to make a better contact - it really is that important and at these low resistances 0.1 of an ohm IS important. So with the meter resistance of 0.3 ohms taken off, the cold filaments of the PX4 valves in good order should be around 0.7 ohms. From the readings you've taken with your valves, the resistance of the filaments is coming out at about 1.0 ohm - a little high if they were stone cold with good contacts from the test leads.

Having said all of the above, I'm tending to now be leaning towards more of a difference in the actual normal emission of the valve due to the emissive coating of the filament rather than an actual heater connection problem, but I still think you need to clean up those valve pins, particularly the heater ones as they're taking an amp each, which is a high current for that type of connection. Also switch cleaner in the sockets of the valve bases on the amplifier chassis to properly rule out any bad connections there. The 'lower' valve could also be a little bit 'gassy'.

Rightly or wrongly, I'm personally not as concerned about this difference in the valves as PJL is, and I'm a bit wary of this getting 'over complicated', but we'll see what he thinks when he posts. It could be that the gram was operated for a long time with one of those coupling capacitors quite leaky that gave one of the valves a bit of stick, or it could even have been a bad heater connection in one of the valves which caused the other one to take more HT current. I think what's happened in the long distant past is that one of those balancing resistors failed open circuit. The difference in the, by then, slightly ageing electrolytics, caused one to get too much voltage across it, so it started to fail. This then caused more ripple further down the line which has given the double can electrolytic that's now getting hot as well, a hard time too. The previous owners probably carried on running it and put up with it 'humming' away until it finally got too much to hear the music over - just guessing, but I bet it was something like that!

I would guess that it's probably only one section of the 'double can' capacitor that's developed a bad leak and is getting hot, but I think it's doubtful that it will recover and will need disconnecting and two of your new 10uf replacements connecting in circuit in its place, just to be on the safe side.

Valve heater testing pictures below:-
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Last edited by Techman; 23rd Feb 2017 at 6:00 pm.
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