valve testing el84 only - Page 3 - World-Designs-Forum

oceanobsession · #21 14th October 2015, 10:00 PM

Thanks richard , I set one socket with a known new valve and tried each of the used ei in that socket , I managed to get 4 around 280 and 4 around 240
the rest were low or high , I put them all in and set the bias , and left it on for an hour or so and the bias setting of 250mv is not wondering much maybe 3mv so all is well and stable, cheers richard for your help me phil.

Richard · #22 15th October 2015, 09:40 AM

Excellent Phil. Once they're all checked you'll be able to see any that go low or high in future.
The Manley looks an interesting beast do you prefer triode or pentode?

oceanobsession · #23 15th October 2015, 06:23 PM

Hi Richard , triode sounds better for me , ive had it 5 or so years with no probs
I had a look at the wiring diagrams you talked about and it makes more sense now , ive come to the conclusion that its best to buy power valves new , ive bought loads of signal valves on ebay with no probs but power valves is a different story , me phil

Richard · #24 16th October 2015, 09:49 AM

Good stuff. Yes used power valves can be a bit difficult unless you know they've been tested etc. Small valves last about 10 times as long. That said your amp runs its el84s gently at 25mA and the manual claims long life which I can believe. Quad IIs did a similar thing and their kt66s could last many years. I also like the low wattage cathode resistors designed to save the OPTX if a valve should run away. Quad did that too so it bodes well for s good long life product

colin.hepburn · #25 16th October 2015, 10:33 AM

Quote:

Originally Posted by Richard

Good stuff. Yes used power valves can be a bit difficult unless you know they've been tested etc. Small valves last about 10 times as long. That said your amp runs its el84s gently at 25mA and the manual claims long life which I can believe. Quad IIs did a similar thing and their kt66s could last many years. I also like the low wattage cathode resistors designed to save the OPTX if a valve should run away. Quad did that too so it bodes well for s good long life product

Hi Richard/All
A question on your Last post Richard
When you say that low wattage cathode resistors designed to save the OPTX does this also apply the Single ended as well. As I use two 10Watt as cathode resistors switched to allow me to run EL34s Kt66 6l6 5881s at there right currents i.e. 470R for EL34s and 680R Kt66s 6l6 5881s

I use a ½ watt resistor on valve socket Anode to G2 as specified on schematic I thought that this low wattage Resistor also acted as the OPTX protection
Any advice on the best way to proceted the OPTXs

Richard · #26 16th October 2015, 06:26 PM

Hi Col, the half watt's would protect against a screen short.
Using a minimally rated cathode resistor would protect against the valve drawing too much current for that or any other reason though (except cathode bypass cap short I suppose).
Quad used a 3W 180R there which is actually a bit below spec for the 144mA so any sign of trouble it blew.
The Manley uses 3W 10R which is more than ample for the current but they say it pops easily in event of a valve fault.
They will both have done their testing so it's probably just something not obvious for us to bear in mind when choosing parts.

Phil Y · #27 19th October 2015, 02:36 PM

Richard,
This is interesting, I never thought of that aspect.
In the 7-8 years I have owned my KEL84 I have had to change, I think 3 cathode resistors that had been over heated and gone high in value. A while back I changed them for 5 Watt Mills as I was afraid I would end up loosing the tracks on the PCB through repeated soldering.

Maybe It would be better to find some stand-offs that would permit changing resistors safely, and go back to 1 watt ones.

Regards, Phil.

oceanobsession · #28 19th October 2015, 05:00 PM

Just a quick update , ive now managed to buy three avo mk2 testers with some spares , ones working and I'm not sure about the other two but there's
no damp or corrosion , I shall renovate the other ones hopefully , ive just got
to learn how to test valves on them now phil.

Richard · #29 19th October 2015, 10:23 PM

Quote:

Originally Posted by oceanobsession

Just a quick update , ive now managed to buy three avo mk2 testers with some spares , ones working and I'm not sure about the other two but there's
no damp or corrosion , I shall renovate the other ones hopefully , ive just got
to learn how to test valves on them now phil.

Wow some commitment there Phil be good to hear how you get on

Richard · #30 19th October 2015, 10:30 PM

Quote:

Originally Posted by Phil Y

Richard,
This is interesting, I never thought of that aspect.
In the 7-8 years I have owned my KEL84 I have had to change, I think 3 cathode resistors that had been over heated and gone high in value. A while back I changed them for 5 Watt Mills as I was afraid I would end up loosing the tracks on the PCB through repeated soldering.

Maybe It would be better to find some stand-offs that would permit changing resistors safely, and go back to 1 watt ones.

Regards, Phil.

Yes could do perhaps next time the board comes out Phil. I suppose it's best to consider what we make the weakest link to be in the event of failure.

They dissipate about half a W and I too have had carbon film resistors failing after a couple of years with similar x2 margin. Perhaps 2W metal film would be more stable and have a better margin than the standard 1W carbon without compromising safety so much as the 5W ones. Maplin do 270R 2W metalfilm for a few pence.