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    Valve odd occurences, thoughts and advice

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    bordonbert

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    Valve odd occurences, thoughts and advice

    Post by bordonbert on Mon Nov 09, 2015 10:13 pm

    I just experienced a strange thing and thought I'd post it here for others to check out.  That gave me the idea of having a thread going for simply posting any useful experiences and thoughts about valves in general.

    Ok, my own issue.  My amp had a couple of days of sounding a little ropey.  Despite calling them up via MIDI, it was difficult to get the sound out of it I was used to at home at lower powers.  Somehow it just seemed flat and lifeless.  I began to wonder if the valves were getting a little rough, despite them being only a year or so old.  They are the usual quad matched JJ EL84s.  Then I turned it on, it came up, but played with an occasional popping sound.  It wasn't happening often, just now and again.  The usual advice is of course "valves going, replace asap".  That's too hasty a solution for me but something definitely had to be done.

    I did the obvious things and checked out the bias with the magic pick.  I was astonished.  2 valves were on 6 flashes, 1 was on 10 and the other on 24!  And the two which matched weren't in the same pair.  Then I noticed that when it was turned on it occasionally would set up with the outer pair being marked as faulty and switched off, but sometimes it was fine.  A duff outer valve I thought and resigned myself to having to buy a new set.  But then I got more picky.

    I decided to simply swap the outer pair for the inner with the thinking that, as I use 18W or below most of the time, the outer pair would hardly have been used.  The outrageous bias reading was given on one of those outers but I figured it would be interesting to try it.  Two minutes later and I had them swapped.  I lit it up, the usual scary light show from the heaters which I find with JJs, and Bob was my mother's brother, it worked perfectly.  No bias issues, a recheck showed them all now within 3 flashes, and the sound was back to magnificent.  It has had days of home use and a full fledged afternoon's rehearsal and not a peep. It behaved better than for months.

    Now I figure the solution is probably the obvious.  Removing and replacing them most likely simply cleaned the pins of tarnishing.  It had never occurred to me that slightly tarnished contacts could be so invasive without actually preventing the amp from working.  My plan now is to remove them every month or two and clean all the pins with a "proprietary fluid".  I also have a tube of special contact grease I use for other types of circuitry, not a cleaning agent but one to lubricate switch surfaces, prevent tarnishing and promote good long term contact.  This is really good stuff, but for use on valves I am waiting for advice from the manufacturer as to its suitability for high voltage circuitry like ours.

    One thing I am curious about is that I use my own home made plinth with forced ventilation for additional cooling. It is just a wooden base with 4 computer fans fitted in. The amp bottom sits with its bottom vents surrounded by a sealing strip and the fans force air into the base and up out of the top. It makes a really big difference to the case temperature so seems to be working on the innards. However, this air will use every path to find its way up into the top section and one of those paths will be up the valve base contacts. I was curious as to whether the airstream was accelerating corrosion in some way. High voltage parts do have their own ways of degrading exposed contacts. Old tube CRT type TVs used to grow notorious "black jam" on their tube's rear pins which was a thick grease like gunk which eventually built up enough to short out the contacts.

    So, anyway, a horror story averted, I didn't need to put more money into the bottomless pit of "music for fun".  Anyone else out there with an amp which is sluggish or misbehaving like mine could do well to simply remove the valves, clean the pins and replace them.  It's the cheapest valve repair you will ever do. They really are much more reliable and long lasting than they are given credit for. (So many myths for us to debunk!!!! Very Happy )

    jitees

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    Re: Valve odd occurences, thoughts and advice

    Post by jitees on Tue Nov 10, 2015 1:14 am

    IMO.......Not a bad idea cleaning the tube pins occasionally with electrical contact cleaner or similiar and re-seating them.  However that said I would go easy with the grease......most of the common, auto parts store electrical greases are NON conductive.  Simply cleaning & reseating should do the trick.  And every 2-3 months is probably a bit much.  Once you get it right yearly should be fine unless you have this problem again sooner.

    Also and again IMO.....the fan could have accelerated tarnish or such....really hard to know for sure.  But one thing for sure is you don't need it.  Trust what H&K says......the GM can take the heat.  I am sure you know it doesn't even generate any additional heat (from attenuation) at the 18 watt setting.  The amp is made to withstand & dissapate what it makes at settings below that.  The air you're forcing thru it probably isn't hurting it (besides maybe causing this problem) but you don't need it either.  That said, it is your amp so if you feel better with the fan have at it.
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    bordonbert

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    Re: Valve odd occurences, thoughts and advice

    Post by bordonbert on Tue Nov 10, 2015 4:32 am

    Haha!  I've made that same argument about not worrying about the heat myself to others.  "Physician heal thyself". Very Happy  The idea of the cooling base was just that it was cheap, easy and had no obvious downside at that time.  It was an extension to a simple base I wanted which would lock in place over the speaker cab handle and offer secure location points for the amp feet.  I've seen amps pulled off the top of stacks before and didn't like it!

    As you said, it can't hurt to cool the base of the amp, other components will have their lives put under a lot less stress that way.  Just 10deg reduction in working temperature roughly doubles electrolytic capacitor life!  It was a curiosity and fun to put together on a rainy day.  With the small computer fans I used and the amp inlet vent size there was never going to be a tornado inside, it's just a steady trickle which you can just feel out of the top vents.  The temperature of the lid really does go down with it working though.  Like you, I do have a little faith in the H&K designers, (at least for most things and apart from their taste in music), though that doesn't stop me checking their work and even trying to improve on it out of interest.

    The grease is definitely NOT auto parts.  Vaseline is generally sufficient to do that job best! Wink  It's a high level electronics product I've used in my work and has to be applied very sparingly anyway.  The only part we are really trying to protect is the tiny contact area between base insert and valve pin and in the scheme of things that is very small.  And it should be non-conductive to do it's job of course.  It must be soft enough to allow contact to be made through its coating but stiff enough to cling around the contact area and exclude air.  The specialist stuff is really very good.  I'll pass on here what the manufacturers say about its suitability once they reply.
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    HwyStar
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    Re: Valve odd occurences, thoughts and advice

    Post by HwyStar on Tue Nov 10, 2015 8:49 pm

    Great tip Bordonbert!


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    bordonbert

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    Re: Valve odd occurences, thoughts and advice

    Post by bordonbert on Tue Nov 10, 2015 9:07 pm

    So on to the more controversial. All of us have amps with Standby switches don't we? Why? To mute them when they are not in use? So why not just switch the Master Volume down, or the volume on your guitar. Usually it is believed it eases the workload of the valves and lengthens their life. At least that was what I always thought.

    Here is an article which questions, (factually not just by his opinion), the wisdom of designing amps with Standby switches and describes why it may originally have been done by Uncle Leo. They cause more damage to the valves than they prevent and the arguments originally used to include them have always been wrong, but seem to have become that thing I love, the "Common Knowledge Audio Myth"!

    Power And Standby Switches

    Enjoy, (or not as the case may be Shocked ), but do let it make you think.

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