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    Another issue raises that ugly head..

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    Ernstmach

    Posts : 82
    Join date : 2015-08-30
    Location : The Great Mitten.

    Another issue raises that ugly head..

    Post by Ernstmach on Wed Nov 01, 2017 9:11 pm

    Having some trouble with the same amp as I have posted about previously.

    Some background:
    Purchased and assembled a pair of MK III's in March 2016. Purchased all of the available upgrades and octal boards
    Had a grounding issue initially but with help all was well.

    Developed a hum and had a fuse die awhile back. Replaced the fuse and the hum decreased to almost silent.
    This week the hum is back so clearly there is an issue.

    Was checking the bias tonight and one of the tube bias points would not give any voltage reading. The other was fine. Adjusted it just a bit and started some music.
    The speaker connected to that amp was clearly not producing like it should. Low volume and very poor SQ. Shut down both amps and the preamp.

    Went to a backup amp and it sucks compared to the MK III.

    I'm hardly an electronics tech but I can follow directions. My expertise lies in other areas..
    So at this point any advice on where to look for issues would be greatly appreciated.
    I should add that the other MK III is still silent and performing very well.
    Please, be gentle..... Assembling these amps was easy but not so much about diagnosing problems.

    Well for me that is.
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    sKiZo

    Posts : 1402
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Another issue raises that ugly head..

    Post by sKiZo on Wed Nov 01, 2017 10:07 pm

    Try swapping the tubes and see if the problem follows?

    Next guess would be a bad bias resistor on the tube that won't fire. Simple enough to replace. I'd first try repointing the solder to see if you're just dealing with a cold connection.
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    Ernstmach

    Posts : 82
    Join date : 2015-08-30
    Location : The Great Mitten.

    Re: Another issue raises that ugly head..

    Post by Ernstmach on Wed Nov 01, 2017 10:29 pm

    sKiZo,
    thanks for that info. All solder joints are good.
    Unfortunately knowing what the bias resistor, is my next question.
    I feel pretty stupid asking these questions. You folks know this stuff.

    I'm sliding backwards on that learning curve. Ask me about machining or Vacuum welding and I'm on it.
    The problem is assembly of these amps is relatively easy. Troubleshooting problems means I need someone to hold my hand.
    I'll gladly send  decent bottle of Bourbon/Scotch, Chardonnay to whomever can help solve this issue.
    If you are of legal age that is..

    Thanks!
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    Peter W.

    Posts : 527
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Another issue raises that ugly head..

    Post by Peter W. on Thu Nov 02, 2017 8:12 am

    First, step back and give yourself some space. We all started knowing nothing-or-less about these things and some of us have progressed from there despite ourselves. There is no reason you should not.

    Next, tighten all the sockets and if you are going to be really thorough, clean them first. I keep several sets of these:

    https://www.widgetsupply.com/product/BER38.html?gclid=CjwKCAjwhOvPBRBxEiwAx2nhLqfP75oBtFucaUW3Y7OY-ccTmBjOZdGg8dinEXthvb5BfP-XLl1xxhoCD9wQAvD_BwE    and use 91% alcohol quite liberally for the purpose, but a round wooden toothpick will do very nearly as well. Don't force anything. Then tighten the pins using a dental pick, or similar.

    https://mosaicartsupply.com/wp-content/uploads/2015/06/tool-dental-picks-big.jpg  

    For giggles, try the amp now. It is unlikely from the symptoms you have described that the solution is this simple, but it sure would be nice if it were. If not:

    You have a significant advantage inasmuch as you have two amps, one good, one temporarily troubled. So, the first thing I would do were these my amps would be to unplug them, mark the tube positions in each amp and then remove the tubes. With the schematics in front of you (or the pictoral diagram if that is easier for you to follow), measure the resistance across each component one-to-the-other amp and mark these values on the schematic. Matters not if the component is a resistor or a capacitor - all you are trying to do is *compare* one to another. Look for significant differences, if any.

    While you are doing this, check _every_ connection for solidity, and location. Keep in mind that often enough connections look good, but the wire may be cracked at the end, or held in place by melted insulation - any number of crazy things. Tug on each connection with that same dental pick and look for any undue flexing.

    If there are any differences, examine the part(s) in question and, if necessary, remove them from the  amp (take LOTS of pictures during the process) for testing out-of-circuit, and replace any that are bad.  Again, you have the good amp as your benchmark.

    I expect that you do not have a tube tester, but as you seem to be in Michigan, you might try someone at MARC (  http://michiganantiqueradio.org/  ) who might be near you and also have a tube tester. Mark Oppat comes to mind if you are near Plymouth. He is "in the book", and runs a very good parts house for vintage equipment.

    I am not going to advise on "Hot" tests as I do not know your level of equipment and understanding of the lethal voltages present in these amps. At the very least, an isolation transformer and excellent lighting are the minimal necessities for any hot tests.

    But, if you are systematic, I suspect that you will find the problem in reasonably short order. Take your time!
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    Ernstmach

    Posts : 82
    Join date : 2015-08-30
    Location : The Great Mitten.

    Re: Another issue raises that ugly head..

    Post by Ernstmach on Thu Nov 02, 2017 9:46 am

    Peter,
    thanks for your message and the linkage.

    In a bit of a time crunch today so will get started on the diagnosis as soon as I can.

    Again, thanks!
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    LeGrace

    Posts : 233
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: Another issue raises that ugly head..

    Post by LeGrace on Thu Nov 02, 2017 9:47 am

    I can relate - things will get better. Built my M125's in Aug 2016 from a zero knowledge base. About a week later there was a bright flash and one of the amps was dead. I was hyperventilating wondering what I had done wrong in the build. I replaced the rectifier tube and fuse but it happened right again. I'm wondering what the heck am I doing wrong..turns out nothing except for using the recommended 5AR4 tube. Research the forum and come to learn most people avoid this tube because they are highly prone to premature failure. After awhile you get to know the character of your amps. Ask a lot of questions. Myself I found that 99% of my issues ended up stemming from poor tube quality, nothing I had done.

    Moving tubes around is one way to isolate if a tube is at fault, see if the problem follows the tube. I also had two bad power tubes develop within the first year. Everyone was saying it must be the amp, but further research and yeah modern production tubes are hit or miss quality wise. When a power tube is off it can generate adjacent issues such as red plating, had that happen as well. Hence the emphasis on buying matched from a reputable vendor that will pretest them. I've got better ones now and what a difference that has made.

    Hang in there!
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    Ernstmach

    Posts : 82
    Join date : 2015-08-30
    Location : The Great Mitten.

    Re: Another issue raises that ugly head..

    Post by Ernstmach on Thu Nov 02, 2017 9:54 am

    LeGrace,
    I have followed your thread with much interest. Have learned a lot from it and have seen the help others here have provided.
    Some great folks here.
    Thanks!

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