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    Wax potting a noisy power transformer on a MKII

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    Chris Scott

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    Wax potting a noisy power transformer on a MKII

    Post by Chris Scott on Wed Jun 13, 2018 4:12 pm

    Just went through a pr. of MKII's, and they now sound fantastic, save for a pretty noisy PT on one of them. I bought the rubber iso washers which helped a bit, but it's still an unacceptable noise level so I'm wondering if I should pot the thing (with wax) as it seems like this is what would quiet it down.

    Is this a solution, or is replacement the only option?

    TIA!
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    PeterCapo

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    Re: Wax potting a noisy power transformer on a MKII

    Post by PeterCapo on Wed Jun 13, 2018 6:04 pm

    I'd question what potting would do to its ability to radiate the heat it generates. Have you reseated the end bells and snugged the bolts? Checked voltages per the chart in the manual to make sure there isn't something causing excessive current draw causing it to buzz/vibrate more than usual?

    If replacing it, keep in mind that some replacements will have a primary winding intended for a different AC mains. An original Mark II PT would likely have its primary wound for 117VAC. Replacements may be different and if so you'd have a channel-to-channel inconsistency.

    Chris Scott

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    Join date : 2017-05-09

    Re: Wax potting a noisy power transformer on a MKII

    Post by Chris Scott on Wed Jun 13, 2018 7:00 pm

    Thanks Peter.

    I checked that the through bolts were tight, don't know anything re. reseating the end bells, nor do I have a clue as to what voltages could be off enough to cause it to vibrate as it does...the two amps are working quite well, with good power, no distortion or hum and a great overall sound.

    Both PT's do make a fair bit of heat however, and as my experience is exclusively with guitar amps I'm not really familiar with how much is *too much*...fwiw, the idle current is set at 65 mV on each JJ E34L, and the PT's on both (the other is dead quiet btw) generate enough heat to make it just bearable to rest your palm on top after 1/2 hr. or so. Both OT's are still cool.

    I do recall hearing this level of noise on older home sound gear back in the day, so I wasn't too worried about it at first. But if you think there could be something amiss that would generate this noise while the unit still performs well I'm all ears!
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    Bob Latino
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    Re: Wax potting a noisy power transformer on a MKII

    Post by Bob Latino on Wed Jun 13, 2018 8:28 pm

    Chris Scott wrote:Thanks Peter.

    I checked that the through bolts were tight, don't know anything re. reseating the end bells, nor do I have a clue as to what voltages could be off enough to cause it to vibrate as it does...the two amps are working quite well, with good power, no distortion or hum and a great overall sound.

    Both PT's do make a fair bit of heat however, and as my experience is exclusively with guitar amps  I'm not really familiar with how much is *too much*...fwiw, the idle current is set at 65 mV on each JJ E34L, and the PT's on both (the other is dead quiet btw) generate enough heat to make it just bearable to rest your palm on top after 1/2 hr. or so. Both OT's are still cool.

    I do recall hearing this level of noise on older home sound gear back in the day, so I wasn't too worried about it at first. But if you think there could be something amiss that would generate this noise while the unit still performs well I'm all ears!

    65 ma on each EL34 output tube is pretty high by today's standards. Dynaco got away with this high bias current because they used the super rugged Mullard EL34's. Today's EL34's will be pushed pretty hard at 65 ma per each output tube. Most of the original Dynaco Mark II's are now over 60 years old. (Most were sold between 1955 - 1957) Personally, I would reduce the bias current to about 50 - 55 milliamps per each output tube.

    Bob

    Chris Scott

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    Join date : 2017-05-09

    Re: Wax potting a noisy power transformer on a MKII

    Post by Chris Scott on Wed Jun 13, 2018 10:11 pm

    Thanks Bob, I was wondering how much these JJ's can take, and though many have said they'll hold up fine @ 65mV ea. I'm inclined to agree with you so I reckon I'll drop the voltage a bit.

    Thanks again!
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    Peter W.

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    Location : Melrose Park, PA

    Re: Wax potting a noisy power transformer on a MKII

    Post by Peter W. on Thu Jun 14, 2018 7:05 am

    Chris Scott wrote:Just went through a pr. of MKII's, and they now sound fantastic, save for a pretty noisy PT on one of them. I bought the rubber iso washers which helped a bit, but it's still an unacceptable noise level so I'm wondering if I should pot the thing (with wax) as it seems like this is what would quiet it down.

    Is this a solution, or is replacement the only option?

    TIA!

    Potting is a bad idea as it would interfere with heat transfer. But:

    If you get ultra-thin super glue (ZAP brand, accept no substitutes): https://www.amazon.com/Zap-Super-thin-Bottle-PT09/dp/B01MTVVDCH/ref=sr_1_7?ie=UTF8&qid=1528974051&sr=8-7&keywords=zap+glue+thin

    And introduce it into the laminates, then clamp them with either a C-Clamp or vice-grips: http://www.airgas.com/p/VIS18 , you will reduce the noise. Note that this will make future rewinding of the transformer more difficult, but not impossible.

    Good luck with it.

    Chris Scott

    Posts : 27
    Join date : 2017-05-09

    Re: Wax potting a noisy power transformer on a MKII

    Post by Chris Scott on Thu Jun 14, 2018 4:31 pm

    Peter W. wrote:
    Chris Scott wrote:Just went through a pr. of MKII's, and they now sound fantastic, save for a pretty noisy PT on one of them. I bought the rubber iso washers which helped a bit, but it's still an unacceptable noise level so I'm wondering if I should pot the thing (with wax) as it seems like this is what would quiet it down.

    Is this a solution, or is replacement the only option?

    TIA!

    Potting is a bad idea as it would interfere with heat transfer. But:

    If you get ultra-thin super glue (ZAP brand, accept no substitutes):   https://www.amazon.com/Zap-Super-thin-Bottle-PT09/dp/B01MTVVDCH/ref=sr_1_7?ie=UTF8&qid=1528974051&sr=8-7&keywords=zap+glue+thin  

    And introduce it into the laminates, then clamp them with either a C-Clamp or vice-grips:  http://www.airgas.com/p/VIS18 , you will reduce the noise. Note that this will make future rewinding of the transformer more difficult, but not impossible.

    Good luck with it.


    Hey thanks Peter. Interesting idea, though tbh I've no idea how wicking CA in there addresses the noise - are the laminates vibrating (as in actually moving), causing the noise? - I'm guessing yes, but I'd like to know for sure what's actually going on in there.

    Beautiful cat btw, quite the furry monster you got there!

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    Peter W.

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    Re: Wax potting a noisy power transformer on a MKII

    Post by Peter W. on Fri Jun 15, 2018 7:17 am

    Hey thanks Peter. Interesting idea, though tbh I've no idea how wicking CA in there addresses the noise - are the laminates vibrating (as in actually moving), causing the noise? - I'm guessing yes, but I'd like to know for sure what's actually going on in there.

    Beautiful cat btw, quite the furry monster you got there!

    Chris:

    It is not uncommon for the laminates to vibrate against each other and to do so without clattering. It is also not uncommon for the laminates to vibrate as a block and transmit that to the chassis. The super-glue-and-clamping breaks up the harmonics generated by these vibrations.

    That is Murphy, a 19-pound, 15 ounce Maine Coon at 8 years old. He enjoys walking up and down the stair handrail, and sitting on the newel posts. Perhaps the gentlest cat we have ever had, he shares the household with two dogs and another cat, but is definitely the "big daddy" of all of them, cleaning the golden's face, or the Scottie's eyes on a regular basis. As to the younger cat (Seamus, 18 pounds), Murphy applies regular pummeling equally balanced by grooming.

    Chris Scott

    Posts : 27
    Join date : 2017-05-09

    Re: Wax potting a noisy power transformer on a MKII

    Post by Chris Scott on Fri Jun 15, 2018 2:22 pm

    Peter W. wrote:Hey thanks Peter. Interesting idea, though tbh I've no idea how wicking CA in there addresses the noise - are the laminates vibrating (as in actually moving), causing the noise? - I'm guessing yes, but I'd like to know for sure what's actually going on in there.

    Beautiful cat btw, quite the furry monster you got there!

    Chris:

    It is not uncommon for the laminates to vibrate against each other and to do so without clattering. It is also not uncommon for the laminates to vibrate as a block and transmit that to the chassis. The super-glue-and-clamping breaks up the harmonics generated by these vibrations.

    That is Murphy, a 19-pound, 15 ounce Maine Coon at 8 years old. He enjoys walking up and down the stair handrail, and sitting on the newel posts. Perhaps the gentlest cat we have ever had, he shares the household with two dogs and another cat, but is definitely the "big daddy" of all of them, cleaning the golden's face, or the Scottie's eyes on a regular basis. As to the younger cat (Seamus, 18 pounds), Murphy applies regular pummeling equally balanced by grooming.

    19 lbs.??!!!!

    Holy crap that's a big puss! - I've never seen a Maine Coon "in the fur", and as my wife and I are serious cat people (she runs an animal rights/spay-neuter/adoption organization down here in Costa Rica) actually getting to hang with one would be a treat...maybe someday.

    I'm going to give the cyano tip a go, and maybe it'll allow the guy to get some more life out of it...someone mentioned that if it was ever disassembled there could be a partial short somewhere - is there anything you can recommend in respect to what I should look out for? - I'd hate to invest more time in trying to repair something that's going to end up as a paperweight!

    Thanks again for your time Peter!
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    Peter W.

    Posts : 838
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Wax potting a noisy power transformer on a MKII

    Post by Peter W. on Fri Jun 15, 2018 2:49 pm

    Holy crap that's a big puss! - I've never seen a Maine Coon "in the fur", and as my wife and I are serious cat people (she runs an animal rights/spay-neuter/adoption organization down here in Costa Rica) actually getting to hang with one would be a treat...maybe someday.

    He is down from 23 pounds on the vet's advice. Now, he is actually quite lean. One more thing about Maine Coons is that they like water, and have webbed feet. His paws are about 2.5 inches across.

    I'm going to give the cyano tip a go, and maybe it'll allow the guy to get some more life out of it...someone mentioned that if it was ever disassembled there could be a partial short somewhere - is there anything you can recommend in respect to what I should look out for? - I'd hate to invest more time in trying to repair something that's going to end up as a paperweight!

    The first thing to do in that case is check all the primary and secondary windings for shorts to the case. You can do that "hot" by testing for current from the case to a good ground. It will not be more than one winding, or the transformer function would be compromised. If you find current, then try to determine *which* winding is the problem.

    Now, here is the thing. If there is a partial internal short, there is also a kill-or-cure. use it with great care as the odds of full cure are as follows: No change 33%. Total winding Failure: 33% Re-isolation of winding: 34%.

    The transformer will have to come out of the amp - carefully so as not to damage the leads.

    The first thing you will do is remove the end-bells and check to see if any of the winding-to-lead connections are visibly compromised. And if you are lucky, you will find it and fix it there. And you will also test with a VOM to the windings. If nothing - you have something touching that should not. If all is visibly good, and the test still shows a short then, and only then:

    You will need a large capacitor and the means to charge it. Something on the order of at least a Farad at low voltage or 10,000 uF at 100V+ DC. You will want to charge it fully, then discharge it from the transformer case through the bad winding, making sure you pick the shortest run (lowest resistance) of the two leads. The idea is to blow up the connection - without breaking the coil.

    Be exceedingly careful when doing this, there is enough energy involved to be fatal under some conditions. And, remember, the alternative is landfill, so heroic measures are permitted.

    https://www.youtube.com/watch?v=WaqBhYNBiGo

    Of course, this is all *AFTER* you have determined that your filter caps are good, the rectifier is good, and there are no faults in the B+ and/or output transformers. Transformers will vibrate when over-loaded. So, if you have a fine-pitch ammeter, see if the troubled unit pulls any more current than the untroubled unit. If so, trouble-shoot from there before blaming the transformer.

    Best of luck with it!

    https://www.dropbox.com/s/u8cem07ows61enw/IMG_1192.JPG?dl=0   Seamus is the smaller cat on the left at 18 pounds.


    Last edited by Peter W. on Fri Jun 15, 2018 3:25 pm; edited 1 time in total (Reason for editing : Added clip of capacitor discharge.)
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    Pillo69

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    Re: Wax potting a noisy power transformer on a MKII

    Post by Pillo69 on Fri Jun 15, 2018 6:08 pm

    Have you looked at whether the screws securing the transformer to the chassis have insulating plastic washers at the sites that hold the transformer?

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