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    Amp Trauma Pt 2: Problem with hum in MK IV

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    Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Guest on Sat Jul 02, 2011 3:14 pm

    I have been trying to trouble shoot hum in a rebuilt pair of MK IV's. I think I may now have the main problem. I am using the VTA boards, configured for 12AT7's. I did find a resistor that needed to have one leg shifted. The two holes were 1/16" a part, and the trace connected the two on one side, but not on the other where it mattered. The VTA's for the MK III/IV has the 3 trimmers, 2 for adjusting the PT bias and one for AC balance. I had purchased a new PEC 10K pot when I was going to use new Dynaco clone boards to replace the OEM pots. I decided that since the PEC's are nice pots, rather than use the on board trimmer as spec'ed for the new boards, I would use the PEC. The problem with this I believe is that I have to run 3 wires that are over 4 inches long to get from the board to the pot. They are not shielded, and AC lines floating in the breeze can't be good. Can you shield something like this, or better to get a trimmer? Thanks!

    Bob Latino
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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Bob Latino on Sun Jul 03, 2011 7:36 am

    Michael,

    IMHO you should get a new 10K trimmer for the board and not use the 10K PEC pot as an AC balance control. You have probably answered your own question regarding the source of hum in your amp.

    Bob

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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Bob Latino on Tue Jul 05, 2011 7:45 am

    Michael,

    1. Does the hum go away if you short the inputs ? If it does, the hum could be external to your Mark IV's and not caused by the external 10K bias pot. If the RCA input jacks are left "open" and not terminated with interconnects plugged in and terminated on the other end of the interconnects (IOW not plugged into your preamp) - you will have hum ..

    2. Did you add or do the amps have a 3 wire cord ? If so - you could have a ground loop? Try floating the ground on one of the amps with a 3 to 2 "cheater plug" which effectively lifts the ground.

    3. In your other post you had mentioned that your input jacks had the ground tabs touching the chassis. This is not an accepted way of grounding the input jacks. The negative side of the input jacks should not touch the chassis and instead should be grounded back to the ground eyelet on the VTA driver board.

    Bob

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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Guest on Tue Jul 05, 2011 8:54 am

    Bob:
    1: Yes the hum does go away when the input jacks are shorted. In fact I have been discussing this with a tech, and he said that if I shorted the input and the hum when away it was not the amp. For some reason he then decided that the problem was in the input stage of the driver board. I had meant to ask him about this but proceeded assuming that the problem was still in the amp. I recall that open inputs can even be a problem in SS amps. The amp with the inputs shorted is actual dead quiet. So I will try that this AM.

    2: I did add a 3 wire cord, but when I floated the ground the hum was the still there. I did however locate the 3rd wire at the terminal strip where everything else is grounded. A friend sent me an article on star grounding, so I figured that would be the place to put it. I even tried isolating the stand offs and screws that secure the board to the chassis. Since that board is grounded through a wire to the terminal ground, I though the additional grounding through the 4 screws might be creating a problem, but it did not change a thing.

    3: I had changed the input jack to being grounded on the chassis, but changed it back on your advice form another post. I realized that I had isolated them in the first place because the OEM jacks were isolated.

    Thanks for all the suggestions you have offered. I think this is a case of my having enough knowledge to stumble through this, but not the level required to intuitively know what to look for. Hence the saying: you have enough knowledge to be dangerous.

    Guest
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    Trimmer made no difference

    Post by Guest on Fri Jul 08, 2011 2:23 am

    I received the trimmers today, installed them and they made no difference. Are the ST-70's, and HK MKIII's dead quiet? I am beginning to believe the MKIV is like the red haired, bastard step child Thanks for your help!
    Michael

    Bob Latino
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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Bob Latino on Fri Jul 08, 2011 6:20 am

    Michael,

    Have you replaced the two quad caps in your amps ? Older, leaky quad caps can cause the amps to hum ..

    See if you can post a photo of the top of the amp and also one of the bottom wiring. One of use may be able to pick up on something in a photo that might not be readily apparent to you.

    Bob

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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Guest on Sat Jul 09, 2011 11:37 am

    Bob: The quad caps are new, which is not to say there couldn't be a problem. The only thing that is not new are the Transformers and the chokes. I will get some pictures, that's a good idea on your part. I have twisted all the wires from the transformers, and boards to sockets, in a tighter twist at the recommendation of a friend. I didn't go as tight as he recommended because these trannys wires are old and stiff, something I can relate to.

    tubes4hifi
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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by tubes4hifi on Sat Jul 09, 2011 3:41 pm

    MK4s are as dead quiet as ST70s and MK3s when wired properly,
    there must be some wiring issue that you are overlooking

    Guest
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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Guest on Sat Jul 09, 2011 10:17 pm

    This doesn't really answer the question I asked. I think we can all agree that there are varying
    levels of quality and cost; the two usually related, of hi-fi gear that has been made through the years. I was under the impression that Dynaco wasn't the hi-end of it's era, perhaps I'm wrong. Usually lower cost necessitates compromises which is why I asked the question. Was/is the Dynaco line of amps very quiet. I think it is a fair question and it was not intended as a rhetorical question. Since I have no idea how quiet an ST-70 or a MKIII are, the statement they are the same really has no meaning. The noise threshold of amps, pre-amps ... can vary from brand to brand, and unit to unit. When I bought the amps, I checked to be sure they worked, and put them a side to rebuild.

    Let me qualify something. The hum in the amps is very low, you have to bend down right next to the speaker in order to hear it. When the music is playing at any level you don't hear it at all. But that doesn't change the fact that it's there.

    As far as the amps being wired correctly, I followed all the directions for the construction of the boards. I verified the transformer and internal wiring with the Dynaco manual for the MKIV that I downloaded. I have rechecked all of this for a 3rd time. I assume this comment is just a back handed way to said I must have screwed up somewhere. Of course this is possible as well. Of course everyone knows that instructional manuals are never wrong so I guess that only leaves one possibility.

    anbitet66

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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by anbitet66 on Sat Jul 09, 2011 10:45 pm

    Michael,

    If you need to put your ear so close to the speaker to hear a hum, there is no problem with your amp. A small amount of hum is normal. The ST-70 I assembled a few months ago has a hum, but only so slight that you need to be no more than a foot from the speaker in order to hear it. I also have a vintage ST-70 that is all original (except for two output tubes) and has the same level of hum.

    I'd recommend you sit back and enjoy your amp.

    Tony

    Bob Latino
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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Bob Latino on Sat Jul 09, 2011 10:49 pm

    Michael,

    Above on post #4 you mentioned that > "Yes the hum does go away when the input jacks are shorted." Shorting the input jacks is a definitive test on the noise/hum/buzz etc. of the amp ALONE. If you get no hum from an amp with the input jacks shorted that is definitive proof that the cause of the hum/buzz/noise is external to the amp. Some interaction with your cables, preamp grounding issue, or something else in your home that is external to the amp has to be the cause of the hum. Mark IV's, as Roy mentioned above, if properly assembled, are every bit as quiet as an ST-70 or a Mark III.

    Bob

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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Guest on Sat Jul 09, 2011 11:19 pm

    Thanks, that's all I wanted to know!

    j beede

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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by j beede on Sun Jul 10, 2011 2:13 am

    I often hear hum in amps that have been described to me as "dead quiet". I have yet to have a Dynaco amplifier on my bench that my frequency counter doesn't immediately lock onto 60Hz at the output. Dynaco power transformers are not the quietest I have encountered. I can easily hear the PT hum when I turn on ST-70 or Mk IIIs. This normally doesn't detract from the pleasure they provide when listening to music. Some people are more sensitive to 60Hz than others.

    Guest
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    Re: Amp Trauma Pt 2: Problem with hum in MK IV

    Post by Guest on Sun Jul 10, 2011 8:22 am

    [b]Thanks for the info. By the way, I like the photo accompanying your nom de guerre. The 57's are wonderful. My first tube amp was a Quicksilver, and it was very quiet, so I guess that was my frame of reference. After checking and rechecking all the work I did on the MKIV's and I still had the hum, I figured something was wrong. I unfortunately never thought to ask if a certain level of noise was normal, which brings me to this point.
    Through out the process I have tried to change one amp at a time in order to have a reference. This AM I just finished re-twisting all the wiring in one amp as tight as I could get them to see if that made a difference. They both sound exactly the same.

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