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    Mk IV Questions

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    harpy

    Posts : 94
    Join date : 2012-02-04

    Mk IV Questions

    Post by harpy on Sat Feb 04, 2012 4:13 pm

    Greetings all,
    I am a new forum member just purchased a pair of Mk-IV's. One is factory wired the other was a kit. I would like to restore the pair. I breifly powered both on and all the tubes glow. I did a quick visual inspection and saw an issue one:



    To me it looks like the Multi-section cap is leaking.

    Also I've read the that power transformers can be problematic.



    Does tha leakage on top of the transformer mean it has run hot or is that normal? My goal is to keep everything original as posible, but move everything over to a Stainless Chassis.

    Bob Latino
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    Location : Massachusetts

    Re: Mk IV Questions

    Post by Bob Latino on Sat Feb 04, 2012 4:56 pm

    Hi and welcome to the Forum. The transformer is not a problem. What you see on the top is just some excess varnish that they use to hold the plates together. That transformer came from the factory looking like that ...

    The quad cap was the weak spot on all the older Dynaco amps. That quad cap looks like an original Dynaco quad cap. Even if the amp works OK now, it will almost surely cause an issue later on especially if you intend to play the amp on a daily basis. Dynakitparts sells replacement quad caps for all the Dynaco amps.

    Bob

    tubes4hifi
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    Re: Mk IV Questions

    Post by tubes4hifi on Sat Feb 04, 2012 6:52 pm

    I agree, replace the quad cap, and replace the 6.8K resistor there with a 3 watt metal oxide type.
    The transformer is probably fine and you can just clean the varnish off.
    I would probably also replace the two capacitors (very center of underside) 47uF 100v

    you can get a copy of the manual here: http://tubes4hifi.com/Dynaco%20MK4%20manual.pdf

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Sat Feb 04, 2012 7:40 pm

    Thanks,

    I see Dynakits offer 2 caps at the 30 20 20 20 uF "consisting of (4) discrete High Quality "Low ESR" high voltage caps mounted in a twist-lock style can" and a regular one. Would it be worth the extra expense "Low ESR" one?
    I would like to test drive them tonight and see what I have.

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Sat Feb 04, 2012 8:32 pm

    I tried them out. The one with the leaking cap makes a loud hum through the speaker. I did swap tubes to rule that out. Might it be the multi cap or do I have other issues? All the tubes light and the hum gets louder as the amp starts up.

    Bob Latino
    Admin

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    Re: Mk IV Questions

    Post by Bob Latino on Sat Feb 04, 2012 8:37 pm

    harpy wrote: I tried them out. The one with the leaking cap makes a loud hum through the speaker. I did swap tubes to rule that out. Might it be the multi cap or do I have other issues? All the tubes light and the hum gets louder as the amp starts up.

    Hi - You have a classic symptom of a bad quad cap with a loud hum like that ...

    As to the quad cap, I would use the 550 volt low ESR cap. Yes - It does cost a little more than the other (CE 525 volt rated) quad cap but it has an extra 25 volts of headroom.

    Bob

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Sat Feb 04, 2012 8:50 pm

    Thanks Bob,

    I'll start there, so nice not to be flying totally blind. I'll do the 6.8k and 47 uF too. Thanks that for advice Tubes. Very Happy

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Sun Feb 12, 2012 1:58 am

    Thanks, you guy were correct it was a bad quad cap.

    [img][/img]

    Quad installed with 6.8 ohm 3 watt metal oxide resister. New Choke.

    [img][/img]

    Outside

    I was able to listen to them for the first time and was stunned by how good they sounded. The urge to change anything was gone. I was wondering though if I should do the same thing to the other amp? I hate to tear up the factory wire one, but I bought 2 quads and thought they should be as similar as posible. Afer hearing them these will be my main amps, so I am more interestd in function and long term use. They have mix set of tubes, so I guess my next step will be to retube.


    j beede

    Posts : 299
    Join date : 2011-02-07
    Location : California

    Re: Mk IV Questions

    Post by j beede on Sun Feb 12, 2012 11:59 am

    Get yourself a brass wire brush so you can burnish the stock Dynaco hardware. It will clean up nicely. I get my brushes at Harbor Freight. I restored a pair of Mk IIIs that were stored outdoors under a picnic table. That same brass brush will clean up your laminations before painting too.

    [img][/img]

    [img][/img]

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Sun Feb 12, 2012 1:25 pm

    Those look great! I tried some metal polish on one the chrome bottoms, but it's a fair amount of work. I did polish the metal around the quad cap while it was removed. I noticed that most transformers have 6 holes where the transformer cover is mounted. One one of the Mk-IV's PA-135s there are single bolts that goes through the middle sides, while the one pictured has bolts in all four corners. I did notice the transformer covers were different and picked up a set from the Triode Store so they would match.
    I have a DQ-LP1 cross-over that sets its high pass filter via the amps input impedence. I thought I read the Mk-iV's input imedence was 147k Ohms. Going to head to Harbour Freight and pick up some brushes.


    j beede

    Posts : 299
    Join date : 2011-02-07
    Location : California

    Re: Mk IV Questions

    Post by j beede on Sun Feb 12, 2012 1:52 pm

    Stock Dynaco chassis are not chrome plated and nothing (in my experience) will produce a mirror finish. Flitz will get the chassis clean--but I think you already mentioned that you are going with a reproduction chassis. Restoring the transformers was a big job but was well worth the effort in my opinion. I think the trick to good transformer appearance is keeping the paint on the laminations very thin. It is easy to "flood" them with paint.

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 48
    Location : Texas

    Re: Mk IV Questions

    Post by denny9167 on Sun Feb 12, 2012 4:02 pm

    harpy wrote: Thanks, you guy were correct it was a bad quad cap.

    [img][/img]

    Quad installed with 6.8 ohm 3 watt metal oxide resister. New Choke.

    [img][/img]

    Outside

    I was able to listen to them for the first time and was stunned by how good they sounded. The urge to change anything was gone. I was wondering though if I should do the same thing to the other amp? I hate to tear up the factory wire one, but I bought 2 quads and thought they should be as similar as posible. Afer hearing them these will be my main amps, so I am more interestd in function and long term use. They have mix set of tubes, so I guess my next step will be to retube.


    I built a pair of reproduction models of the Mark IV's a few months back,and the sound is awe inspiring. I'm using an ipod for the source,a stereo buffer preamp that I built,and a pair of RD-75 planars. I wasn't sure how the Mark IV's would sound,or how they would drive the planars,but they do a really nice job. I think they are rated at 88db. I've listened to Krell and Apogee combo's that Came close to this sound.[img][/img]
    [img][/img]

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 48
    Location : Texas

    Re: Mk IV Questions

    Post by denny9167 on Sun Feb 12, 2012 5:44 pm

    j beede wrote:Get yourself a brass wire brush so you can burnish the stock Dynaco hardware. It will clean up nicely. I get my brushes at Harbor Freight. I restored a pair of Mk IIIs that were stored outdoors under a picnic table. That same brass brush will clean up your laminations before painting too.

    [img][/img]

    [img][/img]

    SWEET!!!!

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Sun Feb 12, 2012 7:00 pm

    I was worried too as the Dahlquist DQ-10's are 86db. I have rebuilt the DQ-10's and was driving them with a Rotel RB-991 200 watt SS amp crossed over at 60 Hz with an old Bryston 6b dumping 800 watts (4 ohms) into the sub. I hooked the Mk-IV's nude (no crossover) just see if I had done the quad cap right and it turned in to 3 hour listening session. I want to try them on the cross-over and see if the magic continues plus that should take a little load off the Mk-IV's as well.
    I use a similar playback setup. I have all my CD's in uncompressed AIFF format with ITunes on Pc in another room. I use an Apple TV2 that pulls the music off the PC over Ethernet and sends it via optical to a Rotel 1098 that does the DA conversion and preamp duties. I run a converter box that converts ATV HDMI to composite video so the Rotel can display the Album art. I use my IPhone as a remote.


    [img][/img]
    Above is the DQ-Lp1 that does the cross over duties.


    j beede

    Posts : 299
    Join date : 2011-02-07
    Location : California

    Re: Mk IV Questions

    Post by j beede on Sun Feb 12, 2012 7:48 pm

    i'd like to hear some details about those ribbon drivers Denny9167.

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 48
    Location : Texas

    Re: Mk IV Questions

    Post by denny9167 on Sun Feb 12, 2012 10:29 pm

    j beede wrote:i'd like to hear some details about those ribbon drivers Denny9167.

    They are made by a company called BG,they stand around 6 1/2' tall, they have a range from 150Hz to 18,000Hz 400W max power handling. I am planning on a powered subwoofer to handle the low end.

    Here is the link to the web page

    www.bgcorp.com

    also check www.partsexpress.com for more imformation. That is were I purchased mine.

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Thu Feb 16, 2012 12:23 am


    One of the Bias caps showed some signs of chemical leakage, so I decided use Dynakit's Bias Circut Kit. It uses higher value and voltage caps with a IN4007 Diode replacing the green one and I choose carbon resistors for whatever reason.


    New Transformer cover to match the other amp. In hindsight in may been a poor idea because the old covers were hard to get off and I think I was lucky not to damage the transformer. Plus the quad cap really needs to be uninsalled, but I able to get it on. Also the four corner holes do not match up, but that was no big deal because the center did.


    Finally some matched 6CA7 electro-harmonix and a lot Flitz. I biased the tubes to 1.43. Think I am done for for now. Will need to read up on upgrades and start looking at the circuit board caps. Thanks all for the help and good ideas.


    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Thu Feb 16, 2012 4:18 pm

    Sorry to be Mr. Spammy, but I was doing the Bias one my factory wired amp and had a question.


    One this one I have pulled yellow green wire from pin 2 and noticed a jumper wire connecting pin 2 to pin 3.


    Here is the kit wired one I just did, but there is no jumper between pins 2 and 3. The yellow and green wire just sits there doing nothing. The amp plays fine, but when I did the upgrade I did wonder why the wire would just sit there not connected to anything.

    Looked like in the schematic it is jumpered any thoughts?

    tubes4hifi
    Admin

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    Re: Mk IV Questions

    Post by tubes4hifi on Thu Feb 16, 2012 5:13 pm

    if you look at the schematic for the MK4 the yellow-green wire should be going to ground, it is the center-tap for the filament voltage

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Thu Feb 16, 2012 8:23 pm

    Thanks Tubes4hifi,

    That makes sense, post 3 goes to ground. The kit amp showed no evidence of this being ever being installed. I should have questioned a transformer wire being soldered to a terminal that doesn't go to anywhere as suspect. Easy fix thanks again. Embarassed

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Mk IV Questions

    Post by harpy on Thu Feb 16, 2012 11:54 pm



    Little jumper installed. The PA-135 was running a lot hotter than the one on the other amp. With it grounded correctly they both run very cool now. That was going to be my next issue to figure out, so I'm glad it was an easy fix. Also moved to transformer leads to the lower terminals like the factory one.



    Cage installed to protect it from Great Dane tail swipes.

    tubes4hifi
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    Re: Mk IV Questions

    Post by tubes4hifi on Fri Feb 17, 2012 12:28 am

    very good! now enjoy!!

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