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    Dynaco MkIII restored, KT120 in it?

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    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Tue Sep 06, 2016 1:23 pm

    My MkIII's with Svetlana 6550 had a problem. A fuse of one of the monoblocks blew out.
    I suspected problems with the tubes. I bought the MkIII with these tubes so I have no information about the history.
    I let the tubes test on a Amplitrex AT1000. One set about 84% gain, the other set 91%. They seem OK.
    I measured all the components and after that I decided to replace all components, also the can capacitor.
    Slowly raise the voltage with a variac. With some music on it, again a fuse blew out.
    The tubes of this amp I put aside. With the tubes of the good block I did measurements; both amps measure the same.
    Now the 'bad' tubes in the good amp and again the fuse blew out. So now I'm sure one of the 6550 is bad though it measured OK on the Amplitrex.

    OK, next....

    Looking for other tubes, I found the Tungsol KT120 from NewSensor. Mr. Bob Latino wrote that the bias current has to be the same value as for the KT88.
    He also wrote that it sound more 'tubier' than a KT88 (more mid bass). What is meant with 'tubier'?
    Does anyone has experience with the KT120 in a MkIII at this moment? How does it sound?

    I ordered the Tungsol KT120 matched quad for $48 each.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Bob Latino on Tue Sep 06, 2016 1:54 pm

    Dynaco_wjw wrote:My MkIII's with Svetlana 6550 had a problem. A fuse of one of the monoblocks blew out.
    I suspected problems with the tubes. I bought the MkIII with these tubes so I have no information about the history.
    I let the tubes test on a Amplitrex AT1000. One set about 84% gain, the other set 91%. They seem OK.
    I measured all the components and after that I decided to replace all components, also the can capacitor.
    Slowly raise the voltage with a variac. With some music on it, again a fuse blew out.
    The tubes of this amp I put aside. With the tubes of the good block I did measurements; both amps measure the same.
    Now the 'bad' tubes in the good amp and again the fuse blew out. So now I'm sure one of the 6550 is bad though it measured OK on the Amplitrex.

    OK, next....

    Looking for other tubes, I found the Tungsol KT120 from NewSensor. Mr. Bob Latino wrote that the bias current has to be the same value as for the KT88.
    He also wrote that it sound more 'tubier' than a KT88 (more mid bass). What is meant with 'tubier'?
    Does anyone has experience with the KT120 in a MkIII at this moment? How does it sound?

    I ordered the Tungsol KT120 matched quad for $48 each.

    Re: "Tubier" > KT88's (less tubey) tend to have a little more extreme bottom end (below 50 Hz) and extreme top end (above maybe 5000 Hz) than a KT120. KT120's (tubier) have more (a little stronger) mid-bass content (maybe 50 to 500 Hz). These two types of output tubes just sound very slightly different from one another at least to my ear. There is no "which is better" because everyone favors different types of music and everyone has different associated audio components in their music system.

    Bob

    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Tue Sep 06, 2016 2:02 pm

    Thank you very much for your answer. Mr. Latino what can you say about the problem with the Svetlana 6550. What is happening inside the tube? Why is the fuse blown out?
    I'm looking forward using the KT120 tubes. Here the price is much cheaper it seems. In the US about $90,- ?

    The grid voltage has raised from 220VAC to 235VAC. What is this doing for the design of the amplifier? Is the plate voltage also higher? Maybe use a varic constantly to let the amp work at the design voltage of 220VAC?

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Bob Latino on Tue Sep 06, 2016 2:14 pm

    Dynaco_wjw wrote:Thank you very much for your answer. Mr. Latino what can you say about the problem with the Svetlana 6550. What is happening inside the tube? Why is the fuse blown out?

    It is hard to say what the problem is inside your 6550 output tube ? On a Mark III, pins 3 and 4 carry about 475 VDC .. The 475 VDC could be shorting to one of the other elements inside the tube ?

    Bob

    Peter W.

    Posts : 83
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Peter W. on Tue Sep 06, 2016 2:45 pm

    [quote="Dynaco_wjw"]My MkIII's with Svetlana 6550 had a problem. A fuse of one of the monoblocks blew out.
    I suspected problems with the tubes. I bought the MkIII with these tubes so I have no information about the history.
    I let the tubes test on a Amplitrex AT1000. One set about 84% gain, the other set 91%. They seem OK.

    Did you go through the leak test? Is there a formal "short" test?

    And, if you run only one output channel at a time, does the fuse still blow? If so, how about if you switch tubes around?

    I ask, because of only one (1) tube has an operational short (vs. a cold short), you may be able to isolate that tube - assuming an infinite number of fuses. Nor is it a good idea to do that to a power-transformer.

    peterh

    Posts : 642
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    Re: Dynaco MkIII restored, KT120 in it?

    Post by peterh on Tue Sep 06, 2016 3:23 pm

    Dynaco_wjw wrote:My MkIII's with Svetlana 6550 had a problem. A fuse of one of the monoblocks blew out.
    I suspected problems with the tubes. I bought the MkIII with these tubes so I have no information about the history.
    I let the tubes test on a Amplitrex AT1000. One set about 84% gain, the other set 91%. They seem OK.
    I measured all the components and after that I decided to replace all components, also the can capacitor.
    Slowly raise the voltage with a variac. With some music on it, again a fuse blew out.
    The tubes of this amp I put aside. With the tubes of the good block I did measurements; both amps measure the same.
    Now the 'bad' tubes in the good amp and again the fuse blew out. So now I'm sure one of the 6550 is bad though it measured OK on the Amplitrex.

    OK, next....

    Looking for other tubes, I found the Tungsol KT120 from NewSensor. Mr. Bob Latino wrote that the bias current has to be the same value as for the KT88.
    He also wrote that it sound more 'tubier' than a KT88 (more mid bass). What is meant with 'tubier'?
    Does anyone has experience with the KT120 in a MkIII at this moment? How does it sound?

    I ordered the Tungsol KT120 matched quad for $48 each.

    Power tubes are "consumables", and is the first item to replace in case of difficulties.
    An amp made for 6550 ( or kt88) will work best with , guess what , 6550 (or kt88).
    And it's like motor oil, better use cheap stuff and change often then expensive ones
    that stays longer in amp/engine.

    ( price difference in tubes has no correlation to quality, it's the distributors matching/testing that matters)

    Eurotubes will charge you $73 for a matched pair, with good warranty, more vendors are "out there".

    Look for decent warranty and strict matching ( you will need matched tubes )

    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Tue Sep 06, 2016 3:44 pm

    An amp made for 6550 ( or kt88) will work best with , guess what , 6550 (or kt88).

    Does this mean that KT88 will give a better result than the KT120? It's a new type tube.

    Did you go through the leak test? Is there a formal "short" test?
    Yes all the test. I read somewhere that the heating time for the Amplitrex is limited and that is maybe why the fault is not found.

    Better not to find out what the exact problem is with these tubes, just end of life, broken.

    Peter W.

    Posts : 83
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Peter W. on Tue Sep 06, 2016 3:51 pm

    Dynaco_wjw wrote:
    An amp made for 6550 ( or kt88) will work best with , guess what , 6550 (or kt88).

    Does this mean that KT88 will give a better result than the KT120? It's a new type tube.

    YES, insofar as that is for which the amp was designed.

    Better not to find out what the exact problem is with these tubes, just end of life, broken.

    *!!GLEEP!!*

    I really hope is not what you meant to type...

    Best to be absolutely sure what the failure mode is - and if it is merely age, that is all good. But if it is NOT, it could get very expensive in a hurry.

    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Tue Sep 06, 2016 4:04 pm

    Best to be absolutely sure what the failure mode is

    Yes I tested thoroughly. First the resistance of the transformers, measured with the good tubes in the good amp all the voltages, measured with the good tubes in the amp with blown fuse al the voltages, both amps measure the same. Tested the amp with the blown fuse with speakers and music.
    Eventually tested the tubes (when the fuse was blown) on the good amp. Slowly rise the voltage with the variac but at 223VAC again the fuse is blown.
    So for me, it has to be one of the tubes. On the small Junior tester it says 'good' but probably the Amplitrex and the Junior have not the same conditions as in the MkIII.

    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Tue Sep 06, 2016 4:07 pm

    What I would like to see is where the working point is in the diagram.
    In the diagram of the KT88 and in the diagram of the KT120.
    I suppose it is working in class AB.
    Strange enough I hardly see a drawing why a certain bias current is chosen.
    Or am I wrong? Is what I write nonsens? Is this lack of my knowledge about push pull configuration?

    Peter W.

    Posts : 83
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Peter W. on Tue Sep 06, 2016 4:21 pm

    Bob Latino wrote:
    The 475 VDC could be shorting to one of the other elements inside the tube ?

    Bob[/b][/size]

    That would be my bet!

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco MkIII restored, KT120 in it?

    Post by peterh on Tue Sep 06, 2016 5:08 pm

    Dynaco_wjw wrote:
    Best to be absolutely sure what the failure mode is

    Yes I tested thoroughly. First the resistance of the transformers, measured with the good tubes in the good amp all the voltages, measured with the good tubes in the amp with blown fuse al the voltages, both amps measure the same. Tested the amp with the blown fuse with speakers and music.
    Eventually tested the tubes (when the fuse was blown) on the good amp. Slowly rise the voltage with the variac but at 223VAC again the fuse is blown.
    So for me, it has to be one of the tubes. On the small Junior tester it says 'good' but probably the Amplitrex and the Junior have not the same conditions as in the MkIII.
    Do yourself a favor : throw away the suspect/bad tubes. Don't use them, ever!

    Tubes are consumables and has a finite lifetime, in a proper amp they should last
    2000-5000h , in a inproper one they could get broken in hours.
    Flashing, redplating or sudden large change of operating parameters is a strong indication that something is wrong.

    corndog71

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    Location : It can get windy here

    Re: Dynaco MkIII restored, KT120 in it?

    Post by corndog71 on Tue Sep 06, 2016 5:41 pm

    Also check for any connections that may be touching (shorting).

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: Dynaco MkIII restored, KT120 in it?

    Post by j beede on Wed Sep 07, 2016 12:44 am

    FYI: I've tried JJ KT-88, Winged-C EL-34, Gold Lion KT-66, and SOVTEK 6550 in my MkIIIs. To my ears nothing sounds better than 6550.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
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    Re: Dynaco MkIII restored, KT120 in it?

    Post by Bob Latino on Wed Sep 07, 2016 8:32 am

    The Sovtek 6550WE is a real good sounding tube at reasonable pricing. It is also very durable. When Sovtek stopped making their KT88 a few months ago, I ordered a quad of their 6550WE output tubes and used the quad in my VTA ST-120 for about a month. They sound great and zero problems. Customers who want a tube set with their VTA ST-120 amp (kit or wired) now get a matched quad of Sovtek 6550WE output tubes with their amp.

    Bob

    wgallupe

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    Age : 62
    Location : Central Mass.

    Re: Dynaco MkIII restored, KT120 in it?

    Post by wgallupe on Wed Sep 07, 2016 8:48 am

    Hi Bob,

    Can you describe the sound of those Sovtek 6550s vs the GL KT88s in the ST120? Where would they 'fit' in your KT88 / KT120 comparison above (post no 2)?

    Thanks,
    Wayne

    Bob Latino
    Admin

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    Re: Dynaco MkIII restored, KT120 in it?

    Post by Bob Latino on Wed Sep 07, 2016 10:20 am

    Hi Wayne,

    The Sovtek 6550WE lean more towards the Tung-Sol KT120 type of sound than a KT88. They have a little more "tubier" sound than a KT88. A KT120 will probably last a little longer than a 6550 or a KT88 in the VTA ST-120. A KT120 is designed to take higher plate voltages and higher current flows than what you find in the VTA ST-120. When you use a KT120 output tube in the VTA ST-120, you are using the tube "as a KT88".

    I would consider the Sovtek 6550WE "a good buy" or a very cost efficient tube. These tubes run about $28 - $35 each. The Tung-Sol KT120 is about $50 a tube ...

    Bob

    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Wed Sep 07, 2016 4:45 pm

    Dynaco_wjw wrote:Does anyone has experience with the KT120 in a MkIII at this moment?

    Nobody yet?

    Dynaco_wjw

    Posts : 8
    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Sat Sep 10, 2016 5:01 am

    The Tungsol KT120 are placed in the completely restored MkIII. I also put new Philips NOS GZ34 in it.
    The bias (Dynaset) value is adjusted to 1,56V.
    I run the amplifiers with a variac on 220VAC. The grid voltage is rised from 220VAC (when the amp was sold) to 235VAC now.
    I thought maybe it is better not to connect it at this higher voltage because maybe the power transformer is running to hot.
    It does sound very nice. There is only some 'mechanical' hum.

    I bought resistors and tubes at Jukebox-Revival, the can capacitor and other capacitors I bought at Dipl.-Ing. Jan Philipp Wüsten. Since I live in the Netherlands it is the most easyest way for me to order.

    Here are some photo's.



    " />



    peterh

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    Re: Dynaco MkIII restored, KT120 in it?

    Post by peterh on Sat Sep 10, 2016 6:40 am

    What an amazing plaze ! Nice restoral and interesting environment.

    Dynaco_wjw

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    Join date : 2013-11-28

    Re: Dynaco MkIII restored, KT120 in it?

    Post by Dynaco_wjw on Sat Sep 10, 2016 7:21 am

    Thank you Peter! With 'interesting environment' you mean the whisky in the corner? Razz

    When I bought the MkIII they where in a extremely good condition, the chassis and the cover.
    The price was €400,- I played some time with it until one of the fuses blew out (start of this topic).
    I decided to restore it, at least the capacitors. The tubes where tested on a Amplitrex, the measured about 90%.
    Eventually one of the tubes was not OK so the fuse blew out. I looked and looked on the internet for KT88, found some quads with prices of about $350,- !!!
    Did not buy these but the KT120 of New Sensor, about $40 each.
    I measured all the resistors and discovered that some of them did not have the right value anymore so I decided to replace these R's too.
    The amps where rewired by the former owner. He used solid brown colored wire of 2,5mm2 !!! The same wire what is used in buildings here for electricity.
    It looks terrible and I could not reach the soldering area's. So I decided to remove this stuff and rewire it the way the factory wanted it. I would like to use about the same wire, black 18AWG but it is so very hard to get. I had to buy it in the US. Eventually I decided to use a good quality wire which was/is not solid.
    I replaced the original Selenium diode by a Silicium diode.

    Now I'm testing the amplifiers. The GZ34 tubes (base) smell a little bit due to the heat Crying or Very sad but I think that is a question of time.
    The MkIII's are connected to a Sony 333ESD and the Swedish Rauna Leira II. It all sounds very smooth and pleasant. Enough punch for the woofers.
    I 'm also comparing the sound at this moment with the ST-35 (completely restored with new boards from 'Dave' with a power supply equipped with  EFB

    I think one of my next projects will be to restore a PAS-3X.

    Here is a photo and a video of the restored amps.

    " />


    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco MkIII restored, KT120 in it?

    Post by peterh on Sat Sep 10, 2016 7:58 am

    I see that you have several of Bo Hanssons speakers! These are gems that won't come back.
    Speakers was designed by listening, no computers involved only trained ears using acustical
    music ( Bo also made acustical high quality recordings under the brand Opus 3. )


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