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    New Member, New ST120!

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    jsl1234

    Posts : 4
    Join date : 2017-08-28

    New Member, New ST120!

    Post by jsl1234 on Sat Sep 02, 2017 12:19 pm

    Hi,

    After several years of back and forth I'm finally dipping my toes in the tube ocean.

    Purchased a VTA ST120 kit from Bob last week. Read all of the reviews all over the net on tube amp kits--Bob's is by far cream of the crop.

    Since I've never owned a tube unit, I decided to start out with Bob's optional tube set Sovtek 6550WE, 3 12AU7, upgraded caps, 21 step attenuator  and and a gz43/5AR4 rectifier. My plan is to run the unit without a pre-amp since my only source is a Bryston DAC and it puts out a fair amount of voltage from the unbalanced outputs.

    So anyhow, got the boxes Wednesday afternoon and spent 3 days of hard work (2 assembling the kit and painting the chassis; 1 day debugging).

    I had a couple of early problems...first I mis-wired the rectifier tube which caused it to arc when i fired the unit up Embarassed  Sorted that out with Bob's help in 10 minutes.

    Then I checked for bias voltage--discovered I was shorting two low-voltage wires on the driver board to chassis (again, with Bob's help).

    Cleaned that up, checked bias voltages (all good), inserted output tubes and fired the unit up to set bias and....rectifier tube arcs a second time Rolling Eyes. That took a little longer but sorted that out after a couple of hours of help from Bob's east coast tech Bill...(another great guy--thanks!)

    Anyhow finally ready to roll at about 6PM last night. Set initial bias, played music for two hours, went to dinner (left the unit on), came back about 2 hours later and the unit was dead (blown fuse).  Sad.

    Sooooo this morning I rechecked all of the voltages, installed a fresh rectifier tube (thinking the first rectifier tube may have gone south after all of the punishment I meted out during the diagnostic / initial start-up runs), installed a new fuse, re-biased the tubes (drum roll please.......)

    So far so good. Been humming along at moderate volume for the last 2 hours without issue.

    Keeping my fingers crossed that the rectifier tube was the issue. However if another fuse pops with the new rectifier tube, Bob suggested I recheck solder joints while I'm cleaning up the rats nest I created when I undid all of the tie wraps during the debugging phase--Bob suggested that heat expansion can cause a poor solder connection to weaken to the point of failure and given this is my first project of this sort I don't doubt my soldering skills are still suspect.

    Quite an adventure, more to come (including pictures if I can ever figure that out...)

    Oh yea, I have to mention that Bob is the most customer friendly, supportive guy--I used to run a very large customer support center for a very large company, so I get customer service/support. Bob (and Bill) get 5 STARS all around for putting the customer first!!!

    Jay

    Dale Stevens

    Posts : 84
    Join date : 2014-07-06
    Age : 68
    Location : Loris, SC

    Re: New Member, New ST120!

    Post by Dale Stevens on Sat Sep 02, 2017 4:20 pm

    Jay . Welcome . Bob and all the folks here are spot on. I would not , however, leave a tube amp on
    and go leave it. As our buddy (loosely put) DeePee has said "don't turn your back on dem bitches" . Dale

    jsl1234

    Posts : 4
    Join date : 2017-08-28

    Re: New Member, New ST120!

    Post by jsl1234 on Sat Sep 02, 2017 4:36 pm

    Dale Stevens wrote:Jay . Welcome .   Bob and all the folks here are spot on.  I would not , however, leave a tube amp on
           and go leave it.  As our buddy (loosely put) DeePee has said "don't turn your back on  dem bitches" .    Dale              

    Haha, understand--I'll shut it down next time we do a pizza run Very Happy

    It has been running continuously for over 6 hours today without issue, so I'm cautiously optimistic that swapping out the first rectifier (which arc'd twice) addressed the problem!

    Cheers and happy Labor Day to all!

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    Tubes4ever

    Posts : 96
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: New Member, New ST120!

    Post by Tubes4ever on Sat Sep 02, 2017 7:10 pm

    jsl1234, One thing you need to know about the 5AR4 tube is that once it starts arcing for whatever reason, it will most likely do it again soon.  Arcing damages the tube immediately. So, whether it was due to faulty wiring or just age, it needs to be replaced.  When it arcs, it will blow the fuse.

    You should keep a 5U4GB tube around for when the 5AR4 fails until you get another 5AR4..  You can get NOS for cheap on Ebay.
    You'll need to re-bias, but otherwise the amp will function normally.

    You might just decide to keep using the 5U4GB because it is a much more robust tube that is more reliable than the 5AR4.
    The only drawback is that you will have a slightly lower B+. Unless you run the system at window rattling levels, you will likely not notice a difference.


    Last edited by Tubes4ever on Sat Sep 02, 2017 9:32 pm; edited 1 time in total (Reason for editing : More information)

    Dogstar

    Posts : 264
    Join date : 2014-06-23

    Re: New Member, New ST120!

    Post by Dogstar on Sat Sep 02, 2017 8:17 pm

    I've never really thought about tube amps being like the women in my life but i can see the similarities now that deepee put everything into the proper perspective.

    And its true. When you are giving them 100% of your undivided attention Rectifiers can get really hot and sensuous...and when you take your eyes (and ears) off of them to focus on something they get all sparky and flare up and go all evil on you.

    jsl1234

    Posts : 4
    Join date : 2017-08-28

    Re: New Member, New ST120!

    Post by jsl1234 on Sun Sep 03, 2017 5:52 am

    Tubes4ever wrote:jsl1234, One thing you need to know about the 5AR4 tube is that once it starts arcing for whatever reason, it will most likely do it again soon.  Arcing damages the tube immediately. So, whether it was due to faulty wiring or just age, it needs to be replaced.  When it arcs, it will blow the fuse.

    You should keep a 5U4GB tube around for when the 5AR4 fails until you get another 5AR4..  You can get NOS for cheap on Ebay.
    You'll need to re-bias, but otherwise the amp will function normally.

    You might just decide to keep using the 5U4GB because it is a much more robust tube that is more reliable than the 5AR4.
    The only drawback is that you will have a slightly lower B+.  Unless you run the system at window rattling levels, you will likely not notice a difference.

    Thanks for the tip. I had already run out and purchased 2 spare JJ GZ34 / 5AR4 at a local music shop here in Richmond knowing I was going to need them Very Happy I figure I'll run through those for however long they last (hopefully a while).

    What do you think of the solid-state cooper cap? It seems like a much more reliable alternative altogether (i did not howver install the circuit delay option so not sure I can through in the SS unit without the modification.)

    jsl1234

    Posts : 4
    Join date : 2017-08-28

    Re: New Member, New ST120!

    Post by jsl1234 on Sun Sep 03, 2017 8:24 am

    Quick question to verify I have my facts right regarding needing a preamp (or not in this case).

    My only source to the amp is my Bryston DAC. According to the Byston spec the DAC outputs 2.3v unbalanced with 100 ohm impedance. As near as I can tell the input sensitivity of the amp is 1.4 volts to reach peak output (60 watts).

    So I guess adding a preamp will not effect the 'loudness' of the amp, only drive it to peak sooner (say 12 o'clock on the volume control vs maybe closer to 3 o'clock.....?)
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    Tubes4ever

    Posts : 96
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: New Member, New ST120!

    Post by Tubes4ever on Sun Sep 03, 2017 11:11 am

    jsl1234 wrote:What do you think of the solid-state cooper cap? It seems like a much more reliable alternative altogether (i did not howver install the circuit delay option so not sure I can through in the SS unit without the modification.)

    They are more reliable, but you lose the nice glow of the rectifier tube.  I prefer only tubes to be the active components on my tube amp. But others here use the Weber Copper Cap and are happy with it.

    You could also perform the "Yellow sheet mod" which is a sticky on the forum.  This involves soldering two diodes and moving two wires around on the rectifier socket.  This will greatly reduce the arcing risk with the 5AR4.  However, in this case the diodes are doing the actual rectification and the tube is simply acting to drop the B+ to its normal level.  I prefer my rectifier tubes to do the actual rectification, but that's just me.

    As for needing a preamp, you don't really need one as what you have puts out more than enough voltage to drive the amp to full output.

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