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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


5 posters

    Near Panic Time

    ggnarley
    ggnarley
    ggnarley


    Posts : 18
    Join date : 2018-05-11
    Location : Columbus, Ohio
    • Post n°1

    Near Panic Time Empty Near Panic Time

    Post by ggnarley Thu Oct 18, 2018 3:32 pm

    Short story: Doing music for our churches fall festival in 2 days (10/20). Was planning on using new vta-st70 as amp. Was playing this morning and went to do last bias and one output tube is dead. No bias reading, and not glowing.

    Amp is currently playing at lower volume levels and I can't even tell tube is out.

    Is this dangerous to keep playing this way?

    Would amp survive an evening at about half load?

    Any emergency source of output tubes?

    Do I need 4 matching or can I get just 2?



    Thanks
    Paul
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    ggnarley
    ggnarley
    ggnarley


    Posts : 18
    Join date : 2018-05-11
    Location : Columbus, Ohio
    • Post n°2

    Near Panic Time Empty may have found 2

    Post by ggnarley Thu Oct 18, 2018 3:41 pm

    Local quitar center has 2 EL34's which is what I have so going to get them now.
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    Peter W.
    Peter W.
    Peter W.


    Posts : 1351
    Join date : 2016-08-07
    Location : Melrose Park, PA
    • Post n°3

    Near Panic Time Empty Re: Near Panic Time

    Post by Peter W. Thu Oct 18, 2018 3:52 pm

    Please note the interpolations.

    ggnarley wrote:Short story: Doing music for our churches fall festival in 2 days (10/20).  Was planning on using new vta-st70 as amp.  Was playing this morning and went to do last bias and one output tube is dead.  No bias reading, and not glowing.  

    You have double-checked the wiring and have used a VOM across the filament pins of the tube? These are pins 2 & 7.

    https://tdsl.duncanamps.com/show.php?des=EL34   Have you switched tubes to prove that the socket is not bad?  

    Amp is currently playing at lower volume levels and I can't even tell tube is out.

    Is this dangerous to keep playing this way?  

    Were this a vintage 70, and at a low volume, and if the bias were correct, the short answer would be NO. I am not sufficiently familiar with the VTA to opine with certainty.

    Would amp survive an evening at about half load?

    As above, but YES instead.

    Any emergency source of output tubes?

    Amp store? Hifi store?

    Do I need 4 matching or can I get just 2?

    Two will be fine - after verifying that it is not an amp issue as noted above.



    Thanks
    Paul
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    ggnarley
    ggnarley
    ggnarley


    Posts : 18
    Join date : 2018-05-11
    Location : Columbus, Ohio
    • Post n°4

    Near Panic Time Empty May be fixed

    Post by ggnarley Thu Oct 18, 2018 5:52 pm

    Peter,

    Thanks for the quick reply. In such a rush I didn't even think of swapping tubes. I swapped the two tubes on the suspect side, and this time the bad one came back on, but the bias was way high in suspect tube, and way low in the other one. Shut system back down and let cool.

    Dialed bias down and plugged in new EL34's and system is back to working now. Both biases are close to one another so I bought all biases to a bit under target. Will let it run for a few hours and keep checking bias, but looks good so far.

    All tubes are EL34, but not same brand, but that is fine for now. Just need to be making music come Saturday. Will be reaching out to Jim McShane to get matched set soon.

    Thanks again for the help
    Paul

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    mazeeff
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    mazeeff


    Posts : 155
    Join date : 2014-01-06
    Age : 69
    Location : Grand Rapids, Michigan, USA
    • Post n°5

    Near Panic Time Empty Re: Near Panic Time

    Post by mazeeff Fri Oct 19, 2018 2:58 am

    Having done a few church festivals, I can offer one suggestion. Bring a backup solid state stereo amp with you. The VTA-70 is not the type of amp that would survive a lot of gigging! There are a ton of things that can go wrong (Murphys law). Blowing a tube, rectifier, resistor, etc. might ruin the show. If the gig is important, bring a backup!

    Mike
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    wildiowa
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    wildiowa


    Posts : 237
    Join date : 2012-03-19
    • Post n°6

    Near Panic Time Empty Re: Near Panic Time

    Post by wildiowa Fri Oct 19, 2018 9:32 am

    Have a since deceased friend who decided to try DJ stuff and at one of his first gigs ran music through two EV 3-ways and an original ST70 he found for $5 at a thrift shop, trying to cover several hundred kids in a ballroom....although the EVs were quite efficient and the ST70 cranked like a mutha, he was a bit naive about new DJ set ups and Class D power. 40 years ago I also used to carry around a Citation II running two Voice of the Theatres for our band PA. Incredibly dependable given we were hauling them around for live gigs from place to place. Only weighed 75 lbs. Only failure was when a Solo cup of Coke and ice vibrated off my amp onto the CIT II and four cooking 6550's...biggest tube amp fireworks I have ever witnessed. My friend had it up and running again the next day. Of course now I would never ever try this...tube amps are best left safe at home. Get a sleazy SS amp for your wing ding!
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    ggnarley
    ggnarley
    ggnarley


    Posts : 18
    Join date : 2018-05-11
    Location : Columbus, Ohio
    • Post n°7

    Near Panic Time Empty Good Advice

    Post by ggnarley Sat Oct 20, 2018 9:06 am

    Good advice from all.

    I am a firm believer in Murphy.  Will see if my 35 year old NAD will fire up and use it for backup 1.  My first Class-D build will be backup 2.   Will be running source from PC,  and if that goes down will shift to Tidal account on phone.

    Wildiowa - The EV/ST-70 beats my first setup by a mile.  Last couple years I have used a pair of the inefficient 3" Bose cubes and their chuffing subwoofer (driven by 20 watts) as the gym sound system.  It made noise and people danced.  

    This time, I am mating the VTA-ST70 to a pair of 15 " 4pi speakers I just finshed.  (98db, 600w, pro style).  Should be quite a bump up.

    The gym is only a mile away, so the amp will ride in my lap and be moved with great care.  By next year I hope to have a SS amp for travel, but for now the VTA is it.


    We are in Buckeye country, so if everything goes south we will just move the TV to the gym and watch the game.Wink

    Thanks
    Paul
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    peterh
    peterh
    peterh


    Posts : 1832
    Join date : 2012-12-25
    Location : gothenburg, sweden
    • Post n°8

    Near Panic Time Empty Re: Near Panic Time

    Post by peterh Sat Oct 20, 2018 9:36 am

    The "panic" schenario is a good reason to have a quad of tubes on a shelf ( and do not use them, keep then unused just like they come from the dealer) Also a GZ34 kept at the same place, and som fuzes .
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    ggnarley
    ggnarley
    ggnarley


    Posts : 18
    Join date : 2018-05-11
    Location : Columbus, Ohio
    • Post n°9

    Near Panic Time Empty Re: Near Panic Time

    Post by ggnarley Sun Oct 21, 2018 10:35 am

    Everything turned out well.  VTA ST-70 held up like a champ and had more than enough power to fill the gym.  The early soft dinner music (Nat King Cole, Frank Sinatra, Etta James) was really sweet sounding and set the mood.  Later pop/dance music had plenty of thump and got people dancing.  

    Will try to round up a suitable SS beater amp for next go around though.  I feel like I will be tempting fate by traveling with the VTA too much.  


    https://ibb.co/dA6VML

    Thanks for all the help.
    Paul
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    peterh
    peterh
    peterh


    Posts : 1832
    Join date : 2012-12-25
    Location : gothenburg, sweden
    • Post n°10

    Near Panic Time Empty Re: Near Panic Time

    Post by peterh Sun Oct 21, 2018 2:26 pm

    ggnarley wrote:Everything turned out well.  VTA ST-70 held up like a champ and had more than enough power to fill the gym.  The early soft dinner music (Nat King Cole, Frank Sinatra, Etta James) was really sweet sounding and set the mood.  Later pop/dance music had plenty of thump and got people dancing.  

    Will try to round up a suitable SS beater amp for next go around though.  I feel like I will be tempting fate by traveling with the VTA too much.  


    https://ibb.co/dA6VML

    Thanks for all the help.
    Paul
    That is a good observation, traveling with a "bare bones" tubeamp has some risks.
    If one want's to use a mobile tubeamp, mounting in a protecting case with all cables connected
    outside is good.
    And a decent sand amp is less sensitive to vibrations and drops.
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    wildiowa
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    wildiowa


    Posts : 237
    Join date : 2012-03-19
    • Post n°11

    Near Panic Time Empty Re: Near Panic Time

    Post by wildiowa Sun Oct 21, 2018 5:42 pm

    Success! Nothing kicks butt like tubes for a live gig...I used Mark IIIs for our PA many years in the 60's along with the Cit II. Unfortunately, for convenience I drilled out the phono inputs and installed 1/4" phone jacks...and added two phone jack outputs to the back of the chassis...pretty much ruined the vintage chassis, much to the disappointment and amazement of the purists...but they were working road amps and quick set up took precedence. Quarter inch phone jacks were the musical standard for input and speaker cable. Who knew 40 years later we would be cuddling them lovingly.
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    Peter W.
    Peter W.
    Peter W.


    Posts : 1351
    Join date : 2016-08-07
    Location : Melrose Park, PA
    • Post n°12

    Near Panic Time Empty Re: Near Panic Time

    Post by Peter W. Mon Oct 22, 2018 8:56 am

    All this reminds me of a time my wife and I took a train excursion powered by a steam locomotive. As we were traveling in main-line tracks, there had to be a diesel locomotive available as back-up. At the end of the trip, we came to find out that the diesel had broken down and the steam locomotive was pulling the entire load.
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