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    I made a tube goof.

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    Sprags

    Posts : 123
    Join date : 2013-02-27

    I made a tube goof.

    Post by Sprags on Thu Apr 18, 2013 1:56 am

    I was testing out my new tube amp and preamp at a friends warehouse. He buys and sells lots of stuff. Anyway I brought my tube and preamp there and and after testing he let me borrow a Harmon Karden solid stat preamp. Only problem is that when I packed up and got home I realized I left the PCB mounted 5814A tubes at his warehouse. He's not going to be back in town until next Monday so without those tubes I'm out of luck for doing any testing. I did however see there are a few shops in town that sell tubes and I figured it wouldn't be so bad to have some extras on hand. The two tube shops didn't list the 5814A tube as being in stock. He does have 12AU7's, 12AX7's and 12AY7's in stock. Can these tubes be used I'm place of the 5814a tubes which I'm pretty sure ill get back in the mean time?

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: I made a tube goof.

    Post by peterh on Thu Apr 18, 2013 2:00 am

    Get yourself a coupel of 12 au7. They will most likely be better!

    from www.tubeworld.com/5814a.htm
    (the 5814A is equivalent to a 12AU7WA, it is ruggedized and redesigned; the filament current
    raised to 175ma per section versus 150ma per section for a 12AU7. 5814A is computer rated, 5814WA is ruggedized)

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: I made a tube goof.

    Post by Sprags on Thu Apr 18, 2013 2:20 am

    I'm going to go to a shop in town that sells them si can continue my testing without too much of a delay. Thanks,,,,Blake

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: I made a tube goof.

    Post by Bob Latino on Thu Apr 18, 2013 10:45 am

    Hi Blake,

    The USA made Sylvania 5814A tubes included with your amp are probably better driver tubes than any currently made 12AU7 tube. I have been including these USA made driver tubes with all the VTA ST-70 and ST-120 kit and wired amps while I have them in stock. You CAN substitute any 12AU7, 12BH7, 5963, 6189 or CV4003 tube in your VTA ST-120 amp. Do not use 12AT7 or 12AX7 as driver tubes in YOUR amp. Both the 12AT7 and 12AX7 have too much gain for your amp.

    Bob

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: I made a tube goof.

    Post by peterh on Thu Apr 18, 2013 12:00 pm

    5963 does not sound good. In fact i think it's more useful in oldtime computers.
    I have 3 here that i ditched and replaced with jj ecc82 and it made life much better!

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: I made a tube goof.

    Post by Sprags on Thu Apr 18, 2013 12:54 pm

    Well...the good news is that the guy with the warehouse says he's pretty sure they are there since he closed up as soon i left. Unfortunatley he won't back in town until Monday.

    There is a big franchise store (Parts Express) in town that does have JJ ECC82 / 12AU7 Vacuum Tubes in stock. So even though they may not sound as good as the 5814A's I'll get three of them to use for testing purposes and I guess it wouldn't hurt to have a few spares on hand.

    sKiZo

    Posts : 1308
    Join date : 2013-04-01
    Location : Michigan USA

    Re: I made a tube goof.

    Post by sKiZo on Thu Apr 18, 2013 3:32 pm

    Bob din't say nutting about having to re-bias after replacing drivers, so I guess that's not necessary? Good idea anyway?

    I'd think "hot swapping" is out too ... tongue

    I've heard pros and cons about the JJ tubes ... then again, it's all in the ear of the beholder. I'd always seen trash talk about US tubes too. Recently got a grab bag of 5670's for my tube dac, and there was a Westinghouse in the bunch. Westinghouse?? Bah!

    Turns out, that's now gone to the top of the rotation, right next to a 1950's WE JW2C51. Go figure. Looks to be NOS, and a triple mica with a D getter no less. Westinghouse!

    So for those JJ's ... Try em ... you may like em!

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: I made a tube goof.

    Post by Bob Latino on Thu Apr 18, 2013 3:58 pm

    In general, the bias may change very slightly but not really significantly when you change driver tubes. When you change either the rectifier tube or the output tubes the bias will change more significantly. You should rebias the amp after any rectifier or output tube change.

    Bob

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: I made a tube goof.

    Post by Sprags on Fri Apr 19, 2013 1:17 am

    Unfortunately Parts Express only had the 12AU7's in stock. I could have ordered the 12AU7WA's but I didn't want to wait. Despite the place being crowded and busy the sales tech was helpful and offered suggestions for some questions I had. Their prices were far better than Radio Shack or the other electronic supermarkets. They had everything there primarily gear for musicians, or pro audio systems and studio setups but they also had home audio equipment including some nice looking speaker kits including ones with the cabinets assembled and finished in high quality looking veneers. I didn't hear them but they were far better finished that the ones you see in Best Buy but reviews claim they sound very good.

    Anyway...getting back to my story...

    I figured being safe than sorry is better than the alternative. And I'm glad I checked. The two left tubes were at about 505 to 525 mV and the right ones were high ...they were somewhere between 580 and 595 when I first started checking after a minute.

    Of course the one thing I noticed is that the variance stabilized as the tubes warmed up. So when they were warmed up after 5 minutes they were all at 550 +/-2 mV.

    I went back and forth maybe 10 times checking and making minor tweaks. Probably because I'm a tube newbie and electronically challenged...I just shelled out a fairly large expense for the amp ( for me ) and I just want check and take care of it.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: I made a tube goof.

    Post by Bob Latino on Fri Apr 19, 2013 7:29 am

    Sprags wrote:Unfortunately Parts Express only had the 12AU7's in stock. I could have ordered the 12AU7WA's but I didn't want to wait. Despite the place being crowded and busy the sales tech was helpful and offered suggestions for some questions I had. Their prices were far better than Radio Shack or the other electronic supermarkets. They had everything there primarily gear for musicians, or pro audio systems and studio setups but they also had home audio equipment including some nice looking speaker kits including ones with the cabinets assembled and finished in high quality looking veneers. I didn't hear them but they were far better finished that the ones you see in Best Buy but reviews claim they sound very good.

    Anyway...getting back to my story...

    I figured being safe than sorry is better than the alternative. And I'm glad I checked. The two left tubes were at about 505 to 525 mV and the right ones were high ...they were somewhere between 580 and 595 when I first started checking after a minute.

    Of course the one thing I noticed is that the variance stabilized as the tubes warmed up. So when they were warmed up after 5 minutes they were all at 550 +/-2 mV.

    I went back and forth maybe 10 times checking and making minor tweaks. Probably because I'm a tube newbie and electronically challenged...I just shelled out a fairly large expense for the amp ( for me ) and I just want check and take care of it.

    It is not necessary to get the bias as fine as a 2 millivolt variation from tube to tube on any VTA amp. You don't have to get the bias that "perfect". If you get all the VTA ST-120 output tubes biased between .540 and .560 VDC - that is fine enough. The bias varys with line voltage. During the day in MANY areas of the USA the line voltage will vary UP and DOWN. When that happens your bias voltage will go up and down also. These small bias fluctuations have no bearing on the sound of the amp.

    When the amp is new and the output tubes are new - you may want to check the bias once a week and make minor corrections. Later when the amp is broken in, check it once a month and make minor corrections.

    Bob

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: I made a tube goof.

    Post by Sprags on Fri Apr 19, 2013 9:32 am

    OK...thank you.
    Blake

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