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    Everything working great, then...

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    SnugDynaco

    Posts : 19
    Join date : 2011-09-11

    Everything working great, then...

    Post by SnugDynaco on Wed May 27, 2015 12:43 pm

    My [Latino kit] ST70 is about a year old; my [Ebay] PAT-4 about two years.  Everything was great until a few days ago.  Power on.  All "on" indicator lights are lit; all of the ST70 tubes lit and glowing properly.  Absolutely no sound, not even the very very faint signal from the Heresy IIs.  No changes from the days and weeks before - system cart not moved, and no one has messed with the system.  All connections from/to components and speakers checked, and all fuses intact.  No lightning storms nor recent neighborhood power disruptions.  Two months ago I added a Marantz 6005 CD/DAC player, but, again, all working fine up until recently.

    Some help from other members would be greatly appreciated.  I have a Bottlehead Foreplay preamp that I could substitute for the PAT4.  Other than that, I don't know where else to start looking for the solution.

    Many thanks in advance.

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    Re: Everything working great, then...

    Post by ArlanB on Wed May 27, 2015 1:03 pm

    My firs thought is check your rectifier.  Just  because the heaters are working does not mean that you have high voltage.
    Swap out for a new rectifier. I know this can happen because about two months ago I was listening to vinyl and suddenly the amp went quiet just like someone had shut if off, however the heaters were still glowing bright red but no sound.  Chaned for a new rectifier and "Viola" back in business.

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Everything working great, then...

    Post by sKiZo on Wed May 27, 2015 2:32 pm

    You could try swapping the pre, but I'd think no hiss from the speakers would indicate an amp problem. Rectifier is also first place I'd look as that's common to both channels.

    DO post back if that helps or doesn't. Always nice to be able to see solutions as well as the problems.

    (Bonus points if you have a spare rectifier on hand.) geek

    Bob Latino
    Admin

    Posts : 2376
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Everything working great, then...

    Post by Bob Latino on Wed May 27, 2015 2:43 pm

    More than likely, as Arlan has mentioned, the rectifier is bad. Just because the rectifier lights up is not an indication that it is good. A no sound AND a no bias situation (you get no bias voltage at any of the four bias measuring points) is almost always a bad rectifier tube. Replace the rectifier tube and you should be back in business ...

    Bob

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Everything working great, then...

    Post by GP49 on Wed May 27, 2015 6:24 pm

    "Dead" giveaway that there is no B+: tubes that aren't hot!

    SnugDynaco

    Posts : 19
    Join date : 2011-09-11

    Re: Everything working great, then...

    Post by SnugDynaco on Thu May 28, 2015 4:28 pm

    I think you hit it on the head - no bias. And, me being me, no spare rectifier on hand. I'll be buying at least two.

    The first lasted only a year. Is this typical? I believe I have correctly biased the amp, checking it when moved to our new home and every 45-60 days. Amp gets a lot of use - not happy with the headphones and iPad.

    The kit came with the Sovtek rectifier. Other brands recommended?

    Thank you for your replies. The forum is very worthwhile.

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Everything working great, then...

    Post by sKiZo on Thu May 28, 2015 5:14 pm

    Hearing good things about the new production TungSol 5AR4 ... Genelex is also getting good reviews.


    nmchiefsfan

    Posts : 33
    Join date : 2012-03-21

    Re: Everything working great, then...

    Post by nmchiefsfan on Thu May 28, 2015 10:51 pm

    Have you checked the bias?

    SnugDynaco

    Posts : 19
    Join date : 2011-09-11

    Re: Everything working great, then...

    Post by SnugDynaco on Mon Jun 01, 2015 11:54 am

    New Sovtek rectifier, and a backup, arrived today, and I am back in business.

    At initial startup, bias range was down slightly to .380-.385. I assume I should let it run for a few hours, then check and adjust the bias to .400?

    Very quiet the last few days around here - a slight amount of withdrawal. I have a set of Grado cans and iTunes, but it just is not the same as the great ST-70.

    Thanks again all, for your help.

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Everything working great, then...

    Post by sKiZo on Mon Jun 01, 2015 12:04 pm

    Every tube will bias up a little differently ... With a new tube, I set it to spec once it's warm, then again after playing it for a bit, then let it idle a while and check it again. You may see some small variation, but it should stabilize after a few hours at most. After that, the occasional spot check is always a good thing.



    Bob Latino
    Admin

    Posts : 2376
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Everything working great, then...

    Post by Bob Latino on Mon Jun 01, 2015 12:40 pm

    sKiZo wrote:Every tube will bias up a little differently ... With a new tube, I set it to spec once it's warm, then again after playing it for a bit, then let it idle a while and check it again. You may see some small variation, but it should stabilize after a few hours at most. After that, the occasional spot check is always a good thing.


    Nearly always the bias will rise slightly as the amp warms up and then stabilize after 30 to 45 minutes. If your target bias point for a VTA ST-70 is .400 volts DC per each output tube, then you should set the bias a little low on the initial bias adjustment when the amp is cold - maybe .375 to .385 VDC. After the 30 to 45 minutes reset the bias on each output tube to the .400 VDC. NOTE - you do not have to get the bias "perfect". As long as all four output tubes bias between .390 and .410 on an ST-70, you are fine.

    Also realize that the bias will vary ..
    1. As your line voltage goes up and down. If your line voltage rises, the bias on all four output tubes will rise slightly. About 1 week ago the bias dropped quite a bit on both of my M-125's here. I checked the line voltage. It was 113.7 VAC What?? I called the power company and told them to get up here and find what was wrong. They found a bad distribution transformer and had to change it out. I am now at 120.2 VAC as I write this.
    2. If you change output tubes. EL34's wont bias at the same point as a KT66 in an ST-70.
    3. As the output tubes wear a little the bias will change. Check your bias about once a month (after a 30 to 45 minute warm up) and make adjustments if necessary.
    4. If you change the rectifier. Even if you go from one 5AR4 to another 5AR4 (even of the same brand) the bias might be slightly different.

    When you set the bias, you are setting the idle current on your amp. An analogy to bias current is like a team of four horses on an old stagecoach being held in rein by the driver of the team. The four horses are your four output tubes. The driver pulling back a little and holding the reins is the like the "bias system" of the stagecoach. He is "keeping control" of those four horses. If he falls off and loses control, then the horses run wild with no control. In a tube amp you will see the output tube's plates turn red as excessive current is drawn. Many times the red plated tube will cause a short. Nearly always the fuse will blow and save the rest of the amp from further damage.

    Bob

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