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    ST-120 First a buzz, then a tube light show....and then dead.

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    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 26, 2014 9:26 am

    Dogstar wrote:Now I'm concerned...despite installing new tubes and a Weber rectifier and not having any hiss tonight I turned on the amp and without a source playing hiss was clearly audible. Once the amp warms up the hiss goes away. I do have a volume control on the amp so tomorrow I will run a source directly to the amp and bypass my preamp. If there s no hiss then it may be time to replace the tubes in the preamp too. It takes 4 12AU7's so that would be a good place for the RCA's. If the preamp isn't the source of the hiss then what should I check next?

    disconnect the RCA cables to the amp and then SHORT the inputs of the amp, use whatever means you have to short the ground, ie outside of both the amps RCA sockets with the inside, turn the amp back on and see if you can hear the hiss. If no, the problem is external, if yes, its still the amp. This is a really quick way to determine where that noise source is.
    Also, can you take a nice, well lit & sharp photo of the inside of your amp and post it here, this will give us a chance to see if there is anything obvious in the wiring etc..

    Dogstar

    Posts : 91
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 26, 2014 12:06 pm

    As I said this was a Latino built amp....the wiring looks impeccable to me...but I will perform the test you suggest and take photos within the next day or two. In the meantime I'll suffer with my vintage Yamaha CA-2010 integrated amp. It doesn't sound as good as the VTA-120 but for SS it sounds OK.

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 26, 2014 12:10 pm

    Dogstar wrote:As I said this was a Latino built amp....the wiring looks impeccable to me...but I will perform the test you suggest and take photos within the next day or two. In the meantime I'll suffer with my vintage Yamaha CA-2010 integrated amp. It doesn't sound as good as the VTA-120 but for SS it sounds OK.

    yes, really need to establish for once and for all if it is the amp or preamp, then we can go further

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by sKiZo on Tue Oct 28, 2014 3:59 pm

    Begs the question ... what do you consider "warm up"?

    Takes around five minutes to stabilize properly, and even then you can hear the amp "bloom" a bit as you listen. I turn mine on, then wander off for a bit. Lap around the yard, cuppa coffee, check the web, or various other odd and sundry tasks you probably don't want to know about ...

    Also ... hiss? It's common to have a bit of feedback with zero signal ... I'd suspect the preamp there, as the circuits usually need something to lock on. I've got a dbx BoomBox that does that on idle, but it goes dead quiet during playback. That's getting a bit long in the tooth, and I do have plans (and parts!) to do a full rebuild soon.

    Maybe a ground issue too. Try running a wire from the amp chassis to a common system ground ... should be a stud somewhere for the turntable anyway. Easy test there would be to touch metal on both the amp and pre at the same time - you should notice a difference if it's a ground loop. You'll DEFINITELY notice if it's a ground fault ... zAP!

    Dogstar

    Posts : 91
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Fri Oct 31, 2014 11:19 pm

    So could the hiss and the tube going supernova be unrelated issues? My plan is to open the amp up Sunday to look under the for cold solder joints or anything else that's been suggested but I'm kinda curious about the hiss as Skizo suggests.

    I've experienced a bad hum from a ground loop that was cured with a ground wire but this is hiss not hum. Could hiss be a ground loop sign also?

    Dogstar

    Posts : 91
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Fri Oct 31, 2014 11:35 pm

    I should have checked my preamp as Skizo suggested....sure enough....since I have the gain control on the amp I plugged my CD player directly into the amp and there was no hiss at all. Dead silent. Looks like I need to spring for 4 more 12AU7's for the preamp.

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    Re: ST-120 First a buzz, then a tube light show....and then dead.

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