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    [ST-70] Noise when heating up

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    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    [ST-70] Noise when heating up

    Post by Nicola on Mon Nov 21, 2011 8:27 am

    First of all: hi everybody! This is my first post, so I think I should spend some words to present myself. Smile

    I'm Nicola from Italy. More than 40 years ago my grampa bought a fully equipped stereo system with radio, record and audio reel players and (above all!) a Dynakit Stereo 70 with a PAS-2 pre-amplifier and two "Esquire" speakers from ElectroVoice.

    About ten years ago I retrieved speakers, ST-70 and PAS-2 and started using them. I had no experience with electronic jobs, so I simply cleaned and powered them up. I used them for ten years.

    Last winter the ST-70 began to emit a 120Hz hum. I searched solutions on the internet and replaced the can capacitor. The new quad-cap is the only piece changed in over 40 years, everything else is completely original.

    I know that I could make a lot of improvements to my Dynakit but I'm not so expert with electronics and I believe that is better not to touch an old thing if it works!

    By the way, some days ago my ST-70 started to make a singolar short noise on the left channel only. Each time I switch it on, after 2 or 3 minutes I can hear this noise once. Apart from that, it works well.

    I've seen the quick troubleshooting guide on this forum, especially point 13: "One channel cuts out completely or drops in volume after time". I don't know if my problem is connected to that, so I recorded the noise from my ST-70 left channel to have your opinion.

    Noise:
    http://www.4shared.com/audio/D4PD0X6o/ST-70-Noise.html

    Waveform screenshot: img38.imageshack.us/img38/1279/st70noise.png

    The noise sounds like when you power up something for a few seconds, then you turn it down. So it has an initial peak and a final peak. Between these two peaks, there are 2-4 seconds of "almost" silence: normally the amplifier has a little operating noise. Between the peaks, the operating noise is a bit louder.

    In order to be sure that this noise depends only on the amplifier, I tested it with no input (rca disconnected) and with different speakers. On more recent speakers, peaks are less loud, but still exist.
    The only coupling of the ST-70 with other pieces is the power cable coming from the PAS-2 output.

    Thanks in advance for your opinion and for this forum, I'm new here but I feel this is a friendly community!

    Cheers
    Nicola


    Last edited by Nicola on Tue Dec 13, 2011 7:57 am; edited 3 times in total

    Greg_M

    Posts : 40
    Join date : 2010-09-18
    Location : Poulsbo, WA

    Re: [ST-70] Noise when heating up

    Post by Greg_M on Tue Nov 22, 2011 8:59 pm

    Try moving the tubes from the one side to the other and see if the noise follows. Mine did when I had that same thing happen. It was bad tubes

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Wed Nov 23, 2011 12:36 pm

    Thank you for your answer!
    I've changed tube sides following these steps:
    1) inverted the 7199 tubes - noise still present on the same channel
    2) inverted the first couple of EL34 - noise still present on the same channel
    3) inverted the second couple of EL34 - noise disappeared!

    After step 1 and 2, at step 3 I was pretty sure to get the noise on the other channel but... fortunately noise has completely disappeared.

    Unfortunately the 7199 tubes are becoming microphonic: hitting them generate a little noise hearable through the speakers. They also sometimes had generated some crackling noise, so I've moved them a little into the plugs and now I get no more hiss/crackle.
    I think I'll wait to replace them: microphonic problem seems to be not big actually, and I'm not so happy to spend a lot of money to get two of the few 7199 still available online, neither to replace them with 6GH8A...

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: [ST-70] Noise when heating up

    Post by Bob Latino on Wed Nov 23, 2011 12:49 pm

    Nicola,

    A 6GH8A is not a drop in replacement for a 7199 tube. The pin outs are different. To use a 6GH8A in place of a 7199 you have to either rewire the socket OR if you want an easier fix, buy two 7199 to 6GH8A adapters ($20 each).

    Bob


    Last edited by Bob Latino on Wed Nov 23, 2011 1:11 pm; edited 1 time in total

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Wed Nov 23, 2011 12:53 pm

    Infact, that was a problem that discouraged me to get 6GH8A!

    jhoak

    Posts : 46
    Join date : 2009-10-16
    Age : 58
    Location : Central Florida

    Re: [ST-70] Noise when heating up

    Post by jhoak on Wed Nov 23, 2011 2:03 pm

    Bob Latino wrote:Nicola,

    To use a 6GH8A in place of a 7199 you have to either rewire the socket OR if you want an easier fix, buy two 7199 to 6GH8A adapters ($20 each).

    Bob

    Better yet buy a VTA board and a set of CV4024s and take your ST-70 to the "next level".

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Wed Nov 23, 2011 2:34 pm

    Yes, that's another possibility but as I said in my first post: I would like to mantain my audio set as original as possible.
    Changing tubes, caps and the board would improve its sound, but remember I'm running it on a couple of vintage speakers from the 1960s that are possibly the bottleneck of my system...
    Anyway I think it sounds very well, probably better than some other completely-original-never-modded ST-70s in the world.
    I'm oriented to just resolve one by one the problems that could happen in the future Wink

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: [ST-70] Noise when heating up

    Post by Jim McShane on Wed Nov 23, 2011 6:14 pm

    Nicola wrote:Thank you for your answer!
    I've changed tube sides following these steps:
    1) inverted the 7199 tubes - noise still present on the same channel
    2) inverted the first couple of EL34 - noise still present on the same channel
    3) inverted the second couple of EL34 - noise disappeared!

    After step 1 and 2, at step 3 I was pretty sure to get the noise on the other channel but... fortunately noise has completely disappeared.

    Sometimes the act of removing and reinstalling a tube will improve a poor/dirty connection enough to eliminate a noise caused by poor contact. That may have been what happened in your case.

    Unfortunately the 7199 tubes are becoming microphonic: hitting them generate a little noise hearable through the speakers.

    Don't do that!! You may have a tube that wasn't microphonic before and make it microphonic by tapping on it while hot. As well, most tubes make some noise when struck - especially if they are in a high gain circuit.

    ALL tubes are microphonic to some degree. If the only time a tube makes noise is when you tap it it is not excessively microphonic.

    They also sometimes had generated some crackling noise, so I've moved them a little into the plugs and now I get no more hiss/crackle.

    Clean the pins and sockets!! Re-tension the output tube sockets if needed.

    I think I'll wait to replace them: microphonic problem seems to be not big actually, and I'm not so happy to spend a lot of money to get two of the few 7199 still available online, neither to replace them with 6GH8A...

    Clean all the tube pins and sockets - and stop rapping on the hot tubes - and I suspect your problem will disappear.

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Wed Nov 23, 2011 7:20 pm

    Jim McShane wrote:and stop rapping on the hot tubes
    Ahahah lol!
    Thank you for your accurate answers!
    I didn't touch tubes too much fortunately, I've just did a quick test and noticed that there's the "microphonic" phenomen in 2 of 7 tubes! And I also concluded that is a negligible problem, because tubes are not "so much microphonic" to capture interferences or other noises.

    I'll clean pins and sockets, that's sure! Any good idea to clean sockets, or just spray cleaner inside?

    Greg_M

    Posts : 40
    Join date : 2010-09-18
    Location : Poulsbo, WA

    Re: [ST-70] Noise when heating up

    Post by Greg_M on Wed Nov 23, 2011 8:11 pm

    You can spray Deoxit on the tube pins and then work them in and out of the sockets a few times

    Ciao

    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: [ST-70] Noise when heating up

    Post by Luddite on Thu Nov 24, 2011 12:22 am

    For the record, there is one instance where tapping on heated tubes is permitted, nay even a necessity. That is when you are performing a "short" test with a tube tester. When testing any suspicious tubes or tubes of unknown condition, a "short" test should be performed before proceeding with further testing. A positive indication of a short inevitably means a rejected tube. Proper testing requires tapping on the tube while switching between the elements. I use a pencil with a rubber pumpkin eraser on top (don't ask Wink ) for my "test instrument".

    Best Regards,
    Charlie

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Thu Nov 24, 2011 8:17 am

    Thanks everybody for your answers.

    I've cleaned all tubes' contacts. Unfortunately the noise explained in my first post is back Sad
    But as Greg_M said, now it is on the other channel (where I moved the supposed-bad EL34). I think I'll have to replace that tube.

    EDIT: no more noise now... That tube is kidding me, I'm sure!

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: [ST-70] Noise when heating up

    Post by Jim McShane on Thu Nov 24, 2011 1:34 pm

    Nicola wrote:Thanks everybody for your answers.

    I've cleaned all tubes' contacts. Unfortunately the noise explained in my first post is back Sad
    But as Greg_M said, now it is on the other channel (where I moved the supposed-bad EL34). I think I'll have to replace that tube.

    EDIT: no more noise now... That tube is kidding me, I'm sure!

    Nicola,

    It seems you have an intermittent issue - so replace the suspect tube.

    BTW, I use pipe cleaners and D5 DeOxit for octal socket cleaning. For 9 pin sockets you can use the tiny brushes they sell at the drug store for cleaning between teeth. Here is one brand: http://www.piksters.com/

    Good luck!

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Thu Nov 24, 2011 2:23 pm

    Thank you for cleaning suggestions! It's hard to find DeoxIT in Italy, so I've used the only product available in my local hardware store, the CRC 2-26.
    It did a good job on switches and tubes. Maybe DeoxIT action is more durable...

    Greg_M

    Posts : 40
    Join date : 2010-09-18
    Location : Poulsbo, WA

    Re: [ST-70] Noise when heating up

    Post by Greg_M on Thu Nov 24, 2011 10:23 pm

    Make sure it is "Safe For Plastic"

    Nicola

    Posts : 12
    Join date : 2011-11-20
    Location : Roma (Italy)

    Re: [ST-70] Noise when heating up

    Post by Nicola on Fri Nov 25, 2011 5:15 am

    It is! Thank you Wink
    The official product description states that:
    CRC 2-26® is a versatile, plastic safe, multi-purpose precision lubricant, penetrant and corrosion inhibitor. Its unique
    viscosity allows it to cover more surface area and penetrate deep into the surface of all metals, including steel,
    copper, brass and aluminum alloys. 2-26® displaces moisture and leaves a thin, long lasting film to protect against
    corrosion.

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