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    Need Help With Stereo 70

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    kygeezer

    Posts : 51
    Join date : 2012-11-30

    Need Help With Stereo 70

    Post by kygeezer on Mon Dec 10, 2012 7:21 pm

    I recently built a Welborne modified "Stereo 70" from scratch, using only two original Dynaco output transformers. Everything else is new, including a new, larger power transformer from Triode (unfortunately, at that time, I had not discovered this forum or dynakitparts.com). The amp sounds great, but I have two concerns: first, the left output transformer gets very warm (can hold my hand on it but does feel very warm) but the right one stays room temp, and second, the power transformer gets downright hot. I am wondering why the new, bigger power transformer is so hot, and if the left output transformer is defective. Do I have problems or is this normal? If so, and if I replace the one Output transformer with a new one, will there be an audible difference between the channels. I am anxious to hear from each of you. I have gained so much from this forum, I wish I had found it long ago.

    nmchiefsfan

    Posts : 33
    Join date : 2012-03-21

    Re: Need Help With Stereo 70

    Post by nmchiefsfan on Mon Dec 10, 2012 8:54 pm

    If it is laid out like a standard ST-70 the left transformer could be getting hotter due to the rectifier being on that side. I have a Latino VTA ST-120 and the power transformer gets pretty hot after it has been on for a couple hours but seems normalize at that point.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Need Help With Stereo 70

    Post by Bob Latino on Mon Dec 10, 2012 9:04 pm

    Hi,

    The left output transformer will always be warmer than the right due to the left transformer's proximity to the rectifier tube. It is more difficult for the left transformer to get rid of the heat because of another warm object nearby. The upgraded power transformer from Triode or Dynakitparts or our VTA ST-70 power transformer are all upgraded ST-70 power transformers with a higher stack. They all will run cooler than the original PA-060 power transformer which only had a 1 1/2 inch stack.

    If your amp is running hot even with an upgraded power transformer I would ask you ...

    1. Is your line voltage much above 120 VAC ?
    2. Is your bias set higher than normal?
    3. Does your amp have proper ventilation?
    4. Does the top of the amp "look at the ceiling" in your music room?
    5. Is it pushed too close to a back wall?
    6. Is there a problem with the quad cap/or underchassis quad cap replacement board if you have one of these ?

    Any of the above can cause an amp to run hotter than normal ...

    Bob

    kygeezer

    Posts : 51
    Join date : 2012-11-30

    Re: Need Help With Stereo 70

    Post by kygeezer on Mon Dec 10, 2012 9:37 pm

    At the moment, my amp is in the open, no cage and at least 18" from any wall, cabinet, etc. I hadn't thought about the rectifier tube, but it does make sense. The power transformer is hot but I can put my hand on it for several seconds. I just can't remember how hot the original was. It's been around 35+ years since I had an original Stereo 70. Well, I feel better about the situation. I now have the Tubes4HiFi PAS mod installed and it is amazing. I am really enjoying my music collection and hearing subtleties I didn't know were on the tape/cd/LP. Actually, the bias is set to 1.40 instead of the recommended 1.56v and this mod has all the caps on the bottom of the circuit board. Again, thanks for all the help.

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Need Help With Stereo 70

    Post by tubes4hifi on Mon Dec 10, 2012 10:06 pm

    Bob, time to repost your great photo of all the temps on the amp taken with your laser temp probe!!

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Need Help With Stereo 70

    Post by Bob Latino on Mon Dec 10, 2012 11:34 pm

    tubes4hifi wrote:Bob, time to repost your great photo of all the temps on the amp taken with your laser temp probe!!

    Ok - Roy ... A repost of this photo. I wanted to make sure that the VTA ST-120 did not overheat using the new (at the time) Tung-Sol KT120 output tubes. Below is the photo Roy is referring to .. I actually forgot to take a temperature reading off the Weber WZ68 Copper cap ? It was pretty warm though. Weber uses that copper tube to dissipate heat. Notice also how much cooler the plates are on the front KT120 output tubes. The front KT120's are about 100 degrees cooler than the rear tubes. The laser thermometer shown is a Cason CA380 that I got on Ebay for about $25. It's not that expensive and it works well. You just point the Cason at the item you are measuring the temperature of and a red dot appears on the object and the temperature is read out digitally on an LCD screen above the pistol grip. Just don't point it at the wife like I did one day. She threw her hands up and remarked "What the hell are you doing?" My answer > "Just using this laser thermometer to see if you are hot stuff?" The joke did not go over well ... Seriously, don't point it in someone's eyes but you can get a readout of someone's skin temperature ..

    Bob



    Pillo69

    Posts : 104
    Join date : 2012-04-11
    Location : Granada (España)

    Re: Need Help With Stereo 70

    Post by Pillo69 on Tue Dec 11, 2012 1:08 pm

    I love this forum as Dynaco amplifiers are discussed.
    Bob, congratulations on the photograph of temperatures.
    A greeting.

    kygeezer

    Posts : 51
    Join date : 2012-11-30

    St 70

    Post by kygeezer on Tue Dec 11, 2012 1:42 pm

    Bob-
    Thanks for posting the temperatures - I thnk my amp is within normal temps. Again, Roy, pat yourself on the back for the PAS mods. The boards were a snap to populate and I now have a new, great sounding preamp. I even used Teflon silver tinned stranded hookup wire. Sure wish they had that back when I did a lot of kit building. Thank you to all of you!

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