I am looking for repair of my amp I live in north Georgia. Thank You , Vincent.
5 posters
ST70 blows 5 amp fuse
vincentdsnt- Posts : 6
Join date : 2012-03-02
Dale Stevens- Posts : 201
Join date : 2014-07-06
Age : 75
Location : Loris, SC
Vincent, look at the "Quick troubleshoot guide" on home page. (3rd from bottom) sticky and
then get back with us. Dale
then get back with us. Dale
Big Harry- Posts : 51
Join date : 2016-06-19
Location : Kingman, Arizona
To begin with, an ST70 should be using a 3 amp fuse and not a 5 amp. Three things that can cause a fuse to fail are a shorted power transformer, shorted filter capacitors or a shorted rectifier tube. You can try another preferably 3 amp fuse but remove the 5AR4 rectifier tube before powering up the amp. If the fuse blows, the power transformer is likely the problem. If the fuse doesn't fail, then either the 5AR4 is bad or the main filter capacitor has shorted. There are other things like a shorted EL34 that can cause a fuse to fail but I would think that isn't your problem.
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Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
Big Harry wrote:To begin with, an ST70 should be using a 3 amp fuse and not a 5 amp. Three things that can cause a fuse to fail are a shorted power transformer, shorted filter capacitors or a shorted rectifier tube. You can try another preferably 3 amp fuse but remove the 5AR4 rectifier tube before powering up the amp. If the fuse blows, the power transformer is likely the problem. If the fuse doesn't fail, then either the 5AR4 is bad or the main filter capacitor has shorted. There are other things like a shorted EL34 that can cause a fuse to fail but I would think that isn't your problem.
Yes - Use a 3 amp fuse on any ST-70 type amp and NOT a 5 amp fuse. Also, make sure that fuse is a SLO-BLO fuse. If you use a fast blow fuse, the surge of current in the first 2 or 3 seconds on an ST-70 type amp may take out a fast blow fuse. A SLO-BLO fuse is made to take the initial surge of current and NOT blow.
Bob
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Solder Slinger- Posts : 36
Join date : 2009-05-24
- Post n°5
Agree with the others, 5 Amp is way to large a fuse
by Solder Slinger Fri Apr 21, 2023 1:28 pm
Agree with the previous posters, Dynaco specified a 3 amp fuse and that is what you should use. Personally, I add a CL-80 NTC for slow(er) turn on between the fuse and the power switch As suggested earlier, remove the rectifier tube and see if that resolves the problem using a 3 amp fuse, in this case, for testing, it can be either a Slo-Blo or regular fuse. If it doesn't, you'll need a new power transformer, if it does resolve the fuse blowing, leave it alone until someone who has the proper testing tools can look at both the rectifier and the capacitor. You could damage the amp even more.
I may be able to help, I live in North Georgia and have repaired and modified Dynaco amps for years and have the tools to diagnose the problem. In what town exactly do you live? I'm in Blairsville.
-Ed
I may be able to help, I live in North Georgia and have repaired and modified Dynaco amps for years and have the tools to diagnose the problem. In what town exactly do you live? I'm in Blairsville.
-Ed
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Big Harry- Posts : 51
Join date : 2016-06-19
Location : Kingman, Arizona
I agree with Solder Slinger, I've installed CL 80's in both of my ST70's and my Mark 3's. The CL80 will reduce the inrush current which I believe will increase the life of the tubes. I also agree that if the fuse doesn't blow with the 5AR4 removed, there are other problems in the amp which if you are not comfortable with working on tube equipment, it should be left to someone more experienced. I would certainly be happy to look at your amp if you wanted to ship it to Arizona. Harry
Solder Slinger- Posts : 36
Join date : 2009-05-24
Big Harry,
The book, "Getting the most out of Vacuum Tubes" by Robert B Tomer ( who was well known in the 1950s ) states that an NTC will increase the life span of the tube heater by a factor of 4 ( starting on P. 12 ). This is because the heater comes on "gently" rather than getting slammed on. The book also states there are other positives but I don't remember what they were.
I'll take a 4:1 increase in heater life, I've had more than a few tubes die from open heaters.
The book, "Getting the most out of Vacuum Tubes" by Robert B Tomer ( who was well known in the 1950s ) states that an NTC will increase the life span of the tube heater by a factor of 4 ( starting on P. 12 ). This is because the heater comes on "gently" rather than getting slammed on. The book also states there are other positives but I don't remember what they were.
I'll take a 4:1 increase in heater life, I've had more than a few tubes die from open heaters.
Big Harry likes this post
Big Harry- Posts : 51
Join date : 2016-06-19
Location : Kingman, Arizona
Solder Slinger, I have either a CL80 or CL90 thermistor installed in most of my tube equipped gear and that includes my audio, antique radios and tube equipped test equipment. I can't remember when I have had a tube fail because of heater burnout. I'm also going to look for Robert Tomer's book as I believe it would be a good read. Thanks for the tip.
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