I now see that I'm not the only one that has experienced problems with the ST-120 when it comes to rectifier tubes not lasting very long. So I was wondering if those of you that have experienced this issue could tell me which kind of rectifier tube you've had the most luck with in your ST-120. The amp that I have is the latest ST-120 kit from tubes4hifi.com (just in case other versions of the 120 lacked this problem).
Someone mentioned that the JJ GZ34 is not a good one and I would tend to believe that since it lasted about 12 hours (about three sessions of about 4 hours each).
My friend has an ST-70 and his does not have this issue...so that's why I was hoping to hear from some ST-120 owners.
Unfortunately, I somehow missed the discussions on this problem before buying the amp. Although I probably would have still purchased the amp, it would have been nice to not have been surprised by this issue.
And yes, I have already ordered the 1N4007 diodes for the "rectifier diode mod". However, when I asked Bob about this mod he said "...there is the possibility of longer life out of your rectifier tube.". And so that doesn't sound like a sure way to make that tube last as long as it lasts on other tube amps that have at least 60 watt per channel. I have listened and sold many tube amps that generate 60 watts or more of power and they didn't have problems with the rectifier tube having a short life.
Your suggestions are greatly appreciated.
Kind thanks,
Eddie
Someone mentioned that the JJ GZ34 is not a good one and I would tend to believe that since it lasted about 12 hours (about three sessions of about 4 hours each).
My friend has an ST-70 and his does not have this issue...so that's why I was hoping to hear from some ST-120 owners.
Unfortunately, I somehow missed the discussions on this problem before buying the amp. Although I probably would have still purchased the amp, it would have been nice to not have been surprised by this issue.
And yes, I have already ordered the 1N4007 diodes for the "rectifier diode mod". However, when I asked Bob about this mod he said "...there is the possibility of longer life out of your rectifier tube.". And so that doesn't sound like a sure way to make that tube last as long as it lasts on other tube amps that have at least 60 watt per channel. I have listened and sold many tube amps that generate 60 watts or more of power and they didn't have problems with the rectifier tube having a short life.
Your suggestions are greatly appreciated.
Kind thanks,
Eddie
That power transformer has a 5A @ 5V rectifier tap. There is no reason you couldn't use a 5V3, 5V3A, or 5AU4 rectifier. These robust rectifiers were made for color TVs and can carry substantial current and voltage. They make the 5AR4 look like a toy! Sadly prices are up to about $15 each, but they are well worth the expense.
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The ST-120 pushes ANY GZ34 right to its limits. Bob Latino himself tends to recommend the solid state Weber Copper Cap or at least the diode mod. It doesn't matter what other amps do, YOUR amp runs the rectifier hard.
In the other forum you wrote that you set the bias to 55 ma. With 4 output tubes that is a draw of 220 ma. right there. Add the current draw of the (3) 12AU7s and you are very close to the limit. Any disturbance that causes the draw to increase even just a bit can be fatal to the rectifier.
I recommend you install the diode mod using UF4007 diodes instead of the 1N4007. Then replace the rectifier tube with one of the new Tung-Sol or Genalex tubes. I think you'll be fine then.
I have one of the ST-120's with all the extra's from Tubes4HiFi. In talking with Bob Latino who sold me the kit years back, he recommended NOT to use the GZ34 if you were going to run it hard. He mentioned that it was well beyond the current capacity that the GZ34 could support.
Also if you have the supplemental cap kit that puts additional load on the tube. I had built and tested the amp when I put it together with the GZ34 as I was waiting for my weber copper cap (WZ68 as I recall). The 1N4007 mod will not help with the GZ34, the ST120 can draw way more current then the tube is capable of supplying.
If you want to give up some power pick up a 5U4GB from EBAY (NOS are not that bad) and give that a try. They seem to be a lot more rugged then a new GZ34 except for the extra loss in voltage drop. When all said and done lot of amp, not so much rectifier tube.
The Webber Copper Cap works well and looks pretty good in the amp. And has had no issues in use.
Dynaco ST-70 – Tubes4HiFi Build | EZ260
I don't have a pic of the amp with the copper cap, just the GZ34. But like yours it had some fireworks inside at some point shortly before being removed. I was using a JJ GZ34, some folks have reported the Sovtek brand a better and more reliable tube if you want to stick with the tube.
Sandy
Also if you have the supplemental cap kit that puts additional load on the tube. I had built and tested the amp when I put it together with the GZ34 as I was waiting for my weber copper cap (WZ68 as I recall). The 1N4007 mod will not help with the GZ34, the ST120 can draw way more current then the tube is capable of supplying.
If you want to give up some power pick up a 5U4GB from EBAY (NOS are not that bad) and give that a try. They seem to be a lot more rugged then a new GZ34 except for the extra loss in voltage drop. When all said and done lot of amp, not so much rectifier tube.
The Webber Copper Cap works well and looks pretty good in the amp. And has had no issues in use.
Dynaco ST-70 – Tubes4HiFi Build | EZ260
I don't have a pic of the amp with the copper cap, just the GZ34. But like yours it had some fireworks inside at some point shortly before being removed. I was using a JJ GZ34, some folks have reported the Sovtek brand a better and more reliable tube if you want to stick with the tube.
Sandy
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