Need someone to look at my Dynaco 70

Happy New year,

My modified Dynaco 70 has started popping the main fuse. It also blew the rectifier tube (GZ34). I installed another fuse and tube and after playing for a while, it blew the same fuse. I didn't install another rectifier tube, deciding it'd be better for someone to see why the tube is blowing before potentially wasting a new tube. I am in the Sacramento area, does anyone know a person who has the expertise to diagnose what is happening?

Thanks in advance.


 Just answer the poor guys question... Do you know a tech that can help him out or not .....?


have you looked into Paradyme? I’m not from the area, but they list Vintage Stereo Repair as one of their services (along with car audio and home automation/installs). They’re on Fulton Ave. in Sacramento.

The ST-70s seem to eat rectifiers with regularity…new market 5AR4s especially. I run vintage 5U4Gs in my VTA (Bob Latino) ST-70 as they seem to have the best reliability.

The suggestion to upgrade the power transformer is a good one if you haven’t done that yet. You mentioned mods, but I’m not sure which ones.

Best of luck with the fix!

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@sparkymaui The weak spot in the ST-70 design is that they really needed dual rectifiers. So if you play the amp at higher power levels the rectifier tube is the most likely tube to fail. When it does so it will likely also blow the main fuse. 

The main concern here is that your filter capacitors are new. ST-70s go back a long way- if you have an older one and the filter caps in the power supply have not been replaced, I would not run the amp until this is done, as bad filter caps have a way of damaging power transformers!

But if the caps are OK, then the rectifier tube itself is the most likely fault.