Dynakit ST70 Hum in the background

So it holds with just the rectifier but not any of the output tubes? I suppose they could all be bad but it seems unlikely. Try it with a dim bulb tester and monitor the voltage across the 10 ohm cathode resistors. Maybe try one tube at a time, and set the bias pots as negative as they will go. A bad tube should do something strange, either the voltage at the cathode will go high, or the bulb will get way too bright. Probably need 100w or so for this to work
 
gadget73 said:
So it holds with just the rectifier but not any of the output tubes? I suppose they could all be bad but it seems unlikely. Try it with a dim bulb tester and monitor the voltage across the 10 ohm cathode resistors. Maybe try one tube at a time, and set the bias pots as negative as they will go. A bad tube should do something strange, either the voltage at the cathode will go high, or the bulb will get way too bright. Probably need 100w or so for this to work
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It will stay on indefinitely with the rectifier tube. As soon as I put any tube in, the fuse pops. I have a quad of EL34's in my 8B, but I am afraid of damaging known good tubes to test.
 
By any tube other than the rectifier, do you mean even with just a driver tube installed, no power tubes?
 
Have you checked the OTs? I had this issue recently, took forever to diagnose and found one of the OT leads was ‘repaired’ and hooked up to the wrong winding. Created tons of issues.
 
Lavane said:
By any tube other than the rectifier, do you mean even with just a driver tube installed, no power tubes?
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6CA7's installed one at a time, fuse pops shortly there after. No 12AU7's installed at any point, I can't get it to beyond about a minute of power up.
 
You really should build a dim bulb tester. They're cheap and easy to build.
There must be a short somewhere if even just putting the driver tubes in will pop the fuse. There's a short somewhere. :dunno:
 
Lavane said:
You really should build a dim bulb tester. They're cheap and easy to build.
There must be a short somewhere if even just putting the driver tubes in will pop the fuse. There's a short somewhere. :dunno:
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Yep. Going to have to build a good one. I am pretty sure I have all the required items.
 
Find someone in your area with a tube tester. There’s a thread somewhere around here where people list their location and which tester they have, and allow other local users to use.

outlawmws

Thread 'Schematic Links, Tube retail links, local tube testers'

Location: California, SF bay area, East bay specifically.

Tester #1: Jackson Mod 598 tester
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Here's a video I made on building a dim bulb tester a few years ago. I think it's the most correct way to build one, at least a simple one with a single socket, bulb and switch:
 
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gadget73 said:
Is there a voltage chart in the assembly manual for this board?
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Late to the game, but here is the voltage chart from my vta st70 instructions.
I am seriously lacking in troubleshooting ability, so for me, sometimes its much less trouble to gut a piece and start from scratch. In the end I know everything is done correctly.
An FYI for new sockets, I bought some from a well known vendor. After problems with them, pin sockets so tight one pulled a pin from the tube, and one had the center hole so tight the indexing pin broke. After a call he told me they were Chinese knockoffs. (Not mentioned in the description online) I got a refund, but now Im gun-shy on replacing sockets.

IMG_1763.jpeg
 
While it would be really cool if you could get it working properly, if you can't I have a couple original PCB's from ST-70s I could send you one and you could rewire in a few hours and have a real ST-70 instead of whatever this monstrosity is, no offense but I like Dynacos because of the elegant and simple Hafler designs. This looks like a ham radio lol
 
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