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Dynakit ST 70. A Restoration Journey

459vdc where I have good resistors. Still waiting for the new resistors. Measuring before the resistors it’s a little higher. Like 462vdc.
 
Okay! So, we have good B+ voltages, good anode plate voltages (pins 3), and good screen voltages (pins 4).

Now, how are the other voltages? On the 7199s or their substitutes?
 
043224bb-e1f7-4ae3-9c2f-c1ab6755d3e1.jpg

Sylvania data sheet. :)
 
Morning guys. All the parts finally arrived and I installed the new resistors yesterday along with a new matched pair of EL34's. Biased them up and listened to some tunes for about an hour. It sounded noticeably better even on my really cheap pair of test speakers.

@Mr Boochie I do have a few more questions though. On the chart that you provided for the voltages it says that when I measure "Across" pin 2 and 7, I should have 6 to 6.5vdc. I get 0vdc on all four tubes. When I measure those two pins with the ground probe attached to ground, I get 44vdc on pin 2 and 7 on the left channel. On the right channel I get only 4.5vdc. There isn't a spec on that chart for what the voltage on those two pins should be, but I find the difference odd. Any clues as to why?

Otherwise all the B+ voltages seem to be Ok. Some are a little high, but my line voltage yesterday was 124vac. I expect that to be the reason why.

Let me know what ya'll think. Thanks, Mark
 
Stereodiver said:
@Mr Boochie I do have a few more questions though. On the chart that you provided for the voltages it says that when I measure "Across" pin 2 and 7, I should have 6 to 6.5vdc. I get 0vdc on all four tubes. When I measure those two pins with the ground probe attached to ground, I get 44vdc on pin 2 and 7 on the left channel. On the right channel I get only 4.5vdc. There isn't a spec on that chart for what the voltage on those two pins should be, but I find the difference odd. Any clues as to why?
Click to expand...
Pins 2 and 7 are the heater pins for the EL34 tubes. Measure the VAC across 2 and 7. It should be around 6.3 VAC, as long as your unit has a stock heater circuit.
 
^^ this. The tubes care about what AC voltage exists from 2 to 7, not about what any of it shows to ground.

as to why they show something different to ground, that depends what has been done with the heater circuit. In stock form, Dynaco ran a capacitor to ground so there isn't really any tie, and it usually doesn't measure much DC voltage. If there is significant DC voltage it may indicate a heater-cathode leak or short, or if the heater circuit has been modified to elevate it above DC thats probably what you are seeing.
 
@Mr Boochie @gadget73 Yup! I get 6.8vac. That chart does say DC and I've been doing my best to follow all directions to the letter!

I'm sure the heater circuit hasn't been modified. Since you guys don't think I have a problem, and the initial listening test went well, I'm calling it GOOD! Back into the big boy system it goes.

Really everyone, THANKS A BUNCH! I'd be lost without you guys! Mark
 
6.8 is a little high. If you can knock down that voltage a bit your tubes will thank you. An external bucking transformer built around a 6.3v filament transformer should serve nicely. That should get the heater supply down to about 6.4 VAC.


Bucking Xfmrs

Bucking Transformers - What they do, how they do it, and how to determine the proper transformer to use
sound-au.com sound-au.com
 
gadget73 said:
6.8 is a little high. If you can knock down that voltage a bit your tubes will thank you. An external bucking transformer built around a 6.3v filament transformer should serve nicely. That should get the heater supply down to about 6.4 VAC.


Bucking Xfmrs

Bucking Transformers - What they do, how they do it, and how to determine the proper transformer to use
sound-au.com sound-au.com
Click to expand...
Would it be worth knocking the AC line voltage down 6V?
 
Its about 5%, so all of the internal voltages will drop by about the same. It could go down a little more, but 6v transformers are pretty available. I guess an 8 or 10v one would do too.
 
Just wanted to wrap this up. I did get the bucking transformer. It does exactly what it’s suppose to do. I have a line voltage of 124vac. Using one outlet produces this:
8CCC1971-47F2-4CC1-8769-CEA2CE9A3485.jpeg
The other outlet:
139095DF-74E0-481D-A21B-5ACDA5B05155.jpeg
I am using the 115vac outlet which gets my heater voltage down to this:
F23322A6-6EB8-471F-A1C3-1744C089B7F9.jpeg
Here is what’s inside the box:

1B6B0CB6-8C0F-4292-88EE-56F16D096954.jpeg
I had a few days of extended listening sessions and so far everything seems to be working properly! I also think the power transformer on the st70 runs slightly cooler,,, maybe.

Hopfully I’m good to go for at least a few years! Mark
 
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