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Dynakit ST 70. A Restoration Journey

spantou01 said:
Hi Mark,
That’s great news!

I can’t think why they blew, but 1/4 watt may a bit low in wattage for transients, although they will provide extra protection. The EFB SCA-35 with Dave’s boards uses 100 Ohms 1/2 watt screen stability resistors, and that is probably the smallest I will go with.

Good luck and happy listening.
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I usually see 1/2 or 1-watt resistors. Remember, they are soldered onto the sockets, so over time, with heat, they can drift. Metal oxide type would be more stable.
 
Thanks again guys for holding my hand through this. Parts are ordered.

I am a little bummed that a JJ EL34 shorted out after maybe a little over a year. I guess that’s just the price one pays for tube gear.

After I get and install the parts I’ll post up and let you all know how it’s going.

Mark
 
Stereodiver said:
Thanks again guys for holding my hand through this. Parts are ordered.

I am a little bummed that a JJ EL34 shorted out after maybe a little over a year. I guess that’s just the price one pays for tube gear.

After I get and install the parts I’ll post up and let you all know how it’s going.

Mark
Click to expand...
I had a JJ KT-77 short out in my ST-70 with low hours on it. After replacing it I didn't have any more problems with those KT-77's for years after.
 
Mr Boochie said:
That is not uncommon.
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Other than a couple of those nasty JJ 5AR4's, that KT-77 is the only JJ power or signal tube I've lost. The KT-88's are holding up just fine. The JJ 6L6's I put in that guitar amp kit I built for my son are getting a lot of use and hanging in there. His band has been practicing at my place every week for awhile now along with the various gigs they've been playing.
They were here yesterday. I checked the bias while he was here and it's still where I set it. My son likes the JJ 6L6's over the other tubes we tried in it.
I really liked the Tung Sol 5881's I had. While they're pretty closely matched, they're getting pretty weak.
I'll probably get a set of JJ frame grid 12AX7's for that amp to replace the standard JJ's I put in the amp when I built it since I had them on hand.
 
@spantou01 @Mr Boochie Hey guys. As long as I was ordering parts I went ahead and ordered some 1n4007 diodes to do the "Yellow Sheet" mod on the rectifier tube. Mouser delivered way quicker than anyone else so I installed them today.

I moved one red wire from pin 6 to pin 7 and soldered the diode between pin 7 and 6. Than did the same on pins 5 and 4.

Now when I measure the voltage I get 360vac on pins 7 and 5 where the wire connects. Measuring the voltage on pins 6 and 4 after the diode, I only get 9vac.

Remember, I'm a self proclaimed idiot that just happens to know how to solder. So, the question is...

Does this sound right? Is this the way it's suppose to work?

Thanks, Mark
 
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Yellow sheet - Dynaco rectifier modification.

Do you have the diode's polarity like the diagram?
 
Those screen voltages are definitely not right. The screen should be within a few volts of the plate on a UL transformer. Being half the voltage says something is wrong. Either that resistor is a lot higher in value than 100 ohms, or there is something strange going on within the transformer. Ohm check on the resistor would be telling, or a voltage drop measurement. Screens should be maybe 5ma or something at idle, over 100 ohms that should give around a half volt of drop. If its significantly more, the resistor isn't what it appears to be.
 
Nope, didn't remember correctly. Just measured again and pin 8 on the rectfier is 472vdc.

@gadget73 Yes, I did find two of the resistors open as I stated above. Where the good resistors are they do measure 100ohms. I just measured across it and get .9vdc. Thanks for chiming in buddy!
 
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