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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jul Tue 16, 2019 10:53 am 
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Joined: May Sat 14, 2011 5:42 am
Posts: 3545
Location: Ft Worth TX
Dyna's 120 transistor amp would do that if that little snubber cap across the output was the least off-spec. I don't remember the SCA having that feature, so:

Scrupulously examine the feedback loop for that channel. Appears as though at some freq the FB is positive. Hope not, but certain output transf anomalies can cause that.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jul Wed 17, 2019 2:28 am 
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Joined: Aug Sat 01, 2009 2:23 pm
Posts: 1679
Shane1 wrote:
Okay, so I have re-capped this amp. I got a new set of tubes, including using 6GH8 with adapters instead of the hard-to-find 7199s. I've also modified the circuit by separating the two channels... each has its own capacitor and 220ohm resistor.

I'm having similar issues as I was before I did all of this. One channel seems to work okay, but the other channel makes a high-pitched squealing noise as soon as the tubes warm up. It's loud enough to make me worry I'm going to blow a speaker, so I quickly shut the thing off as soon as it happens. It seems to be the same no matter the volume level, or input source. Am I going through all the resistors in this thing to check for drifting values, or is there something else I should look at? Something is very not-happy in the circuitry.


Is it possible that the output transformer primary or secondary is wired incorrectly? E.g., the plate or screen leads to the transformer are swapped? A loud squeal can indicate that the feedback loop is positive instead of negative.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jul Tue 23, 2019 2:51 pm 
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Joined: Sep Wed 10, 2014 7:13 pm
Posts: 264
Location: Central Illinois
The SCA-35 was sold as a kit. So incorrect wiring is a possibility.
Do you have the complete Dynaco assembly instructions?
Dynaco always did a great job on their wiring diagrams.

I doubt that testing the amplifier would destroy any output tubes that aren’t dodgy to begin with.
That said, I always recommend using a dim bulb tester on the workbench. The series light bulb may need to be 100W, 150W, or 200W in order to provide enough power. An old radio consumes only 30 to 50W. But a tube type stereo amp will have a mains power consumption of 100 to 200W. That’s why the dim bulb tester needs a larger light bulb.

Always watch for any sign of “red-plating” of the output tubes while bench testing and be prepared to cut power immediately at the first sign of a glowing tube plate.

-EB


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jul Wed 24, 2019 1:16 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4807
Location: Sunbury, Ohio 43074
I’d start by narrowing down where the problem is. Ground both inputs for starters. If it persists follow the signal path through either bypsssing points along the way with a .1 cap to ground, disconnecting things etc.... find out if you have an input issue or an output issue first, and proceed from there

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