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Tubes used at the broken resistor is not damaged by that.dtwright wrote:Update! I found the multimeter and checked the 15.6 resistors right after posting the above. The one on the channel that's out is reading "open circuit" so it's clearly bad; sounds like that's the culprit. Thanks for the help! Is there anything else I should check that might have been damaged by this failure? Would having tubes (not the ones that were in during the failure) installed on that channel for about 30-60 seconds of operation (as described above) have damaged them?
I just noticed that the tube that was installed in that socket when the channel went out seems sort of loose in its base, like it has come unglued or something; I will definitely be junking that one...
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Solder Slinger wrote:A couple of things you might want to do that would extend the life of the tubes and the system:
1.) Measure the voltage across pins 2 and 7 of the output tubes with a meter set to the 20 volt or so AC scale on a voltmeter. I'm going to assume that it is higher than 6.3 volts. Since you have a new power transformer on that unit (can see that by the lack of bell covering on the underside and the plastic covered leads from the power transformer) you might want to put a 3 amp CL-80 NTC on the power cord hot (black) lead (maybe between the power switch and the fuse). Measure again, and if still above 6.3 volt, put a second NTC across the white return lead. Having 6.3 volts or slightly less leads to long life of the tubes, additionally the NTCs inhibit the turn on power surge, putting less strain on the power transformer, tube heaters and the circuit in general.
2.) Put a 3-5 watt resistor between the output transformers screen lead and the screen of the power tubes (Pin 4). While this may very slightly reduce power output, the tubes will be much less likely to arc and the overall effect is a better sound. Minimum value is 100 ohms but I personally like about 500-750 ohms. See the following websites for more on this issue:
And much more on this issue with some other enhancements:
3.) Finally, I would highly recommend getting the individual tube bias adjustment kit from DynaKit Parts. This makes it possible to adjust the current flow to each tube rather than the pair which may not be balanced:
Good luck, I really like my ST-70 which is highly modded and very listenable.
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