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Old 24th March 2006, 09:49 PM   #41
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Join Date: May 2005
Location: Pretoria, South Africa
Stuart,

Loud plopping when switching is bad news, especially for your loudspeakers. It can also cause pulses (transients) with unpredictable results in feedback circuits. I don't want to criticise unnecessarily, but plops should really not exist. It is easy and normal to use make-before-break switches (also in relays), or in push buttons where the switch is usually a double-throw, use the "in rest" contact as common. (In that way the input shorts momentarily to common while the next input is brought in, apart from other advantages.)

In an existing amplifier you may not be able to do much about this, but just for information's sake.

With regard to using a cathode resistor as a safeguard measure, I see no problem except for possibly exceeding the safe heater-cathode voltage when the cathode goes high. A further bypass resistor (higher than the operating cathode resistor) might be a good idea - one also gets fair high-voltage zeners these days. In high power amplifiers (over say 50W) I usually chickened out and sensed over-current somewhere in the power supply or at the cold end of the cathode resistor, fed to a simple relay circuit that cut the mains and held it off until everything had been restored. (It will be ironic to save a tube by way of a cathode resistor blowing, only to lose it because of heater-cathode trouble.)

Regards.
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Old 29th April 2006, 10:30 PM   #42
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Join Date: Jan 2005
Location: Las Vegas
Just a couple of questions. What fuse for the AC input was being used at the time of 6CA7 failure? Was the value being used the manufactures recomended value?
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Old 11th May 2006, 07:15 PM   #43
stueyp is offline stueyp  United Kingdom
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Join Date: Jan 2006
Location: Sussex
Default Hi again

Hi. I just checked the mains fuse. It was 13amp. I have no idea what it is supposed to be as the manual does not state! However, one of the left channel tubes went into thermal runaway. I replaced the fuse with 3amp as a long shot. Now the other left el34 is hotting up..... Nothing has blown YET.....*SIGH*...I will solder a fuse in line with the cathode resistor. Can I just ask what value this should be? Also, how do I hard wire UL? Can anyone tell from looking at the pictures of the relay?
I strongly urge people to do their research before buying one of these "amplifiers".
Thankyou guys for your help,
Stuart.
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Old 11th May 2006, 09:23 PM   #44
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Join Date: Jan 2005
Location: Las Vegas
I suspect that your 3 amp fuse will be fine. The 13 amp fuse is almost no protection at all. If one of your tubes goes really bad, it should start drawing enough current to take out the main amp fuse. Well, that's usualy the plan anyway. And this all should happen long before the transformers are damaged, if the transformers were made properly.
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